[advances in agronomy] volume 85 || crop residue management for nutrient cycling and improving soil...

CROP RESIDUE MANAGEMENT FOR NUTRIENT

CYCLING AND IMPROVING SOIL

PRODUCTIVITY IN RICE-BASED CROPPING

SYSTEMS IN THE TROPICS

Yadvinder-Singh,1 Bijay-Singh1 and J. Timsina2

1Department of Soils, Punjab Agricultural University, Ludhiana 141 004, India2CSIRO Land and Water, Griffith NSW 2680, Australia

I. Introduction

II. Availability of Crop Residues in Rice-Based Cropping Systems

III. Management Options for Crop Residues

IV. Crop Residue Decomposition

A. Kinetics of Crop Residue Decomposition

B. Factors AVecting Residue Decomposition

C. Fallow Period and Crop Residue Management

V. Crop Residue Management EVects on Nutrient Availability in Soils

A. Nitrogen

B. Phosphorus

C. Potassium

D. Sulfur

E. Micronutrients

VI. EVect of Crop Residues on Soil Properties

A. Soil Fertility

B. Chemical Properties

C. Physical Properties

D. Biological Properties

E. Crop Residues for Reclamation of Salt-AVected Soils

VII. Biological Nitrogen Fixation

VIII. Phytotoxicity Associated with Crop Residue Incorporation into

the Soil

IX. Weed Control and Herbicide EYciency

X. Emission of Greenhouse Gases

A. Methane

B. Nitrous Oxide

C. Mitigation Strategies

XI. Agronomic Responses to Crop Residue Management

A. Rice–Wheat Cropping System

B. Rice–Rice Cropping System

C. Rice–Legume Cropping System

D. Other Rice-Based Cropping Systems

269

Advances in Agronomy, Volume 85Copyright 2005, Elsevier Inc. All rights reserved.

0065-2113/05 $35.00

270 YADVINDER-SINGH ET AL.

I. INTRODUCTION

Rice is the most important crop in Asia, where more than 90% of all rice is

grown and consumed (Blake, 1992). About half of the total area planted to

rice is irrigated, and it accounts for the three-fourths of global production

(Huke and Huke, 1997). In tropical Asia, rice–rice constitutes an important

annual crop rotation. In the subtropical Asia, rice and wheat are grown in

rotation in more than 13 million hectares in the Indo-Gangetic plains of

South Asia (India, Bangladesh, Nepal, Pakistan) and on similar hectarages

in the basin of the Yangtze river in China (Timsina and Connor, 2001). In

addition to wheat, other crops grown in rotation with rice are barley, oats,

maize, sorghum, legumes (mung bean, peanuts, soybean, lentil, chickpea),

oilseeds (mustard, rapeseed), potato, sugarcane, and cotton.

Nutrient cycling in the soil–plant ecosystem is an essential component of

sustainable productive agricultural enterprise. Although during the last three

decades fertilization practices have played a dominant role in the rice-based

cropping systems, crop residues—the harvest remnants of the previous

crop—still play an essential role in the cycling of nutrients. Incorporation

of crop residues alters the soil environment, which in turn influences the

microbial population and activity in the soil and subsequent nutrient trans-

formations. It is through this chain of events that management of crop

residues regulates the eYciency with which fertilizer, water, and other

reserves are used in a cropping system. Another feature of rice-based crop-

ping system in the tropics is the inherent conflict between maximizing short-

term production at minimum cost versus providing sustainable health and

long-term productivity of the soil. One reason for this conflict is the general

below-average economic condition of the farmers practicing rice-based crop-

ping systems. In the tropics, crop residues have, in fact, played a pivotal role

in the maintenance of soil resources at acceptable levels because these are the

major sources of C inputs.

Tropical agricultural ecosystems are distinct from temperate ones in

terms of biological degradation of soils, which results in reduction in organic

matter content due to decline in the amount of C inputs from biomass

(Stewart and Robinson, 1997). Tropical soils vary widely in their properties

and are generally poor in native soil fertility and productivity. The removal

of crop residues leads to low soil fertility and thereby decreased crop

XII. Summary and Conclusions

XIII. Research Needs

References

CROP RESIDUE MANAGEMENT 271

production. Organic materials such as crop residues oVer sustainable and

ecologically sound alternatives for meeting the nutrient requirements of

crops. In addition to their role as the primary source of C inputs, crop

residues, and the way they are managed, have a significant impact on soil

physical properties (Boyle et al., 1989).

Future increase in food production in the tropics will only be possible

through improvement in soil productivity. Increased concern for the envi-

ronment and increased emphasis on sustaining soil productivity has resulted

in major interest in the maintenance and improvement in soil organic matter

in recent years. Proper management and utilization of crop residues and

other agricultural wastes will constitute an important factor in achieving this

objective. With widespread use of combine harvesters, crop residues (mainly

rice and wheat) largely remain in the field and must be managed to provide

the greatest advantages.

The increasing constraints of labor and time under intensive agriculture

have led to the adoption of mechanized farming in rice-based cropping

systems. For example, under highly intensive rice–wheat cropping system in

northwestern India, combine harvesting of rice and wheat fields, which leaves

large amounts of crop residues in the fields, is now a common practice. As

crop residues interfere with tillage and seeding operation for the next crop,

farmers often prefer to burn these residues. In addition to causing environ-

mental pollution, burning results in large losses of organic carbon and plant

nutrients. In recent years, the concept of soil quality has been suggested as a

tool for assessing the long-term sustainability of agricultural practices

at local regional, national, and international levels. Crop residue manage-

ment is known to aVect either directly or indirectly most of the soil quality

indicators—chemical, physical, or biological. It is perceived that soil

quality is improved by the adoption of sound crop residue management

practices. For example, Karlen et al. (1994) evaluated several soil quality

indicators and developed a soil quality index, which had values of 0.45, 0.68,

and 0.86 for removal, normal, and double residue treatments, respectively.

In comparison with green manures and legume residues, cereal straws are

relatively poor with respect to N and P content. Thus, crops sown immedi-

ately after the incorporation of residues of cereal crops suVer due to defi-

ciency of plant-available N. Addition of fertilizer N to the decomposing

residues only partially oVsets the immobilization process. Therefore, a major

problem encountered in the profitable utilization of cereal crop residues is

the occurrence of microbial immobilization of soil and fertilizer N (Mary

et al., 1996). Suitable manipulations of processes such as nutrient immobili-

zation are an important component of an eYcient crop residue management

program. For example, allowing adequate time for decomposition of crop

residues before planting the next crop can be beneficial in alleviating adverse

eVects due to N immobilization and phytotoxicity.


According to one estimate, more than 1000 million tonnes of cereal

residues are being produced annually in the developing world (FAO,

1999). If crop residues could be better managed, this would directly improve

crop yields by increasing soil nutrient availability, decreasing erosion, im-

proving soil structure, and increasing soil water holding capacity. Crop

residues can also lead to negative eVects on crop production in the short

term because of N immobilization and possible release of phytotoxic com-

pounds. Considerable research has been conducted in the last few decades

relating residue management to soil chemical, physical, and biological prop-

erties and consequent fertilizer management practices needed for successful

crop production. In this chapter, we have tried to use this knowledge to

make recommendations and conclusions for crop residue management in

rice-based cropping system. We have not attempted to review all available

literature; only pertinent data sets have been used to substantiate diVerent

conclusions that emerge from the literature. There may be two diVerent

systems of crop residue recycling: (1) when residues are applied directly to

the soil and (2) when residues are first allowed to decompose and are used as

compost. We have focused our attention mainly on the in situ incorporation

of crop residues left naturally in the field under rice-based cropping systems.

A special situation is created by the planting of green manure crops in a crop

rotation and increasing the amount of crop residues at planting time of the

next crop. The use of crop residues as green manures has already been

reviewed extensively (Buresh and De Datta, 1991; Yadvinder-Singh et al.,

1991) and is not included in this chapter. Similarly, depending on the scale

considered, although manures, diVerent organic by-products, animal and

human wastes, and food processing wastes originate mainly from harvested

plants, these are not categorized as crop residues in this chapter. The

challenge is to (1) scientifically understand the short- and long-term eVects

of diVerent crop residue management on the cycling of C, N, and other

nutrients, and (2) develop technologies for crop residue management that

are agronomically beneficial, environmentally friendly, and do not add extra

costs.

II. AVAILABILITY OF CROP RESIDUES IN RICE-BASEDCROPPING SYSTEMS

Rice, wheat, corn, soybean, barley, rapeseed, and potato are the major

residue-producing crops. Asia is the major producer of crop residues—

52.6% of the world residues production occurs in Asia. Rice, wheat, and

corn are the major crops, contributing about 84% of the total production of

crop residue in Asia. On a global basis, these seven crops produced 2956

Table I

Residue Production (�103 t) by Rice and DiVerent Crops Grown in Rotation with Rice in the

Tropics in 1998a

Crop Asia Africa South America World

Rice-straw 771,804 25,968 24,153 844,782

Rice-husk 154,361 5194 4831 168,956

Wheat 379,788 27,395 25,539 946,734

Barley 34,097 6753 2141 208,229

Sugarcane 53,855 8561 41,880 125,227

Cotton 6378 315 69 6801

Oats 2424 342 1604 51,604

Corn 166,205 38,729 54,626 604,013

aData pertaining to residue production was computed by multiplying grain yield data reported

by FAO (1999) with straw:grain ratios reported by Larson et al. (1978) for South America and

by Bhardwaj (1995) and Beri and Sidhu (1996) for Asia and Africa.


million tons of residues in 1998; rice residues were around 1000 million

tons (FAO, 1999). Production of residues by di Verent crops that can be

grown in rotation with rice in di Verent countries is shown in Table I. Rice

contributes about 34.3% of the total residue production, which is 1.2 times

more than wheat. Reliable quantitative estimates of crop residues in tropical

countries are, however, lacking. Below-ground residue production has often

been ignored due to the di Yculty in measuring it. In India, an attempt has

been made to arrive at a figure based on estimates of crop yields and

knowledge of the harvest index of di Verent crops. For example, an estimate

for India was made by Bhardwaj and Gaur (1985) by assuming that all

residues generated were left in the field and that nutrient availability from

this component followed mineralization of 50% per cropping cycle. Average

yields of irrigated rice will have to increase from 4.9 t ha �1 in 1991 to about

8 t ha�1 in 2025 (Cassman and Pingali, 1995). If rice cropping is intensified at

this scale, grain yield and total biomass production will increase by about

60% during the next 30 years. Our rough estimates indicate that the expected

increase in biomass production will potentially increase the amount of

C remaining in straw and roots by about 90 million t year �1 and that of

N by about 1.8 million t year �1 (Doberman and Witt, 2000). This represents

an enlarged sink for CO2 but also a greater potential source for CH4

emission.

Globally, about 31, 26, and 154% of N, P, and K, respectively, of the

fertilizer consumption in 1998 were found in crop residues (FAO, 1999).

Residues of seven leading crops in all the continents contained about

18.8 million tons of N, 2.9 million tons of P, and 24.0 million tons of

K. An estimate of the quantity of N, P, and K contained annually in

Table II

Estimates of N, P, and K ( � 103 t) in Residue Produced by DiVerent Crops Grown in Rotation with

Rice in the Tropics in 1998a

Crop

Asia

Africa and

South America World

N P K N P K N P K

Rice 4862.4 771.8 4600.0 327.8 52.5 446.1 5345.6 849.5 5321.7

Wheat 1898.9 265.9 2354.7 308.1 37.1 417.6 5650.9 662.7 7758.2

Barley 221.6 30.7 235.3 59.9 8.4 73.4 1520.9 220.9 2374.1

Sugarcane 226.2 43.1 360.8 211.9 40.4 338.0 526.0 100.2 839.0

Cotton 64.4 9.6 63.8 4.4 0.6 4.2 69.5 10.3 68.5

Oats 15.3 3.9 40.0 12.3 3.1 32.1 325.1 82.6 851.5

Corn 781.2 216.1 1229.9 788.4 148.7 1013.1 5393.0 984.8 6824.3

aEstimates of N, P, and K in crop residues were computed by multiplying residue yield data

given in Table I with N, P, and K contents in straw reported by Larson et al. (1978) for South

America and by Bhardwaj (1995) and Beri and Sidhu (1996) for Asia and Africa.


the residues of major crops grown in rice-based rotations in di Verent con-

tinents is presented in Table II. These estimates are based on average

nutrient concentrations in crop residues as reported by Larson et al.

(1978) for Europe, South and Central America, and Oceania and by Beri

and Sidhu (1996) and Bhardwaj (1995) for Asia and Africa. The values

reported in Table II do not include nutrients contained in roots. The crop

residue N is available to the extent of 41% in Asia followed by 28% in North-

central America, 15% in Europe, 11% in South America, 4% in Africa, and

only 1% in Oceania. In addition to N, P, and K, crop residues also contain

substantial amounts of secondary and micronutrients. The fertilizer equiva-

lent value of field residues for nine Indian crops worked out to be 760,000

tons, a sizeable and significant figure (Bhardwaj and Gaur, 1985). In China,

straw yield of cereals has been calculated as 621.6 million tons per year, and

20–30% is commonly returned to fields following harvest (Compilatory

Committee, 1990).

III. MANAGEMENT OPTIONS FOR CROP RESIDUES

There exist several options for managing crop residues. These include

being removed from the field, left on the soil surface, incorporated into the

soil, burned in situ, composted, or used as mulch for succeeding crops.

Throughout the tropics there is little recycling of crop residues in the

field—these are either harvested for fuel, animal feed, or bedding or are


burned in the field. Crop residues removed from the field can also be used

as bedding for animals, a substrate for composting, biogas generation or

mushroom culture, or as a raw material for industry. Local conditions

determine the disposal method. Currently, in China, North Vietnam,

India, Bangladesh, and Nepal, complete removal of straw from the field is

widespread in areas with hand harvest and great demand for straw as fodder,

as fuel, or for industrial purposes, causing large nutrient export from rice

fields. Open-field burning of rice straw is predominant in areas with combine

harvesting (northern India, Thailand, parts of China) or where manual

thrashing is done in the field (Indonesia, Malaysia, Myanmar, Philippines,

southern Vietnam). In many parts of the tropics, crop residues are burned in

the field due to the ignorance of farmers about their value and lack of proper

technology for in situ incorporation of residues (Samra et al., 2003). For

example, in the intensive rice–wheat cropping system in the Indo-Gangetic

plains of South Asia, crop residues, particularly rice straw, are not used as

animal feed and are disposed of by burning. This is a cost-eVective method

of straw disposal and helps to reduce pest and disease populations resident in

the straw biomass, but it also causes pollution by releasing CO2, N2O, NH3,

and particulate, leading to global warming and health concerns (Kirkby,

1999). It also reduces the number and activity of soil microbes. The magni-

tude of C and nutrient loss during burning is influenced by the quantity of

residue burned and the intensity of the fire. Complete burning of rice straw

at 470 8C in muZe furnace resulted in 100, 20, 20, and 80% losses of N, P, K,

and S, respectively (Sharma and Mishra, 2001). The corresponding losses

due to burning of wheat straw were 100, 22, 22, and 75%. The losses of

nutrients were less due to incomplete burning of the crop residues in open air

under field conditions: 88.6% N, 1.8% P, 17.5% K, and 25.3% S for wheat

straw and 89.2% N, 5.5% P, 19.9% K, and 20.5% S for rice straw, as

compared to complete burning. No loss of micronutrients was noted

during incomplete burning of straw. The temperature of heating was more

important than the duration of heating.

In the Philippines, Indonesia, and parts of China, heaping of rice straw in

the field at threshing sites is common. Heaping the straw in successive

quadrants of a field each season is recommended to even out nutrient

distribution. The straw decomposes slowly, largely aerobically, and can be

easily spread and incorporated into the soil at the beginning of the next

season. Not much is known about the rate at which straw in heaps decom-

poses or about the loss of N via denitrification or loss of N and K through

leaching.

Because of air pollution concerns and nutrient losses, the burning of

residues is now being reconsidered in many regions of the world (Ocio

et al., 1991; Miura and Kanno, 1997). However, in double- or triple-cropped

rice-based systems with sustained flooding, incorporating straw may reduce


yields (Cassman et al., 1995). The crop residues can impede seedbed prepa-

ration and contribute to disease and weed problems. There are currently few

options for rice straw because of its poor quality for forage, bioconversion,

and engineering applications (Jenkins et al., 1997). Rice growers are there-

fore seeking alternative disposal options, such as incorporation of the straw

into the soil. The incorporation of rice residues and continuous flooding has

become common in tropical soils through intensification of rice cropping

practices (Cassman and Pingali, 1995).

In addition to introducing an extra cost, rice straw incorporation

in association with flooding likely impacts soil fertility through nutrient

and pest interactions (Cassman et al., 1995, 1997; Olk et al., 1996) and

environmental quality through greenhouse gas emissions (Bossio et al.,

1999; Delwiche and Cicerone, 1993). In rice–wheat cropping systems, too,

management of rice straw, rather than wheat straw, is a serious problem,

because there is very little turn-around time between rice harvest and wheat

sowing.

Incorporating the crop residues into the soil and allowing them to decom-

pose returns to the soil almost all the nutrients in the straw. The common

practice of burning the residues can have a net short-term beneficial eVect on

the N supply to subsequent crops but a deleterious eVect on overall N supply

and soil C. In North America and Europe, incorporation of cereal straw is

being considered as an alternative to burning because of concern over the

adverse environmental impacts of burning (Prasad and Power, 1991).

With the advent of direct drilling, there is now much interest in the

possibility of direct drilling of wheat into rice stubble—either the full stub-

ble, or after removal or burning of the header tailings. Current research in

Punjab, India shows that sowing into stubble using no-till seed drill is

impaired by blockages with the loose straw and inadequate closure of the

seed slots. Bed planting provides new opportunities and challenges for

stubble management in rice–wheat systems, which need to be addressed.

Conservation tillage and mulch farming techniques have proven useful

in the highly erodible soils of the Loess Plateau of China (Zhiqiang et al.,

1999). Keeping in mind both socioeconomic and biophysical factors, there is

a need to develop conservation tillage systems for a wide range of rice-

based cropping systems, soils, and agroecological environments. Use of

crop residues as mulch is important to the development of soil-specific

conservation tillage systems in the upland soils of the tropics.

DiVerent residue management technologies or strategies need to be devel-

oped at a regional level to fit diVerent rice-based cropping systems and to

accommodate the management diversity required within a single farming

enterprise. Estimates of relative costs of diVerent options must be developed,

as the most attractive choices might have significant impacts on environmen-

tal quality through their eVects on microbial processes that determine


the magnitude of C storage in the soil, methane emission into the atmo-

sphere, and long-term soil fertility. Incorporation of straw in the soil is the

management option dealt in detail in this chapter. After the crop is har-

vested, the straw is spread on the land and incorporated into the soil by

disking or plowing.

IV. CROP RESIDUE DECOMPOSITION

Decaying of crop residues starts as soon as the residues come into contact

with the soil. The process of decomposition is controlled by the interaction

of three components: the soil organisms or biological processes, the quality

of crop residues, and the physical and chemical environment. The combina-

tion of these components determines not only the rate of decomposition of

crop residues but also the end product of the decomposition process. The

amount of plant materials decomposed in the soils is determined by the loss

of dry weight of these plant materials buried in the soil or by the evolution

of CO2 from plant materials, either unlabeled or 14C or 13C. Burying of

rice straw in soil has been reported to accelerate the decomposition in

comparison with placing the straw on the soil surface (Kumar and Goh,

2000). Residues are managed diVerently; e.g., residues can be placed on the

surface, mixed into the soil, or confined in mesh bags within the soil. Surface

placement or heterogeneous distribution reduces the residue–soil contact as

compared with a homogenous distribution. This may aVect the decomposi-

tion dynamics. Knowledge of such eVects is important when results from

diVerent studies are being compared and is essential when developing

and calibrating decomposition models. It is also important when assessing

the eVects of tillage practices resulting in diVerent degrees of residue–soil

contact, e.g., no-till ploughing and rotovating. The degree of contact be-

tween crop residues and the soil matrix, as determined by the method of

residue incorporation, aVects decomposition dynamics under both natural

and experimental conditions. A dearth of information exists regarding straw

decomposition under upland conditions and its eVect on long-term

N availability in temperate regions.

A. KINETICS OF CROP RESIDUE DECOMPOSITION

Crop residues left in the field after harvest are the raw materials for

humus formation and may represent a significant supply of nutrients to

subsequent crops. Knowledge about residue decomposition is, therefore,


essential for management of agroecosystems. Most of the work on decom-

position of crop residues has been carried out in temperate soils (Kumar and

Goh, 2000). Soil C content depends on the amount of C that leaves the soil

through decomposition, erosion, or leaching. Under normal circumstances,

most of the C is lost from the system through decomposition. Kinetic models

of decomposition have commonly used some form of the first-order equation

(Molina et al., 1983). Although a single rate constant has been used to

describe decomposition over the long term in the field or laboratory (Havis

and Alberts, 1993; Schomberg et al., 1994b; Kuo et al., 1997), most short-

term laboratory studies have shown that crop residues contain two or more

decomposition fractions (Gilmour et al., 1985; Ajwa and Tabatabai, 1994).

In cases in which two or more fractions exist, decomposition can be de-

scribed by a sequence of first-order equations that allow all fractions to

decompose at the same time (simultaneous model).

The change of C in the soil can be expressed mathematically in one kinetic

rate constant of decomposition:

Ct ¼ C0e�k1t þ Cae

�k2t; ð1Þ

where Ct is the amount of soil C at time t, C0 is the amount of soil C at time

0, k1 is the decomposition rate constant (day�1) of the total soil C pool

before amendment of C added, Ca is the amount of C (crop residue) added,

and k2 is the added C. The decomposition process is often viewed as a series

of first-order reactions for various C fractions, each with its own size and

decomposition rate decomposition rate constant (day�1) (Jenkinson and

Rayner, 1977; Parton et al., 1988). The rapid and slow fractions with a

characteristic slope and intercept can be mathematically represented as two

simultaneous first-order reactions:

% decomposed ¼ % rapidð1 � exp½�k1t�Þ þð100 �% rapidÞð1 � exp½�k2t�Þ; ð2Þ

where % rapid is the amount of crop residue organic C in the rapid fraction,

(100�% rapid) is the amount of crop residue organic C in the slow fraction, k1

is the rapid-fraction first-order rate constant, k2 is the slow-fraction

first-order rate constant, and t is the elapsed time. The percentage of the

crop residue C remaining is 100 minus % decomposed in Eq. (2).

The rate constant increased with temperature and was significantly lower

under flooded conditions (Devevre and Howarth, 2000). The rate of decom-

position of rice straw at 25 8C under flooded conditions was as low as the

rate of decomposition of rice straw at 5 8C under nonflooded conditions.

Under nonflooded conditions at 15 and 25 8C, the model described two pools

of decomposable C (C1/k1 and C2/k2). The first pool (C1) in the nonflooded

treatment at 25 8C represented 30% of the straw C with a turnover time


of 12.5 days; the second pool (C2) represented 36% of the straw C with

a turnover time of 100 days. The corresponding values at 15 8C were

24% with a turnover time of 17 days and 53% with a turnover time of 333

days.

Under flooded conditions, only one pool of C could be described using

the equation. At 25 8C, C mineralized represented 52% of the straw C with a

turnover time of 50 days against 46% of the straw C at 15 8C with a turnover

time of 100 days. The most apparent reason for only one pool of decompos-

able C in flooded treatment was that recycling of waste products from

fermentative metabolism extended the availability of labile sources of C.

From the amount of C mineralized in the flooded treatments, it is evident

that the C2 pool (recalcitrant C compounds: cellulose, lignin, and microbial

melanins) was partially degraded and contributed to the total C mineralized.

The study indicated that the conversion of C2 straw components to C1

components under fermentive conditions most likely increased the C utiliza-

tion eYciency of the more recalcitrant C2 pool under flooded conditions.

The larger biomass, the simultaneously lower total amount of C mineralized,

and the higher eYciency of substrate C conversion to microbial biomass

(yield factor) in the flooded soil supported these observations (Devevre and

Howarth, 2000). These researchers reported values of C1 and C2 of 1086 and

1324mg C g�1 and k1 and k2 values of 0.08 and 0.01 day�1 under nonflooded

conditions, respectively. The C1 and k1 values for flooded soil were 1916 mg

C g�1 and 0.02 day�1 incubated at 25 8C.

If management changes are desired to achieve an increase in total soil C or

soil organic matter (SOM), Eq. (2) allows for two major options: increase

C input or reduce decomposition rates. Virtually no studies have explored a

reduction in decomposition rates in intensive lowland rice systems, and

hence no literature review can be presented.

Table III shows that rates of crop residue decomposition depend on

residue type, length of decomposition period, and climatic conditions.

Cheng and Wen (1998) studied the decomposition of rice straw over a

10-year period in two soils with diVerent mineralogical characteristics

in fields under upland and submerged conditions in China. Using the

first-order equation for residue decomposition, they calculated annual min-

eralization rates (k) of 0.127 under upland and 0.106 under submerged

conditions in yellow-brown soil (pH 7.7). The corresponding rates in red

soil (pH 4.6) were 0.159 and 0.0948. The half-lives of residual C in the two

soils were 4.4–5.5 years under upland and 6.5–7.3 years under submerged

conditions. The percentage of organic C retained in two soils under upland

and submerged conditions was 29.3–31.3% after year 1 and 7.92–11.6% in

year 10 in the yellow-brown soil and 33.5–35.2% in year 1 and 7.48–13.0%

in year 10 in the red soil. Mineralization of residual organic N followed

the same pattern as residual C. More N from plant material was retained in

Table III

Decomposition of DiVerent Crop Residues in DiVerent Laboratory and Field Studies

Reference

Type of

study

Method used

and experimental

conditions

Crop

residue Duration

Decomposition

(%)

Cookson et al.

(1998)

Field Litter bag Wheat straw 90 days 36

Barley straw 90 days 42

Saini (1989) Field Litter bag Rice straw 197 days 73

Reddy et al.

(1994)

Field Litter bag Rice straw 330 days 90

Ghidey et al.

(1985)

Field Wheat straw 10 months 36

Soybean 10 months 74

Sunflower 10 months 61

Saviozzi et al.

(1997)

Field Litter bag Soybean 1 year 66

Sunflower 1 year 73

Rape 1 year 75

Kaboneka et al.

(1997)

Laboratory Soybean 30 days 67

Wheat straw 30 days 39

Martin et al.

(1983)

Laboratory Rice straw 14 days 14

90 days 46

1 year 90

Yadvinder-Singh

et al. (2004b)

Field Litter bag Rice straw 10 days 23

20 days 35

40 days 51

77 days 61


the yellow-brown soil than in the red soil. Rice straw mineralized more

slowly under submerged conditions than under upland conditions.

Buyanovsky and Wagner (1997) described the C decomposition from

wheat straw using nonlinear regression to fit a two-component exponential

model. This relationship could be used to calculate the percentage of the

residue C remaining in the soil at any specified time. The first component

represents the rate of mineralization of the readily decomposable fraction.

This includes the simple sugars, soluble proteins, hemicellulose, and cellu-

lose. The second component represents the mineralization of the resistant

products of microorganisms. The half-life of the first component was 18 days

and that of second component was 433 days. The humification coeYcient for

wheat straw was 0.24.

Mishra et al. (2001a) studied the C and N mineralization from wheat

straw using the nylon mesh bag technique in a silty loam paddy soil. Wheat

straw decomposed at a faster rate initially for 2 weeks, and the rate of

decomposition (measured as loss in weight of straw) slowed down thereafter,

probably when soluble carbohydrates and proteins were exhausted. Within


2 weeks, 30.7 and 25.3% of wheat straw in the bags had decomposed in the

wheat straw plus green manure and wheat straw alone plots, respectively.

The green manuring accelerated the decomposition of the wheat straw by

lowering the C:N ratio of the decomposing material and by stimulating the

microbial population to carry out the decomposition. By the end of 22 weeks,

82–86% of the wheat straw was decomposed. The practice of wheat straw

incorporation in conjunction with green manure holds promise for improv-

ing the soil productivity in rice–wheat cropping systems. The decomposition

of wheat straw followed the first-order kinetic model. The decay rate con-

stant (k) for the wheat straw was 0.013 day�1 and the half-life was

60 days. The C loss could account for 48–49% of the weight loss during

22 weeks. The other constituent of weight loss from wheat straw could

be soluble components such as K, Cl, and organic substances released as

intermediate products during decomposition. Up to 90% of K can be lost

from the crop residues within a few weeks after incorporation into the soil.

Zhu et al. (1988) reported that about 75% of wheat straw was decomposed in

about 150 days under field conditions in China.

Using the nylon mesh bag technique, Mishra et al. (2001b) noted three

phases in the decomposition of rice straw in a silty loam soil during wheat

growing season. The first phase lasted for 5 weeks, during which the rate of

decomposition was relatively faster (38% of the rice residue decomposed),

followed by the second phase of slow rate of decomposition from the 6th to

the 15th weeks (22% of the rice residue decomposed), which may partly be

attributed to the prevailing low air temperatures, followed by the third phase

of fast rate of decomposition up to 23 weeks (19% of the rice residue

decomposed) due to the rising air temperatures. By the end of 23 weeks,

79% of the rice straw was decomposed. The decomposition of rice straw

was satisfactorily described by the Douglas and Rickman (1992) model

(R2 ¼ 0:97). The computed values of fN and k were 1.356 and �0.00045

CDD�1, respectively, where CDD is cumulative degree days. Witt et al.

(1998), using the litter bag technique, reported a 56% decrease in rice crop

residues within 56 days after incorporation at IRRI, Philipines.

Kanazawa and Yoneyama (1980) observed that the decomposition of rice

straw occurred in two phases in a clayey soil under flooded and upland

conditions in a laboratory at 30 8C. During the first 2 to 4 months of

incubation, the dry weight decreased by half. This was followed by a long

period of very gradual weight decrease. After 12 and 24 months, about 70

and 75% of the initial weight of rice straw, respectively, was lost under both

flooded and upland conditions. The above decay pattern has two phases:

rapid C loss in the first few months and then slow loss in the subsequent long

period. The period of rapid decay seems to be almost coincident with the time

of high population of bacteria and fungi. This study showed that the fungi

are the main agents in the decomposition of organic materials under upland


conditions. In contrast, anaerobes are the main agents in the course of

residue decomposition under flooded conditions, and the breakdown of

cellulose and lignin may be slow under such decomposing systems. Under

field conditions, soil moisture conditions may change from time to time, and

this may cause the shifting participation of diVerent microorganisms during

plant residue decomposition. The rate of decomposition was slightly higher

under flooded than under upland conditions, but the pattern of residue

decomposition was similar under the two moisture regimes. The residue

decomposition trend was closely related to the changes in the microbial

population (bacteria, fungi, and actinomycetes) under flooded conditions.

On the other hand, under upland conditions, significant correlations were

observed between residue weight loss and bacteria or fungi, but not between

residue weight and actinomycetes. This suggested that the fresh crop residues

added to soil are a good substrate for microbial activities. Though the

numbers of microbes were small under flooded compared to upland condi-

tions, aerobic microbes may play some role in the decomposition process of

rice residues in the early incubation periods when the O2 concentration in the

soil is high. In the early incubation period, addition of rice straw caused an

increase in the microbial numbers under both flooded and upland condi-

tions. The decomposing ability of the anaerobes under flooded conditions

may be high enough to decompose the plant materials to a similar extent as

that of the aerobes under upland conditions. The numbers of microorgan-

isms in upland soil were larger than under flooded conditions, and the

addition of rice residues brought about a vigorous increase in the numbers,

especially of actinomycetes and fungi.

B. FACTORS AFFECTING RESIDUE DECOMPOSITION

In tropical systems, mineralization rates are potentially higher because of

high soil temperatures during cropping season, particularly at the time of

incorporation of residues. Several reviews have summarized the factors

aVecting crop residue decomposition, particularly in temperate climates

(Kumar and Goh, 2000; Parr and Papendick, 1978; Prasad and Power,

1991; Smith et al., 1992). We include here only a brief commentary on the

pertinent factors of crop residue decomposition that have relevance to rice-

based cropping systems in the tropics. Carbon and nitrogen cycling are

mainly caused by changes in the frequency, amount, type, and mode of

recycling of crop residues; the frequency, length, and intensity of wetting

and drying cycles, that is, disturbances that cause severe shifts in microbial

activities and also aVect soil physical properties; and the O2 supply to the soil

during rice growth, that is, the amount of irrigation water percolating

through the soil and the intensity of soil reduction processes.


There are three main factors that aVect crop residue decomposition in the

soil: (1) crop residue factors, (2) edaphic factors, and (3) management

factors. The development of an eVective crop residue management program

depends on a thorough understanding of the ways in which these factors

influence the decomposition process. It has been recognized that organic

residue decomposition and hence soil organic matter dynamics are a

direct result of the physiocochemical environments, e.g., aeration (aerobic/

anaerobic, soil structure) and the quality of the resource acting through their

regulation of the decomposer community.

Crop residue chemical composition plays an important role in determin-

ing decomposition rates. Thus, in order to predict the decomposition and

nutrient mineralization patterns of plant residues, it is essential to under-

stand their constitution in terms of soluble and resistant fractions. Early in

the decomposition process, rapid loss of simple sugars and amino acids may

occur within a few hours to a few days, while polysaccharides, proteins, and

lipids decompose at much slower rates. Lignin makes up 5 to 30% of crop

residue material and is more resistant to decomposition than other plant

constituents. Lignin is an important substrate for soil humus formation due

to its resistance to decomposition.

Janzen and Kucey (1988) found that diVerences in decomposition rates of

crop residues were positively correlated with crop N content. There was no

significant relationship reported between decomposition rates and C:N ratio,

water-soluble C, lignin, hemicellulose, and cellulose content of crop residues.

Using a perfusion system, Villegas-Pangga et al. (2000) observed that the

CO2 release rates in 30 rice varieties varied; the percentage of C released

from straw ranged from 15.4 to 38.4% in 42 days. There was an inverse

relationship (R2 ¼ 0.6) between cumulative C release and C:N ratio and a

direct relationship between digestible organic matter (DOM) and cumula-

tive C release. A straw quality index (SQI) was developed to describe the

decomposition rate of the rice straw as follows:

SQI ¼ �56:85 þ ð11:68 �% NÞ þ ð1:25 �% DOMÞ þ ð2:59 �% ligninÞ;R2 ¼ 0:81: ð3Þ

These findings suggested that SQI is a practical tool for assaying the quality

of the straw materials to predict their usefulness in crop residue management

systems. Despite a twofold diVerence between varieties in the amount of

C evolved over 20 days, the proportion of nutrient release did not diVer

significantly between them.

The availability to microbes of C and N contained in crop residues along

with lignin content greatly influence decomposition rates and N availability

to plants (Vigil and Kissel, 1991). It is generally accepted that residues with

low N content or a high C:N ratio decompose more slowly than those with


a low C:N ratio or high N content (Magid et al., 1997; Parr and Papendick,

1978). Christensen (1986) found that 44% of wheat straw containing 0.92%

N decomposed during the first month but only 7% of the straw containing

0.4% N decomposed during the same period of incubation. Luo and Cheng

(1991) found that the number of days required for 50% mass loss of crop

residues was significantly correlated with the N content of the residue.

Decomposition rates are normally greater for legume residues (low C:N

ratio) than those for cereal residues (high C:N ratio) (Ladd and Foster,

1988).

Although N content and C:N ratio are useful in predicting residue de-

composition rates, they should be used with some caution. Reinertsen et al.

(1984) and Stott and Martin (1989) indicated that the C:N ratio of straw was

not a good decomposition index. De Haan (1977) found no relationship

between percentage of N in added plant residue and the rate of decomposi-

tion. Gilmour et al. (1998) observed that initial (0–2 weeks) decomposition

was related to crop residues N and C:N ratio, while subsequent decomposi-

tion was not related to these factors. Since C:N ratio does not indicate

the availability of the C and N to the microorganisms, crop residue decom-

position based on available C and N seems to relate more closely to

field observations than decomposition based on total C and N contents

(Mtambanengwe and Kirchmann, 1995).

The concentration of polyphenol is generally greater in mature resi-

dues than in green leaves (Fox et al., 1990; Palm and Sanchez, 1991).

The rate of plant residue breakdown depends on the relative proportion

of these fractions. Hagin and Amberger (1974) estimated the half-life of

sugars, hemicellulose, cellulose, and lignin as 0.6, 6.7, 14.0, and 364.5 days,

respectively. Other factors such as lignin, hemicellulose, and polyphenol

content should also be considered for predicting decomposition of crop re-

sidues. Lignin is known to be a recalcitrant fraction and is highly resistant to

microbial decomposition (Mellilo et al., 1982). Many workers have found

that increasing lignin concentration reduces the decomposition rate and

nutrient release from plant residues (Fox et al., 1990; Tian et al., 1992).

Saini et al. (1984) reported that the rates of decomposition of stubbles of

rice, wheat, and rape were lower than those of their straws due to high

ash and lignin contents. Polyphenol concentration in plant tissue also

reduces its rate of decomposition by binding to protein and forming com-

plexes resistant to decomposition (Vallis and Jones, 1973). Since polyphenols

have diVerent properties with respect to binding N-containing compounds

depending upon their molecular weight (Scalbert, 1991), these govern de-

composition and N release in some studies but not in others (Vanlauwe et al.,

1996).

The decomposition rate of plant residues cannot be predicted from a

single property of the organic material. When considered simultaneously,


these properties can predict the decomposition rate from a wide range of

plant residues. In some studies, polyphenol:N and (lignin þ polyphenol):

N ratios have been correlated with residue decomposition and nutrient

release (Constantinides and Fownes, 1994; Fox et al., 1990; Palm and

Sanchez, 1991; Tian et al., 1995). It has been suggested that the polyphe-

nol:N ratio may serve as a short-term index for green manures, while the

(lignin þ polyphenol):N ratio could be used for more mature or woody plant

materials (Palm, 1995).

1. Residue Particle Size

The accessibility of plant residues to soil microbes is of primary impor-

tance in their rate of decomposition. The particle size of the residue can

provide diVerent degrees of accessibility, which in turn aVect residue decom-

position rates as well as the mineralization-immobilization process. Gener-

ally, small particles decompose faster than large particles because the

increased surface area and better distribution in soil will increase the suscep-

tibility to microbial attack (Jensen, 1994). Angers and Recous (1997) studied

the eVect of particle size (0.03 to 10 cm) of wheat straw (C:N ¼ 270) on the

decomposition in a silt loam soil incubated at 15 8C. Early decomposition

(3–17 days) was faster for the small-sized particles (0.06–0.1 cm), followed by

the large-sized particle (5 and 10 cm). After 102 days, the very fine particles

(<0.1 cm) showed the greatest and the intermediate-sized classes (0.5–1.0 cm)

the lowest amount of C mineralization. On the other hand, finely ground

particles of ryegrass (C:N ¼ 9) decomposed at a lower rate than intermedi-

ate-sized classes. It was hypothesised that greater accessibility and availabil-

ity of N were responsible for the higher rate of decomposition observed for

finely ground wheat straw, while a physical protection of finally ground

residues was probably involved in the observed reverse eVect of ryegrass

with a low C:N ratio. Puig-Gimenez and Chase (1984), however, observed

no significant eVect of length of straw on its decomposition. The eVect of

plant residue particle size on C and N mineralization may thus be an

interaction between clay content, secondary metabolic products, plant resi-

due chemical composition, period of decomposition, and fanual activity

(Kumar and Goh, 2000).

2. Environmental Factors

Environmental conditions can aVect residue decomposition rates (Parr

and Papendick, 1978). Generally, decomposition rates are faster in tropical

areas and decrease as water availability and temperature decrease. Maxi-

mum activity of decomposers is near 30 to 35 8C and thus supports maximum


residue decomposition in this range (Roper, 1985; Stott and Martin, 1989).

Singh et al. (1995) found that 24.8–29.0% and 39.5–43.4% of the applied

C through rice and wheat residues was decomposed in 60 days at 258 and

40 8C, respectively.

Soil water content can dramatically influence crop residue decomposition

and nutrient cycling (Doel et al., 1990). The optimum water potential for

residue decomposition lies in soil water potentials between �30 and �100

kPa. Soil dried to a water potential of 10 MPa evolved CO2 at about one-half

the rate of soils incubated at optimum water content (�20 to �50 KPa)

(Sommers et al., 1981). Pal and Broadbent (1975) showed that the maximum

rate of decomposition of plant residues occurred at 60% water holding

capacity (WHC) and the rates decreased at either 30 or 150% WHC. The

lower rates of straw decomposition under lowland conditions are probably

due to limited aeration for microbial activity. However, Villegas-Pangga et al.

(2000) reported that there was 27–45% reductions in C evolution in rice straw

under anaerobic conditions compared to aerobic systems. These results sug-

gested that under flooded conditions, depletion of O2 decreases the decompo-

sition rate of straw but the initial rate of nutrient release is unaVected. This

uncoupling of C and nutrient release appears to be related to the more labile

components of the nutrients present in the plants and their physiological role.

Devevre and Horwath (2000) reported that flooding had a tendency to

reduce C mineralization, and the study showed that anaerobes recycled

fermentation waste products during the long-term incubation, resulting in

a lower net residue C mineralization in flooded systems compared to non-

flooded conditions. As a result, similar microbial production was observed

under flooded and nonflooded conditions even though anaerobes decom-

posed less straw C than aerobes. These results indicate that a significant

amount of decomposition occurred under flooded conditions, but because

substrate use eYciency was higher, less straw C was mineralized than under

aerobic conditions. Kinetic analyses of C mineralization curves confirmed

that C mineralized in the flooded treatment was mainly from labile pools,

with significant amounts coming from more recalcitrant pools, such as

cellulose and lignin depending on temperature.

Li and Lin (1993) reported that in the Wuxi province of China, decom-

position rates of rice straw were similar under upland and submerged con-

ditions. Drying and re-wetting conditions encountered under field situations

may also influence the decomposition of plant residues. Thus, Gestel et al.

(1993) found that soil drying and wetting increased the turnover of14C-labeled plant material. Nyhan (1976) summarized published data,

which showed that Q10 (the change in the rate of reaction for each 10 8C

change in temperature) for plant material decomposition averaged about 2.6

for temperatures from 12.5 to 40 8C. Later on, Howard and Howard (1993)

reported Q10 values from about 2.0 to 2.8 for soil respiration within the


10 to 20 8C temperature range. They also noted that the eVect was similar

across various soil moisture regimes.

3. Management Factors

The eVect of management is predominant in controlling the amount and

kind of plant residues returned to the soil and in determining the degree of

soil disturbance through tillage. Residues are managed diVerently; e.g.,

residues may be placed on the surface, mixed into the soil, or confined in

mesh bags within the soil. Surface placement or heterogeneous distribution

reduces the residue–soil contact as compared with a homogenous distribu-

tion. This may aVect the decomposition dynamics. Knowledge of such

eVects is important when results from diVerent studies are being compared

and is essential when developing and calibrating decomposition models. It

is also important when assessing eVects of tillage practices resulting in

diVerent degrees of residue–soil contact, e.g., no-till ploughing and rotovat-

ing. The degree of contact between crop residues and the soil matrix, as

determined by the method of residue incorporation, aVects decomposition

dynamics under both natural and experimental conditions.

Evidence from laboratory and field studies has suggested that the rates of

the decomposition of plant materials added to soil are proportional to the

amounts initially added (Larson et al., 1972). Generally, small amounts of

crop residues will decompose more rapidly than large amounts (Novak,

1974). However, Ladd et al. (1983) found the amount of 14C-labeled plant

material (Medicago littorallis) added was greater; the proportion of residual

organic C and N in soil was smaller.

Residue placement may be paramount in controlling the rate of crop

residue decomposition and nutrient cycling. Crop residue placement influ-

ences soil temperature, and water regimes indirectly influence microbial

activity and residue decomposition. Residue management practices that

involve intensive tillage and incorporation of residues increase residue de-

composition rates and loss of SOM. Residue incorporation eVects are diY-

cult to separate from tillage eVects because incorporation is accomplished

through some type of tillage operation. Decomposition rates of incorporated

residues (rice and wheat straw) faster than those of surface residues resulted

from greater soil–residue contact, a more favorable and stable micro-

environment, particularly soil moisture regime, and increased availability

of exogenous N for decomposition by microorganisms (Cogle et al., 1987;

Schomberg et al., 1994a). Important secondary eVects of tillage on the soil

microclimate include the influence of surface residue coverage on soil tem-

perature, water interception and infiltration, and the eVects of tillage-

induced changes in porosity and the eVects of soil structure on soil aeration


and water relations. Observed increases in organic matter with reduced

tillage systems are largely attributed to reduced decomposition rates of

surface residues compared to the rapid decomposition of incorporated resi-

dues (Schomberg et al., 1994b). However, the rate and degree of organic

matter accumulation associated with surface residues vary widely due to

diVerences in climate, soil type, and residue quality. Under conditions of

warm temperatures and increased water availability, SOM accumulation

from surface residues is reduced.

The depth of residue incorporation has also been shown to aVect

the decomposition of residues. Increasing the depth of residue incorpora-

tion from 50 to 200 mm resulted in a decrease in breakdown rate due to

less biological activity (Kanal, 1995). Likewise, the decomposition rate of

rice straw was reduced by 13% by increasing the depth of incorporation

of residues from 0–10 cm to 20–30 cm (V. Beri, Department of Soils, PAU,

Ludhiana, personal communication). In contrast, Breland (1994) found that

increasing the depth of incorporation up to 300 mm increased the decompo-

sition rate of residues, due to more favorable moisture regimes in lower

layers. Puig-Gimenez and Chase (1984), however, reported that straw

decomposition was not aVected by the depth of placement.

Crop residues managed previously can also significantly aVect the decom-

position of freshly applied crop residues. For example, Cookson et al. (1998)

observed that on a soil where wheat, barley, or lupin residues were previous-

ly managed for 3 years, wheat straw from the incorporated treatment had a

decomposition rate 50% greater than that from the burned or removed

treatment during 90 days.

4. Availability of Nutrients

Incorporation of crop residues increases the populations of all types of

macro- and microorganisms, and the new cells require all the essential

nutrients for their growth and activity. Residues of cereal crops such as

wheat, rice, barley, and maize have larger C:N ratios (low N contents) that

may require the addition of exogenous N in order for decomposition to

proceed. Mary et al. (1996) concluded that mineral N availability in soil is an

important factor controlling decomposition under field conditions. Accord-

ing to Jenkinson (1981), it is unusual for nutrients other than N to limit the

decomposition of plant materials in normal soils. Enhanced decomposition

rates of crop residues due to the application of N have been reported by

several workers (Bangar and Patil, 1980; Debnath and Sinha, 1993). In

contrast, no or even negative eVects of N added on decomposition and

microbial activity have also been reported (Cheshire and Chapman, 1996;

Clay et al., 1990; Hassink, 1994). The reduction in decomposition in the


latter studies was probably due to the adverse eVects of NH3 toxicity on

decomposers.

5. Soil Properties

Soil texture may aVect the decomposition of plant materials. Decomposi-

tion of crop residues was more rapid in soils with less clay content because

clay protected the organic matter from decomposition (Jenkinson, 1977;

Merckx et al., 1985; Ladd et al., 1996). As clay content increases, soil surface

area also increases which results in increased organic C stabilization poten-

tial. The role of clay in stabilizing organic matter appears to be more

important in warm soils, where decomposition occurs at higher rates.

Skene et al. (1997) showed that for high-quality substrates, physical protec-

tion by inorganic materials is a major limiting factor to decomposition,

whereas for low-quality substrates, chemical protection is the major limiting

factor. Hassink et al. (1993) reported that the texture influences the soil

physical environment, which further aVects microbial activity. Li and Lin

(1993) also stated that soil properties controlled the decomposition rates of

crop residues during the first 2 years at two diVerent locations in China. On

the other hand, Amato et al. (1987) could not observe a significant eVect of

soil properties on decomposition of wheat residues.

Soil pH aVects both the nature and the size of the microbial population,

both of which ultimately aVect the residue decomposition (Paul and Clark,

1989). In general, decomposition of crop residues proceeds more rapidly in

neutral than in acid soils. Consequently, the treatment of acid soils with lime

enhances the decomposition of plant residues (Condron et al., 1993). Like-

wise, soil salinity also aVects the residue decomposition through its direct

influence of osmotic potential on microbial activity or through alterations of

pH, soil structure, aeration, and other factors (Nelson et al., 1996). In the

absence of pH and aeration eVects, sodicity increased and salinity decreased

the decomposition of plant residues, but with no significant interaction

(Nelson et al., 1996). Jenkinson (1971) reported that the decomposition

pattern is not substantially aVected by soil properties or crop residue type

included in the study, and the patterns obtained in diVerent climates can

essentially be superimposed by using an appropriate rate constant factor.

Using the buried mesh bag technique, Henriksen and Breland (2002)

found that soil type had no eVect on decomposition of the easily degradable

clover residues, but cumulative C mineralization of barley straw after 52

days was less in the subsoil than in the topsoil by 12% of initial C. This study

showed the necessity of ensuring realistic conditions for the decomposer

microflora when studying eVects of substrate quality on decomposition

and when extrapolating the results for hemicellulose rich residues to field


conditions. The diVerences in decomposition as aVected by residue–soil

contact in laboratory experiments could be due to soil treatment before

incubation, for example, sieving, which seems to damage fungi in particular.

Thus, the diVerences may not be distinct under natural field conditions with

an undisturbed microflora.

C. FALLOW PERIOD AND CROP RESIDUE MANAGEMENT

In a traditional single-crop rice system, fallow periods between crops are

long and the soil is allowed to dry, resulting in re-oxidation of reduced

substances, more complete decomposition of added organic matter, and

large shifts in microbial communities. Similar changes occur in rice followed

by wheat or other upland crop systems. In contrast, in a double- or triple-

crop rice system, fallow periods are short, the soil is not allowed to dry and

re-oxidize completely, and large amounts of crop residues are returned to the

field more than only once per year. Fallow periods may range from very dry,

to dry-wet, to completely wet conditions and diVer in their length, but only

under drier conditions did a shallow early tillage accelerate soil N minerali-

zation during early growth stages of the rice crop grown thereafter

(Dobermann and Witt, 2000). It is therefore likely that years of intensive

rice–rice cropping lead to a decline in the steady-state soil redox potential

along with a gradual accumulation of reduced substances (Fe and organic

compounds), a change in the qualitative composition of SOM toward more

phenolic compounds, and possibly even an increase acidification of the

rice rhizosphere resulting from greater root-induced Fe2þ oxidation. Unfor-

tunately, we lack long-term experiments with rice-based systems of diVerent

cropping intensity in which such subtle changes with time have been

measured over periods more than 10 years.

In systems in which declining SOM content is a concern (rice-upland

crop), straw incorporation may be the only choice for maintaining or in-

creasing SOM content. In rice–rice (-rice) systems with short aerobic peri-

ods, organic matter management must focus on managing the quality of

soil organic matter, that is, avoiding the accumulation of highly complex

organic matter with slow N mineralization rates. In this case, the time of

incorporation of organic materials is more important than the amount.

Compared with the traditional method of wet incorporation shortly

before planting of the next rice crop, the potential benefits of shallow

incorporation of the rice straw shortly after the harvest of rice include

(1) accelerated aerobic decomposition of crop residues (about 50% of the

carbon within 30–40 days), leading to increased N availability (Witt et al.,

1998) and reduced CH4 emission; (2) re-oxidation of ferrous iron and other

Figure 1 EVect of decomposition period on the mass remaining of litter bag rice residue.


reduced substances, leading to increased P availability; (3) reduced weed

growth; and (4) savings in irrigation water during land soaking for rice by

reducing cracking and bypass flow water losses in heavy clay soils. Early

incorporation of residues allows additional time for phenol degradation to

occur under aerobic conditions, thus possibly altering the rates of soil

organic matter formation and subsequent decomposition.

Using the litter bag technique, Yadvinder-Singh et al. (2004b) observed

that time of incorporation had a large eVect on the decomposition of rice

residue during the fallow phase (October–November) after rice harvest

(Fig. 1) in rice–wheat rotation in Punjab, India. At wheat seeding, the

mass loss of rice residue was 51% for a 40-day decomposition period,

compared with 35% for a 20-day decomposition treatment and 25% for a

10-day decomposition treatment. The amount of mass loss remained signifi-

cantly higher for the 40-day decomposition period compared to the 10-day

or 20-day period up to 72 days after seeding of wheat. At the end of the

study, no significant diVerence, however, was noted among the three

treatments.

The data for all three decomposition periods could be best described

using a single logarithmic equation:

Y ¼ 14:9 lnðXÞ � 8:74; R2 ¼ 0:951 ðn ¼ 15Þ; ð4Þwhere Y is the total decomposition period (days) and X is the percent mass

loss of rice residue. Equation (4) may prove useful in predicting residue

decomposition under soil and environmental conditions similar to those in

this study. Rice straw decaying for 0–10 days lost 2.45% of its initial mass


each day. In comparison, the values were 1.0% day�1 for 10–20 days and

0.8% day�1 for 20–40 days during fallow.

The N concentration of the rice residue increased continuously during

the 190-day decomposition period, indicating loss of C as CO2 and/or N

immobilization in the residue by microorganisms, which build up new micro-

bial protein from plant and soil N. The N content of rice straw at the time of

incorporation was 5.6 g kg�1, which increased to 14.8 g kg�1 at the end of the

study period. At the time of wheat sowing, residue N was significantly

lower (7.0 g kg�1) in the 10-day decomposition treatment than in the 20-day

(7.9 g kg�1) or 40-day (8.1 g kg�1) treatment. At the end of the study (150 days

after wheat sowing), the N concentration in rice residue was observed to be

similar (14.8 g kg�1) for 10-, 20-, or 40-day decomposition treatments.

Yadvinder-Singh et al. (2004b) observed substantial immobilization of

fertilizer N with straw incorporation at 10 days after fertilizer application in

treatments in which rice straw was incorporated at 0 and 10 days before

application of fertilizer compared to the no-straw treatment (Fig. 2). The

magnitude of immobilized N was influenced by the decomposition period of

rice straw prior to fertilizer application. Interestingly, immobilization of N in

the treatment where fertilizerN was applied concurrentlywith straw incorpora-

tion (0 day) always remained lower than the treatment without straw (Fig. 2).

Mineral N in the soil was significantly higher under the 20- and 30-day

pre-decomposition periods than under the no-straw treatment at all sampling

times. These data clearly demonstrated that incorporation of rice straw at

20 days or more before wheat sowing will minimize any adverse eVects on

crop growth due to N immobilization after straw incorporation (Fig. 2). This

Figure 2 EVect of pre-decomposition period of rice straw on mineral N (NH4 þ NO3)

dynamics in soil amended with 100 mg N kg�1 and incubated at 75% field capacity moisture

regime at 30 8C.


study suggested a lower amount of N immobilization by rice straw containing

6.7 g N kg�1 when allowed to decompose for 20 or 30 days before fertilizer

N application comparedwith that reported for wheat and barley strawbyMary

et al. (1996). The results from the decomposition and N mineralization studies

suggested that rice residue is likely to have little adverse eVects on N availability

in the soil when it is allowed to decompose under aerobic conditions for at least

10 days before sowing of the next upland crop.

V. CROP RESIDUE MANAGEMENT EFFECTS ONNUTRIENT AVAILABILITY IN SOILS

Rice straw is characterized by a high C:N ratio and abundant K, Si, and

C (Ponnamperuma, 1984). Wheat straw has comparable properties except for

low Si and low K concentration. The successful utilization of crop residues as

a nutrient source relies on manipulating the biological processes in the soil so

as to optimize nutrient availability with respect to plant demand.

A simplified model of the regulation of nutrient flux in the agoecosystem is

presented in Fig. 3. This conceptual model depicts the flow of carbon and

nutrients among organic residues, organic and inorganic pools in soil, and the

plant. Pathways of loss are also included. Decomposition and mineralization

of plant residue are mediated by both soil faunal and microbial populations.

Some of the carbon and associated nutrients are mineralized immediately

(pathway 1a) or are immobilized in the soil microbial pool (pathway 2a), later

to be transformed into other soil organic pools via microbial by-products

(3a). Recalcitrant plant material also may enter the soil organic pools directly

(3b). The carbon and nutrients held in the various soil organic matter pools

are subsequently decomposed and assimilated by soil biomass, resulting in

Figure 3 Conceptual model of nutrient pathways in crop residue amended soils (Myers

et al., 1994).


additional mineralization (1b). The inorganic nutrients released by minerali-

zation may be assimilated by soil biota via immobilization (2). Immobilization

occurs simultaneously with mineralization, and the rate at which nutrients are

available for plant uptake depends on the net balance between mineralization

(1a plus 1b) and immobilization (2). The inorganic nutrients may also be taken

up by plants (pathway 3), lost by leaching or volatilization (pathway 4), or

remain in the soil (Myers et al., 1994). The size of the inorganic pool depends

on the balance of the various processes that add to the pool (mineralization)

and those that subtract (immobilization, plant uptake, and losses).

The proportion of N transferred from the residue to the plant and the rate

at which it occurs are determined by the balance between the rates of the

various processes represented by these flux pathways. This balance is regu-

lated by a hierarchy of factors. Environment, which includes climate and

soil, is an overriding control and determines the rate of the transfer between

pools. The rates also vary depending on the quality of the decomposing

substrate. By manipulating the quality of crop residues, it should be possible

to manage nutrient release to coincide with the time course of the nutrient

requirements of the crop (Swift, 1987). When low-quality crop residues (low

N and P, high lignin or polyphenol contents) are incorporated into the moist

soil, nutrients become available to the plants. With high-quality residues,

nutrients are initially released rapidly in excess of plant demand with a risk

of nutrients such as N being lost via leaching or denitrification or a nutrient

such as P becoming chemically unavailable (Anderson and Swift, 1983).

A. NITROGEN

1. Kinetics of Nitrogen Mineralization–Immobilization

Mineralization and immobilization of N occur simultaneously in the soil.

Net rates of mineralization and immobilization are an integration of a

number of soil N processes and a number of factors, which act on the

interacting processes as well as directly on mineralization–immobilization.

Residues from rice, wheat, barley, maize, and other small grains with large

C:N ratios are noted for their initial N immobilization, which can negatively

aVect crop yields (Cassman et al., 1997; Christensen, 1986). Microbes

using the crop residues as an energy source compete with crop plants for

available N.

The addition of cereal residues often results in a net N immobilization

phase followed by a net re-mineralization phase (Muller et al., 1988; Nieder

and Richter, 1986; Robin, 1994; Yadvinder-Singh et al., 1988). Generally,

mineralization of N from low-N residues occurs only after 50–60% of

residues is decomposed or after the C:N ratio is below 30 (Christensen,

Table IV

Maximum Immobilization of Soil Mineral N Measured During Decomposition of Wheat/Rice

Straw under Laboratory Conditions

Reference Immobilized N (mg N g�1 C)

Simon (1960) 28.5

Guiraud (1984) 30.8

Reinertsen et al. (1984) 18.0a

27.0b

Bakken (1986) 15.1

Nieder and Richter (1986) 25.8

Robin (1994) 27.4

Beri et al. (1995) 35.0

aWithout N.bWith mineral N.

Modified from Mary et al. (1996).


1986). A large number of papers have focused on N immobilization imme-

diately after straw incorporation under laboratory conditions and indicate a

narrow range of N immobilization values (Table IV). Under field conditions,

Mary et al. (1996) derived N immobilization values of 13.0, 24.3, and 32.0 mg

N g�1 added carbon under N0, N180, and N330 treatments, respectively. The

latter value was equal to the immobilization potential found with straw in

incubation studies. Mishra et al. (2001a) reported that the N content of

wheat straw increased from the initial level of 0.50% to 0.73% after 10 weeks

and to 0.88% after 22 weeks. The C:N ratio of wheat straw decreased from

90 to as low as 32 by the end of 22 weeks of decomposition period. During

first 2 weeks, 24% of the total N in wheat straw was released into the soil.

There was net immobilization between 3 and 10 weeks. After 10 weeks, net

N mineralization was noted in the study. By 22 weeks, apparent

N mineralization from wheat straw was 71%. The rate of N mineralization

was lower than that of C mineralization from wheat straw. It suggested that

incorporation of wheat straw at about 8 to 10 weeks before rice transplant-

ing can help to alleviate the adverse e Vect of wheat straw on rice growth due

to N immobilization. In another study, Mishra et al. (2001b) noted that

during the decomposition of rice straw, N content increased from 0.60 to

1.21% and the C:N ratio decreased from 70 to 21.3 with time. About 4.5% of

the total N present in rice straw was released within 5 weeks of its incorpora-

tion into the soil, and apparent N mineralization after 23 weeks was 57.5%

of the total N in the rice straw, of which more than 50% was mineralized

during the third phase.

Similarly, Yadvinder-Singh et al. (2004b) reported that the N content of

rice straw at the time of incorporation was 5.6 g kg�1, which increased to


14.8 g kg�1 at the end of the 190-day study period. At the time of wheat

sowing, residue N concentration was significantly lower (7.0 g kg�1) in the

10-day decomposition treatment than in the 20-day (7.9 g kg�1) or 40-day

(8.1 g kg�1) decomposition treatment. Rice residue appeared to have a brief

initial period of N release after incorporation. This study showed that for

every 10% increase in mass loss, there was about 2.75% (1.07 kg N ha�1)

release from the applied residue N. In this study, despite a substantial mass

loss of 69% residue, the amount of N release was small (6–9 kg N ha�1).

Residues go through several phases in their decomposition, with N dynamics

related to the stage or extent of mass loss. Using 15N-labeled rice residue,

Yoneyama and Yoshida (1977a) reported that 8% of the N in the rice leaf

sheath (8 g N kg�1) was mineralized in 30 days at 30 8C under upland

conditions in the laboratory.

In the tropics, the turnover of organic matter in flooded soils can be as

fast as that in aerobic soils. This causes net immobilization of N in flooded

rice soils after rice or wheat straw incorporation. A review of work on

interaction between decomposition of plant residues and N cycling in soils

showed that the amount of N immobilization can be large, and the intensity

and kinetics of N immobilization and subsequent mineralization depend on

the nature of plant residues and the type of associated decomposers (Mary

et al., 1996).

In a laboratory incubation study, rapid production of NHþ4 –N occurred

during the first week of incubation of untreated flooded soils, while in a soil

treated with rice straw only traces of NHþ4 –N were formed (Phongpan,

1987). However, during the subsequent 6 weeks, the accumulation of

NHþ4 –N was considerably higher in soil treated with rice straw than in

untreated soil. This suggests that NHþ4 –N was immobilized in the soil

amended with rice straw during the early stages followed by remineralization

in later stages. Similar observations have been made by Nishio et al. (1993),

who estimated a remineralization constant of 0.023 day�1 when 0.25 mM of

ðNH4Þ2SO4 was applied along with rice straw. Azmal et al. (1997) observed

that in rice straw amended soil (200 mg C per 100 g soil applied every 6

weeks) mineral N was immobilized immediately after each application of

rice straw due to its high C:N ratio. The immobilization increased until the

third application. This behavior was ascribed to the gradual accumulation in

soil of a portion of undecomposed organic N. Nieder and Richter (1989)

observed immobilization of about 30–40 kg N ha�1 in the soil after straw

incorporation.

Toor and Beri (1991) observed almost compelete immobilization of native

as well as applied N (120 mg kg�1) by the seventh day in a soil amended with

rice straw. At the end of 60 days of incubation, however, about 40 mg

mineral N kg�1 soil was remineralized. Patel and Sarkar (1993) observed

initial rapid rate and magnitude of immobilization of 15N-labeled urea in


three soils amended with wheat straw, and remineralization of immobilized

N occurred by 60 days. Singh et al. (1992) also found that about half of

N immobilized by rice straw was remineralized in 90 days after straw

incorporation. Yadvinder-Singh et al. (1988) observed that remineralization

of immobilized N in a soil amended with rice and wheat straw occurred

4 weeks after incorporation of residues at 35 8C under laboratory conditions.

Using 15N-labeled rice straw, Zhu et al. (1988) reported that about 1% of the

rice straw N was mineralized in 112 days.

Kawaguchi et al. (1986) observed that eventual mineralization of

rice straw N during 28 weeks of incubation period ranged from 17 to 24%.

Addition of rice straw also increased the mineralization of soil N due to a

priming eVect, which was estimated to be 30–100 mg N kg�1 soil. The degree

of the priming eVect correlated with the amount of mineralized rice straw N.

Incorporation of crop residue with low cellulose and high water-soluble

N contents results in more N being mineralized than had been added in

the residues, demonstrating this priming eVect (Bending et al., 1998).

Using 15N-labeled rice and maize straw, Wang et al. (2001) reported

that after a 112-day incubation period under submerged conditions, net

recovery of mineralized N from soil only accounted for 1.0–1.3% of the

straw N added to diVerent soils. About 2.0–4.3% of straw N was immobi-

lized by microbial biomass, and 0.2–14.2% was fixed by clay minerals.

Vertisol fixed a markedly higher amount of straw N than Ultisol. The loss

of straw 15N ranged from 29.7 to 46.3% during the incubation period. The

total amount of straw N mineralized during the 122-day incubation ranged

from 47.7 to 51.7%.

Kushwaha et al. (2000) reported that in residue removal treatment,

N mineralization rates were maximal during the seedling stage of crops

and then decreased through the crop’s maturity. In residue-retained treat-

ments, however, N mineralization rates were lower than in the residue-

removed treatments at seedling stage of both crops. At grain-forming

stage, the N mineralization rates in residue-retained treatments considerably

exceeded the rates in corresponding residue-removed treatments. Microbial

immobilization of available N during the early phase of crops and its pulsed

release later during the period of greater N demand of crops enhanced the

degree of synchronization between crop demand and N supply.

Using the first-order kinetics relationship, Mary et al. (1996) calculated

the maximum net immobilization at 51 kg N ha�1 and a rate constant of

0.031 day�1 in soil amended with wheat straw under field conditions. Islam

et al. (1998) estimated N mineralization potential (N0) under flooded condi-

tions at 35 8C for 12 weeks. The values of No for rice straw were 7 to 15 times

that of the control soil. The first-order rate constants for unamended soils

ranged from 0.35 to 0.52 mg N kg�1 week�1, and for rice straw-amended

soil, from 0.75 to 1.22 mg N kg�1 week�1.


Dynamic computer models can be used to distinguish between immobili-

zation and mineralization of N and between availability and loss through

leaching and denitrification in soils treated with crop residues under field

situations.

2. Factors Affecting N Mineralization

The residue quality and availability of soil N are important determinants

of N mineralization–immobilization occurring during residue decomposi-

tion. Mineralization of organic N depends on the N requirements of the soil

microbial population, the biochemical composition of the decomposing crop

residue, and several soil and environmental factors. Crop residue manage-

ment can aVect N immobilization and stabilization processes important to

eYcient utilization of N from fertilizers, crop residues, and soil organic

matter. The availability of nutrients from crop residues depends to a great

extent on mineralization of nutrients from the crop residues in relation to

crop demand. Nitrogen mineralization is a crucial process of nutrient dy-

namics in the soil–plant system. It has been reported that the larger and

sustained microbial biomass found under flooded compared to aerobic

conditions may act to immobilize more N and make it less available for

plant uptake. Together with N losses, such as denitrification and leaching,

sustained flooding of soil may reduce the size of available N pools, as seen in

some areas of the tropics (Cassman et al., 1995).

a. Crop Residue Quality. It is well established that the chemical

composition of crop residue influences its rate of decomposition and

mineralization–immobilization in the soil and subsequent N uptake by the

crops (Jensen, 1997; Magid et al., 1997; Mary et al., 1996). The quality of

organic residues has been assessed by measuring diVerent biochemical prop-

erties that have been shown to delay or enhance the decomposition and

N mineralization processes. Incorporation of residues with low N contents,

such as rice, wheat, and barley, may result in microbial immobilization of

soil and fertilizer N, eVectively reducing N availability to plants.

Using 15N-labeled crop residues, Norman et al. (1990) found that N

mineralized from the time of residue incorporation until the rice harvest

from rice, wheat, and soybean straw was 9, 38, and 52%, respectively. The

respective amounts of residue N recovered by rice was 3, 37, and 62%

from those residues. Several workers have reported higher rates of N miner-

alization from crop residues with a lower C:N ratio and higher total

N content (Bending et al., 1998; Vigil and Kissel, 1991). Regression analysis

of the data showed that 72–75% of the variability in the measurement of

N mineralization from crop residues can be explained by using either the

C:N ratio or the square root transformation of the N concentration of


residues (Vigil and Kissel, 1991). The break point between net N mineraliza-

tion and net N immobilization was calculated to occur at a C:N ratio of 40,

which corresponds to 10 g N kg�1 residue (assuming residue carbon as 400 g

kg�1). The prediction of N mineralization was improved when regression

analysis included total N and the lignin:N ratio as independent variables.

The fitted equations provided an estimate of the maximum amount of N that

will potentially mineralize in a season from the incorporation of crop resi-

dues of diVerent N contents. Generally, mineralization of N from low-N

residues occurs only after 50 to 60% is decomposed or after the C:N ratio is

below 30.

But others have found quite diVerent results (Haynes, 1986). It is to be

expected that the significance of diVerent chemical parameters will depend

strongly on the condition (temperature and soil moisture content) under

which the experiments were conducted, the range of decomposability and

N concentration in the plant material, the N availability in the soil, and

whether the shorter or longer results are considered. When plant materials

with very diVerent decomposabilities are compared, lignin content is likely

to be an important parameter, as reported by Neely et al. (1991), whereas

when comparing young plant materials from young green manures it is

unlikely to show much relation to N release.

Polyphenol compounds have been shown to react with residue N and

render it unavailable for plant uptake (Palm and Sanchez, 1991), while lignin

is a not easily decomposable cell wall polymer. Azmal et al. (1996) observed

a negative relationship between N mineralized from crop residues and

the amount of cellulose and hemi-cellulose of added organic material. So

far, particle size has not been considered a quality parameter, but needs

attention, as a smaller particle size leads to higher substrate-decomposer

community contact, resulting in high decomposition and N mineralization.

Wide variation in the proportion of various constituents in crop residues

and their diVerential behavior in the soil after incorporation makes the

description of N release solely based on the C:N ratio of the residue too

simplistic (Jarvis et al., 1996). In a flooded soil, net N mineralization from

crop residues (legumes as well as cereals) was not correlated with N and the

C:N ratio, but it was correlated with the lignin:N ratio of the residue under

greenhouse conditions (Becker et al., 1994b). Similarly, initial soil NHþ4 –N

accumulation rates under field conditions were higher from residues

with a relatively low lignin:N ratio, suggesting that the lignin:N ratio of

the applied residue may be a suitable index for predicting N mineralization

rates in flooded soils. Quality components controlling N mineralization from

crop residues also change during decomposition (Bending et al., 1998).

While water-soluble phenolic content significantly correlated with net

N mineralization at early stages, C:N ratios and total N content were

correlated with net N mineralization toward the end of incubation


(6 months) only. In a field experiment, Clement et al. (1995) observed that

immediately after incorporation into wetland rice soil, N mineralization was

positively correlated with crop residue N content. However, at tillering, the

tannin:N ratio was best correlated with the rate of N release. Grain yield was

best predicted by the (lignin þ polyphenol):N ratio. This study emphasized

the importance of the interaction among chemical constituents of crop

residues in the dynamics of N release and uptake by rice.

b. Environmental Factors. Nitrogen mineralization is profoundly influ-

enced by temperature changes that are normally encountered under field

conditions. The majority of soil microorganisms are mesophyllic and prefer

moderate temperatures with optimum activity between 25 and 35 8C. Thus,

turnover of nutrients in plant residues would generally be accelerated in

tropical soils. Pal et al. (1975) suggested that the initial Q10 (2–3 days) for

N mineralization of crop residues recently incorporated into soil is > 2. Vigil

and Kissel (1995) reported that measured Q10 for N mineralization depended

on the C:N ratio of the residue and incubation time, indicating that for

predictive purposes a single Q10 value is inadequate for describing the eVect

of temperature on crop residue N mineralization. Honeycutt and Potaro

(1990) field tested the application of thermal units for predicting N minerali-

zation from crop residues. It was found that thermal units are valid for

predicting commencement of net mineralization of N from crop residues,

despite the harsh environmental conditions and wide temperature variations

to which these residues and soils were subjected.

Nitrogen transformations in flooded soils under rice are markedly diVer-

ent from those taking place in upland soils. The diVerence in the behavior of

N in upland and submerged soils is due to the diVerence in activity of

microorganisms functioning under aerobic and anaerobic conditions. In an

incubation experiment, Yoneyama and Yoshida (1977b) found that net

mineralization of soil N was depressed by the addition of rice straw, except

that the addition of leaf blade under lowland conditions gave more mineral

N at later stage than the unamended control (Table V). Under lowland

conditions, the amount of N immobilized in soil amended with rice straw

was small during the first week but increased substantially after 2 to 3 weeks.

Under upland conditions, the immobilized N reached its maximum during

the first week, but the amount of N immobilized was smaller than that under

lowland conditions. At 30 days of incubation, 26, 20, and 17% of total

N under lowland conditions and 14, 7, and 8% under upland conditions

were mineralized in leaf blades, stems, and leaf sheaths, respectively. This

suggests that mineralization of rice residue N takes place throughout the

decomposition of residue even if the net mineralization of N was not ob-

served by the incorporation of residue low in N content. The amount of

absorbed soil N in rice residue (influx) and the remaining original rice

Table V

Mineralization of Rice Residue in Soil under Upland and Lowland Conditions

Rice residue

Total N

added

(mg kg�1)

Incubation time (days)

2 5 10 30

Lowland conditions

Leaf sheath 32 2.8 4.5 5.3 5.3

Leaf blade 60 3.9 9.2 14.1 15.6

Stem 16.8 0.3 1.1 1.8 3.2

Upland conditions

Leaf sheath 32 0.8 1.3 1.4 2.5

Leaf blade 60 1.0 2.1 6.6 10.4

Stem 16.8 0.5 0.6 0.6 1.1

From Yoneyama and Yoshida (1977b).


residue N (outflux) is more vigorous under lowland than under upland

conditions. Therefore, rice yields and N uptake will be greater under lowland

than under upland conditions. Kanazawa and Yoneyama (1980) observed

that mineral N in soil amended with 15N-labeled rice straw under upland

conditions remained at low levels compared with untreated control through-

out the 24 months of incubation. In flooded soil, the mineral N was lower in

straw-treated soil during the first 4 months, and the diVerences were small

between unamended and amended soil thereafter.

It is generally found that N mineralization is higher under anaerobic

conditions than under aerobic conditions (Ono, 1989). According to Liu

et al. (1996), higher mineral N levels in rice straw amended soil under

anaerobic compared to aerobic conditions possibly occurred because the

minimum need of microorganisms for release of ammonium N from crop

residues in flooded soil is about 0.5% compared with 1.7% in aerobic

systems. Thus, inorganic N is released in larger quantities in anaerobic

than in aerated soils, although the release rate may be slower. Mineralization

rate of N in crop residues is reduced at low soil water contents.

The eVect of soil temperature and water content on N mineralization can

be calculated by using a relationship derived by Andren and Paustin (1987).

A normalized time (equivalent to Qsum) was calculated as

TðnormalizedÞ ¼ tðrealÞ fðTÞ gð*Þ; ð5Þ

where f(T) is a correction factor due to soil temperature and g(*) is a

reduction factor due to soil water potential. The factor f(T) is a multi-

exponential function of temperature. It is set at 1 at a temperature of

25 8C. The eVect of soil moisture on N mineralization was described as

an exponential function of soil water potential. The combined eVect was


calculated as the product of the two terms, assuming that there were no

interactions between temperature and moisture. This approach enables the

comparison of field experiments diVering in climatic conditions to laborato-

ry experiments conducted under constant temperature and moisture. Mary

et al. (1996) found that when the normalized days were substituted to

real days, the diVerence in the kinetics of net N immobilization and

C decomposition in the soils where wheat straw was incorporated under

field conditions were not significant between two years (Fig. 4).

c. Placement of Crop Residues. Using 15N-labeled crop residues, Smith

and Sharpley (1990) found that surface placement of residues reduced

N availability as compared to soil incorporation, but the diVerences

were only equivalent to 1 to 7 kg N ha�1. Residue placement influences

N mineralization through an eVect on the microclimate of the residue.

Slower decomposition rates of surface residues may result in greater poten-

tial for immobilizing N for longer periods than for incorporated residues.

Schomberg et al. (1994b) reported that N immobilization period was longer

than 1 year for surface applied wheat and sorghum residues and about

4 months for buried residues. The maximum value for N immobilization

was 50% lower for buried residues. Although greater N immobilization may

occur with surface residues, subsequent N mineralization can occur within a

period that is optimum for crop utilization.

Residue incorporation with conventional tillage agroecosystems can

be characterized as bacterial-based food webs with fast rates of litter

Figure 4 (A) EVect of wheat straw on net immobilization of soil mineral N versus ‘‘normal-

ized’’ time. (B) Wheat straw C (fraction > 1mm) remaining in soil in the field experiments versus

‘‘normalized’’ time (Mary et al., 1996).


decomposition and nutrient mineralization, while surface residues under no

tillage systems support fungal-based food webs that result in slower decom-

position and greater nutrient retention (Beare et al., 1996). Placement of

residues may play an important role in determining availability of soil N to

subsequent crops during the N immobilization-mineralization process.

d. Soil Type. Soil texture controls mineralization by (1) influencing

aeration/moisture status, (2) aVecting the physical distribution of organic

materials and hence potential for degradation, and (3) conferring some

degree of ‘‘protection’’ through an association of organic materials with

clay particles (Hassink et al., 1993). Becker et al. (1994b) observed that

residue N release in clayey soil was approximately twice that of sandy soil.

DiVerences in mineralization rates between soils would have an impact on

the fertilizer N requirement of the subsequent crop and the potential for

N loss due to leaching or denitrification. Using 15N-labeled wheat straw and

legume residues, Amato et al. (1987), however, observed that eVect of soil

properties and climate on the residual organic 15N was small. Decomposi-

tion and mineralization of crop residues, however, are inhibited under

strongly acidic conditions. For example, Fu et al. (1987) indicated that

N mineralization increased as soil pH increased from 5 to 7.

e. Soil and Fertilizer Nitrogen. Cereal residues generally possess low

N content and may require addition of exogenous N for decomposition to

proceed. From a series of experiments, Yoshida et al. (1973) inferred that

N mineralization in soil amended with rice straw increased with increasing

NHþ4 –N concentration up to 300 mg N kg�1 soil, but N mineralization

decreased when rates greater than 300 mg N kg�1 were applied. Mary et al.

(1996) concluded that immobilization intensity of crop residues expressed

per unit of mineralized carbon is reduced and N remineralization is delayed

in soils with low mineral N concentrations. Nitrogen availability in soil can

therefore strongly modify the mineralization–immobilization kinetics by a

feedback eVect. On bare plots, immobilization of mineral N by wheat straw

incorporation increased markedly by the addition of mineral N throughout

the decomposition. A better prediction of the evolution of mineral N in

soil may, therefore, require description and modeling of the respective

localization of both organic matter and mineral N in soil aggregates.

3. Effect of Crop Residues on Utilization of N by Crops

Availability of N from crop residues to subsequent crops is highly depen-

dent on decomposition rate, residue quality, and environmental conditions

(Fox et al., 1990). Application of crop residues has been shown to depress


the NHþ4 –N concentration in soil and flood water due to N immobilization

and consequential lower N uptake by rice compared with control (Huang

and Broadbent, 1988; Nagarajah et al., 1989). Rice is known to take up more

organic N than any other crop because (1) it takes up NHþ4 , amino acids, or

relatively high molecules of organic N, preferentially; (2) it has stronger

activity in competing with soil microorganisms than the other crops; (3)

it secretes organic substrates that support multiplication of microfauna,

resulting in rapid decomposition of organic residues; and (4) it has superior

Km (Michaelis constant), Vmax (maximum uptake velocity), and Cmin

(minimum concentration of a nutrient) for N uptake (Yamagata et al.,

1996). Thus, rice is expected to respond to crop residue N better than

other crops. Yoneyama and Yoshida (1977a) found that N uptake by rice

from residue-amended soil was at its peak during the intermediate stages of

growth, and N uptake from the fertilizer was rapid during early growth.

They recorded 25% N recovery from straw N by rice plants in 130 days.

Although contribution of 5 t straw ha�1 to the current N needs of rice is

relatively small, the long-term eVects may be substantial. For example,

Tanaka (1974), Chatterjee et al. (1979), and Kosuge and Zulkarnani (1981)

reported that continuous application of straw builds up soil organic matter

and ensures high N content and uptake and partial substitution of straw

N for fertilizer N.

Jiang et al. (1998) observed that N utilization by wheat in the presence of

wheat straw (4.5 t ha�1) was highest when N was applied in three equal splits

at sowing, tillering, and stem elongation. Guirad and Berlier (1971) reported

that the reduction in the N uptake from Ca(NO3)2 in the wheat straw-

amended plots was due to higher losses of NO3–N by denitrification, and

from (NH4)2SO4 it was caused by immobilization of N in the soil.

Malik et al. (1998) found that incorporation of wheat straw along

with green manure enhanced nutrient availability; and synchrony between

N release and plant uptake was best achieved in soil receiving straw

along with green manure. A temporary lag in N immobilization and

mineralization provided a N-conserving mechanism for the system. Broad-

bent and Nakashima (1965) followed mineralization and plant uptake of

N immobilized by application of straw. When N was added with the straw,

there were indications that remineralization of immobilized N was faster

than mineralization of N in the unamended soil. However, when no N was

applied with the straw, the results did not support the synchrony concept.

Support for the synchrony concept is found in the results of a field experi-

ment with flooded rice (Amarasiri and Wickramsinghe, 1988) in which rice

receiving a 60 kg N ha�1 fertilizer along with straw yielded about the same as

that receiving 90 kg N ha�1 as fertilizer alone. This role of straw may be

interpreted as one of N recycling in a system where losses from the mineral

N pool are potentially large and as such is a type of synchrony.


Tanaka and Nishida (1996) observed that wheat straw decreased N uptake

by rice and increased the amount of 15N remaining in the soil at 17 days after

transplanting. At the booting stage, 6 days before heading, N uptake was

higher and the 15N remaining in the soil was lower in the treatments in which

wheat straw was applied than in unamended control treatment. It was

concluded that decrease in N uptake by wheat straw was caused by

N uptake inhibition and not by N deficiency in the early stages of rice

growth.

In a greenhouse study using 15N-labeled fertilizer, Masayna et al. (1985)

found that rice plants recovered 50–69% of applied fertilizer in the unamend-

ed soil and 45–53% in the rice straw-incorporated soil. In the second and

third crops of rice, recovery of residual N was slightly higher from rice straw-

amended soil than from unamended soil. Islam et al. (1998) found that large

amounts of mineral N pools were lost during the incubation that could not

be accounted for by microbial immobilization under field conditions. To the

contrary, Xu (1984) reported higher fertilizer utilization eYciency in rice

straw-amended soil (75.5 and 82.6%) than that in unamended light clay and

sandy loam soils (51.8 and 47.7%). This could be due to the increased

N immobilization and decreased losses of N via denitrification in residue-

amended soil (Craswell, 1978). Available data suggest that 10 to 20% of

N freshly supplied through cereal residues with a high C:N ratio (rice and

wheat straws) is assimilated by the rice crop, 10 to 20% is lost through

various pathways, and 60 to 80% is immobilized or stored in the soil under

field conditions (Koyama, 1981).

4. Losses of N

The presence of crop residues with high C:N ratios may also lead to

transformation of fertilizer or soil N into slowly available forms, which

may act as slow-release fertilizer and thereby improve N use eYciency.

Bird et al. (2001) reported that the total loss of N fertilizer, based on the15N isotope balance, was approximately 50% and was largely independent of

straw management practice. An increase in total soil microbial biomass in

combination with a large amount of added straw could have led to a

temporary strong sink for N fertilizer. The ensuing immobilization process

could lead to lower N fertilizer losses. Eagle et al. (2001) reported a decrease

in fertilizer N use eYciency with a concomitant increase in the plant avail-

able soil N following change in straw management from burning to in-

corporation. Only 1.8 kg ha�1 (3.5%) straw N was directly available to the

crop in the year following incorporation, and total N uptake increased by

23 kg N ha�1 5 years after straw incorporation. Huang and Lu (1996)

reported that heavy application of rice straw in combination with


N fertilizer at a C:N ratio greater than 40 would have a determental eVect on

the rice growth. Using 15N–labeled fertilizer, it was observed that total

recovery of N was reduced from 40.8% in no straw to 6.1% in straw

treatment with a C:N ratio of 40, but the total N loss was decreased from

13.7 to 5.5%. It was concluded from this study that for eYcient management

of rice straw and N fertilizer in flooded rice cultivation, it is advisable to

incorporate rice straw with a C:N ratio adjusted to <25.

a. Urea Hydrolysis and Ammonia Volatilization. Urea, the major fer-

tilizer N source, can be significantly less eYcient when used under soil and

climatic conditions conducive to NH3 volatilization. The rate of urea hydro-

lysis, a process mediated through soil enzyme urease, has a direct bearing on

losses of N via NH3 volatilization. The incorporation of crop residues causes

a significant increase in the soil urease activity (Gill et al., 1998b; Phongpan,

1987). Khind and Bajwa (1993) also observed that urea hydrolysis proceeded

more rapidly in the crop residue-amended soil than in the control soil. The

rate of urea hydrolysis increased with the increasing rate of crop residue and

with the length of decomposition period. The first order rate constants for

urea hydrolysis ranged from 0.021 to 0.024 h�1 after the application of

200 mg N kg�1 in the unamended soil and from 0.071 to 0.250 h�1 in the

rice straw-amended soil, depending upon the length of decomposition peri-

od. Gill et al. (1998a) reported that in wheat straw-amended soil, hydrolysis

of urea was completed in 6 days, compared to 12 days in the unamended soil

under flooded conditions.

The increased rate of urea hydrolysis on residue-amended soil may lead to

increased loss of N via NH3 volatilization. In a silt loam soil, application of

soybean straw increased the urea hydrolysis from 42.8% in control to 90.0%

2 days after incubation. The ammonia loss in the first 4 days was 12.3 and

28.2% of the applied N in the control and residue-amended Guthrie soil,

respectively (Caramona et al., 1990). The cumulative N losses were, howev-

er, similar (45 and 44%) under the two treatments. McInnes et al. (1986)

found that cumulative loss of NHþ4 –N was 7.6 and 16.6% of the N applied

from unamended and wheat-straw amended soil, respectively. Wheat straw

was found to have a urease activity of about 1830 mg urea kg�1h�1, a pH of

near 8, and a H ion buVering capacity of 53 mmol kg�1 (pH unit)�1. These

factors undoubtedly contributed to NH3 loss from upland soils. Singh and

Singh (1991), however, observed lower cumulative NH3 volatilization loss

from a saturated calcareous soil amended with rice straw than that from

unamended soil due to the lowering of soil pH by rice straw.

Gill et al. (1998a) measured more losses of N via NH3 volatilization from

flooded soils amended with wheat straw (11.6%) than from unamended soils

(7.1%) in 16 days. Application of nBTPT, a urease inhibitor, to the soils

amended with wheat straw reduced losses of NH3, although much less than


in the soil not amended with organic materials. These studies showed that

soils amended with crop residues would require two to four times more

nBTPT than the soils not amended with crop residues for eVectively reduc-

ing NH3 volatilization from flooded soils. Caramona et al. (1990) has also

drawn similar conclusions.

Residue left on the surface under no-till agriculture may also aVect NH3

volatilization. McGarity and Hoult (1971) found that plant material on the

soil surface influenced NH3 volatilization and ascribed this to urease asso-

ciated with the ureolytic phylloplane and litter microorganisms. Bacon et al.

(1986) reported that stubble management involving cultivation lost 7–8 kg

N ha�1, while zero cultivation treatments lost an average of 15 kg N ha�1.

They further reported that partial burial of urea reduced ammonia volatili-

zation from 36 kg under broadcast onto the surface of stubble retention

plots to 7 kg N ha�1. Retaining the stubble resulted in higher soil water

content increasing urea hydrolysis, leading to greater change in pH and

ammonium concentration. Additionally, some urea prills are retained within

the stubble above the soil surface. Under favorable conditions, urease activ-

ity associated with residue surfaces would enable urea hydrolysis in prills

held above the soil. Ash of crop residues at the soil surface also increased

ammonia volatilization due to its high pH (Bacon et al., 1986).

b. Leaching and Denitrification. Nitrogen immobilization by high C:N

ratio residues represents a potential temporary sink to reduce N loss from

leaching (Savant and De Datta, 1982). Immobilization of soil N within

surface residues may have a positive influence on subsequent crop growth

in that N remains near the root zone. However, leaching and denitrification

losses of N with in the soil profile may increase where surface-placed residues

result in increased water infiltration and reduced evaporation rates.

When residues are incorporated, depending on the placement and type, or

soil texture and water content, the potential for denitrification can be

increased dramatically (Aulakh et al., 1992). Generally, in lowland soils,

the rate of NO3 formation rather than available C limits denitrification.

Bacon et al. (1989) observed that incorporation of rice stubble in wheat

increased apparent denitrification of fertilizer N from an average of 34 to

53 kg N ha�1. The N loss occurred over several months, suggesting that

denitrification was maintained by continuous release of metabolizable car-

bohydrates from the decomposing rice stubble. Nugroho and Kuwatsuke

(1992b) found that under upland conditions (60% WHC), denitrification

hardly occurred when the level of NO3–N in soil was <5.5 mg g�1 soil.

The rate of denitrification substantially increased with the increase in the

level of NO3-N in the rice straw-amended soil, while in the soil without rice

straw, it only slightly increased with the increase in the level of NO3–N. The

maximum rate of denitrification was as high as 6.5 mg N2O–N g�1 day�1 in


the rice straw-amended soil. In a laboratory incubation study, Patrick and

Gotoh (1974) recorded reductions in fertilizer N losses by 30–40 mg kg�1

from the soil amended with rice straw. In pot and microplot studies, Xu

(1987) observed that application of rice straw with inorganic N enhanced

immobilization and reduced N losses via denitrification. Denitrification

losses from soil can be influenced by residue N content. Aulakh et al.

(1991) observed that denitrification losses from the soil at 90% water-filled

pore space were 87 to 127% of initial soil NO �3 and increased further with

increasing residue N content.

In a 2-year field study in subtropical Australia, Cogle et al. (1987) ob-

served that after 15 months, only 44% of applied 15N urea was recovered

from wheat residue incorporation treatment, compared to 55% from surface

retained treatment. The greater losses in incorporated straw treatment were

possibly due to greater availability of carbon to the denitrifying population

compared with treatment where wheat straw was retained on the soil

surface.

B. PHOSPHORUS

Phosphorus mineralization from crop residues is determined by the rate

of residue decomposition and microbial immobilization (Stevenson, 1986).

The activity of enzymes (phosphatases) that mineralize P is influenced by the

same factors that aVect microbial activity. Phosphorus content of the added

residue is perhaps the most important factor in regulating the mineralization

of P in crop residues. In general, net immobilization of P occurs following

addition of crop residues with less than 0.2 to 0.3% P, while net mineraliza-

tion occurs with higher P contents (Stevenson, 1986). During early stages of

residue decomposition, net immobilization of P can conserve a substantial

amount of P in slowly available organic forms. Cycling of P in soil is not

easily measured, since mineralized P may be removed from the soil solution

via adsorption to colloidal surfaces or precipitation as Ca, Fe, or Al phos-

phates or immobilized into organic P (Stevenson, 1986). Availability of P to

plants in crop residue-amended soils is, therefore, a function of organic

matter turnover, concentration of inorganic P in soil solution, and P

requirements of microorganisms.

Black and Reitz (1972) and Qiu and Ding (1986) reported that application

of wheat straw decreased the NaHCO3-extractable P in all the soils used in

the study. In other studies, application of rice and wheat straw (with C:P

ratio >300) caused immobilization of P during the first 15 days and then

progressively increased the available P content in soil from the 30th day

onward (Mukherjee et al., 1995; Yadvinder-Singh et al., 1988). Addition


of crop residues to waterlogged soil, however, significantly increased the

Olsen-P content (Yadvinder-Singh et al., 1988). Mishra et al. (2001b)

reported that during the decomposition of rice straw, P content

increased from 0.10 to 0.195% with time. About 22.5 and 59.4% of the

total P present in rice straw was released within 5 and 23 weeks, respectively,

after its incorporation into the soil. McLaughlin et al. (1988) indicated that

crop residue P may not significantly contribute to the nutrition of the

subsequent crop but becomes incorporated into organic P forms. In that

study, only 5.4% of the legume residue (Medicago truncatula L.) P was

recovered by wheat plants, while 22 to 28% was recovered in microbial

biomass. The microbial biomass, therefore, controlled the rate of organic

P accumulation.

In a rice–wheat cropping system, Hundal and Thind (1993) reported that

incorporation of wheat straw (6 t ha�1) depressed labile P and dissolved

P but enhanced organic P content during the initial stages of plant growth.

In a rice-potato-groundnut rotation, application of crop residues increased

the available P content and reduced the depletion of P reserve of soil

(Chatterjee and Mondal, 1996).

In a typic xerofluvant soil, application of barley straw and other organic

materials decreased the P sorption of the soil (Berton and Pratt, 1997). Ohno

and Erich (1997) reported that management systems that return crop resi-

dues back into the soil may increase the availability of P by decreasing the

adsorption of P on the soil surface in an acid soil. The release of aluminum

into the soil increased linearly with increasing rates of crop residues. On the

other hand, P adsorption increased with addition of rice straw in several soils

during the first week of incubation under flooded conditions (Phongpan,

1989). Willet and Higgins (1978) also observed increase in P sorptivity of two

rice soils amended with rice straw under flooded conditions. The increase in

P sorptivity was due to a rise in the levels of oxalate Fe under waterlogged

conditions and was dependent on the free iron oxides content of the soils. In

a long-term study (1979–1991), soils amended with crop residues exhibited

more P adsorption and a greater P adsorption maxima than crop residues

that were removed or burned (Table VI). However, the value of the aYnity

coeYcient, i.e., the bonding strength with which P is adsorbed onto the soil

surface, was the least for the residue incorporation and largest for the residue

removal. Similarly, the Frendulich adsorption isotherm showed the largest

amount adsorbed P but the slowest rate of P adsorption when crop residues

were incorporated.

Organic matter in the surface of no-tillage soils has been shown to

influence P distribution and the availability of P in cropped soils. Higher

P availability in the upper layers in notillage soil was attributed to the

absence of mixing of added fertilizer P, increased quantities of organic P,

Table VI

Langmuir and Freundilich P Adsorption Factors for DiVerent Crop Residue

Management Practices (Average of 6.0 and 8.0 t ha�1 of Wheat and Rice Straw Applied

for 11 Years, Respectively)

Crop residue

management

practice

Langmur adsorption Freundlich adsorption

Adsorption

maxima

(mg g�1)

AYnity

coeYcient,

(mg mL�1)

Extent of

adsorption

(mg g�1)

Rate of

adsorption,

(mg mL�1)

Burned 58.8 3.82 13.7 0.65

Removed 66.7 5.00 13.7 0.74

Incorporated 71.4 3.00 32.7 0.29

From Beri et al. (1995).


and possibly shielding P adsorption sites on soil colloids (Schomberg et al.,

1994a). Additionally, surface application and reduced mixing of P fertilizer

in no-tillage systems may reduce P fixation, thus allowing the accumulation

of unreacted phosphate under those conditions.

Residue management plays an important role in determining distribution

and availability of P in cropped soils. Microbial activity, climate factors, and

soil chemical status all influence the cycling of P in soils. Future research on

residue influences on P availability to plants should consider changes in

organic P, microbial P, and inorganic P transformations in soil. Continued

research on interactions between these pools should help improve the

eYcient utilization of P in cropping systems.

Phosphorus is a costly plant nutrient, since both rock phosphate and

S sources are scarce in developing countries such as India. Because of the

low grade of rock phosphate deposits, about 260 million t in India, it is not

recommended for soils with pH > 7.0. Narayanasamy and Biswas (1998)

reported that application of organic matter along with rock phosphate

increased the P eYciency. The suggested reasons were (1) formation of

plant-assimilable phosphorus-humic compounds, (2) anion replacement of

P ion by humate ion, and (3) coating of sesquoxide particles by humus,

which reduces P fixation. Sharma et al. (2001) reported that Mussoorie rock

phosphate (MRP) and diammonium phosphate (DAP) proved equally

eYcient in increasing grain yield and P uptake of rice when residues of the

preceding wheat were incorporated before rice transplanting or rice residue

was incorporated before sowing of the preceding wheat (Table VII). Without

residue incorporation, MRP (8.1% total P, 12% as citrate soluble) had no

significant eVect on grain yield and P uptake of rice. Similarly, MRP and

DAP proved equally eYcient in increasing wheat yield on residue-amended

plots. Available P in soil did not diVer under no P control and MRP when

Table VII

EVect of Crop Residue Management and Phosphorus Source on Crop Yields in Rice–Wheat

Rotation in India

Residue management Rice yield Wheat yield

Rice Wheat 1993 1994 1995 1993–94 1994–95 1995–96

Removed Removed 4.6 4.3 4.0 4.9 4.7 4.4

Incorporated Removed 4.9 4.5 4.5 4.8 5.0 4.3

Removed Incorporated — 4.8 4.5 4.9 4.8 4.9

Incorporated Incorporated — 4.7 4.6 4.5 4.6 4.8

LSD ( p ¼ 0.05) ns 0.31 0.18 ns ns ns

P source

Control Control 4.4 4.5 4.3 4.8 4.6 4.3

DAP DAP 4.9 4.8 4.4 4.8 4.8 4.7

MRP MRP 4.9 4.8 4.5 4.8 4.8 4.7

LSD ( p ¼ 0.05) 0.38 0.19 0.15 ns 0.18 0.33

DAP, diammonium phosphate; MRP, mussoorie rock phosphate.

From Sharma et al. (2001).

Table VIII

EVect of Crop Residue Management and Phosphorus Source on Olsen-P in Rice–Wheat

Rotation in India

Straw treatment

P source (kg ha�1)

Control DAP MRP

Both straws removed 14.8 19.6 15.6

Wheat straw incorporated 16.4 18.0 18.2

Rice straw incorporated 18.6 22.0 21.4

Both straws incorporated 19.2 23.6 27.6

LSD. ( p ¼ 0.05) Residue management � P source ¼ 5.95

DAP, diammonium phosphate; MRP, mussoorie rock phosphate.

From Sharma et al. (2001).


residues were removed, but the incorporation of crop residues resulted in

similar levels of available P in soil under DAP and MRP. The eVect was

more pronounced when both the residues were incorporated as compared to

incorporation of rice or wheat straw alone (Table VIII).

Biswas and Narayanasamy (2002) evaluated composts prepared by mix-

ing rice straw with diVerent sources of rock phosphates collected from within

India. Cow dung slurry and Trichoderma viridii, a cellulytic fungus, were

inoculated to hasten the composting process. A phosphorus-solubilizing

microorganism (Aspergillus awamori) was also introduced 1 month after


the start of composting. The study showed that composting enhanced the

mobilization of P from rock phosphate as evidenced through increases in

water-soluble, citrate-soluble, and organic P fractions. Verma and Mathur

(1990) found that incorporation of rice straw along with cellulytic micro-

organisms and rock phosphate at 15 days before wheat sowing resulted in a

significant increase in wheat yield over recommended fertilizer management

practices. Tian and Kolawole (1998) reported that application of diVerent

crop residues increased the P uptake by Crotolaria ochsolenca from rock

phosphate. For eYcient use of P from rock phosphate in the low-fertility

soils, it is suggested to apply plant residues with high polyphenol and low

lignin contents.

C. POTASSIUM

Crop residues contain large quantities of potassium, and their recycling

can markedly increase K availability in soils (Chatterjee and Mondal, 1996;

Ning and Hu, 1990; Patil et al., 1993; Sarkar et al., 1989). Recycling of crop

residues can improve crop yields at low rates of K application and decrease

the crop response to the K applications. The role of crop residue recycling in

K balance in the rice–wheat cropping system has been dealt with in detail by

Bijay-Singh et al. (2003).

Yadvinder-Singh et al. (2004b) reported that release of K from rice straw

occurred at a fast rate, and within 10 days after incorporation, available soil

K contents increased from 50 mg K kg�1 in the untreated control to 66 mg

K kg�1 in straw-amended treatments. Tian et al. (1992) reported that most

of K in the rice residue was released in less than 41 days. The amount of

K released from organic materials in the first month was highly correlated

with the water-soluble K (Patil et al., 1993; Sarkar et al., 1989). Potassium is

not bound in any organic compound in the plant material, and thus its

release does not involve microorganisms.

Mishra et al. (2001b) reported that during the decomposition of rice straw,

K contents decreased from 1.30 to 0.28%. About 79% of the total K present

in rice straw was released within 5 weeks after its incorporation into the soil,

and 95.3% of K from straw was mineralized by the end of 23 weeks.

D. SULFUR

Sulfur is a critical nutrient for crop growth, and its deficiency is accen-

tuated in soils of the tropics by intensive agricultural practices, less use of

organic manures, removal of crop residues, and leaching of SO4 by heavy


rains. It is generally accepted that plants assimilate S almost entirely in the

form of SO4, which is produced by the mineralization of organic S. Unlike

phosphates, sulphates are easily leached. Incorporating crop residues into

the soil is one way of reducing S losses by leaching.

Mineralization of S in soils is mediated by biological activity. Very limited

information is available on S mineralization rates and potentials of wetland

soils amended with crop residues. In an incubation study, application of

wheat and barley straw to two soils (pH > 7.0) increased the S concentration

in equilibrium solution, suggesting that the addition of crop residues to soil

would increase available S (Choi and Rossi, 1978). Organic materials with

high C:S ratios such as wheat straw and rice husk caused considerable

immobilization of S, particularly during the early stages of decomposition

(Somani and Saxena, 1975). Addition of inorganic S fertilizers may,

therefore, be necessary. Islam and Dick (1998b) observed that addition of

wheat straw with a low C:S ratio (100:1) had a significantly higher accumu-

lation of SO4–S than the control or the higher C:S ratio (400:1) wheat

straw treatment. The cumulative amount of C mineralized was linearly

related to S mineralization. Islam and Dick (1998a) reported that the

S mineralization from crop residues followed first-order kinetics and that

the amount of SO4 in flooded soils amended with crop residues would

depend on the soil type, the nature of the crop residues, and the time of

decomposition. Crop residue management is a major determinant of long-

term S fertilizer requirements. Singh and Sharma (2000) observed a signifi-

cant increase in the availability of S in soil with the incorporation of crop

residues. The burning of straw or straw removal from rice paddies increases

the demand of the cropping system and will lead to increases in

S requirements in long term. Whitbread et al. (1999) reported an improve-

ment in the S balance with the incorporation of rice straw over removal.

Long-term studies are needed to enable measurement of the eVects of

recycling crop residues and the impact of environmental inputs on

S dynamics in the soil–plant system.

E. MICRONUTRIENTS

A ton each of rice and wheat removes 96, 777, 745, 42, 55, and 4 g ha�1 of

Zn, Fe, Mn, Cu, B, and Mo, respectively. The total crop residue production

in India stands at 105 million tons, and based on micronutrient contents of

the residues, the micronutrient potential associated with crop residues would

be about 35.4 thousand tons (Prasad, 1999). About 50 to 80% of Zn, Cu,

and Mn taken up by rice and wheat crops can be recycled through residue

incorporation (Prasad and Sinha, 1995b). Therefore, recycling of crop

residues can help improve the availability of micronutrients in soil.


1. Iron and Manganese

The application of crop residues to flooded soils leads to a reduced redox

potential (Eh) and, as a consequence, increases the Fe and Mn concentra-

tions in the soil solution. Katyal (1977) observed that not only did the

maximum concentrations of Fe and Mn occur earlier but also their concen-

trations were significantly higher in flooded soils amended with rice straw

compared to control. Yodkeaw and De Datta (1989) also noted that appli-

cation of rice straw increased Fe2þ and Mn2þ concentrations in soil solution,

resulting in increased uptake of Fe and Mn by rice crop. Under controlled

Eh and pH conditions, Atta et al. (1996) observed that at an Eh value of

�330 mV, soil suspension contained approximately double the amount of

water-soluble plus exchangeable Fe as compared with at Eh values of �150

to þ300 mV. Addition of wheat straw to soil suspension decreased the

exchangeable Fe fraction at pH 8.0, while it increased the same fraction at

both pH 6.0 and 7.0. Exchangeable and water-soluble Mn fractions were

reduced due to application of wheat straw at pH 8.0, while the easily

decomposable fraction increased at pH 7.0 and 8.0 and decreased at pH

6.0. In a greenhouse experiment, Sharma et al. (1989) measured significantly

higher leaching losses of Mn2þ and Fe2þ with increasing rates of rice straw

and percolation rate. As much as 111 kg Mn ha�1 and 110 kg Fe ha�1 were

lost through leaching in one cropping season.

2. Zinc

Kang (1988) observed that the availability of Zn in diVerent pools (water-

soluble, exchangeable, weakly and tightly complexed to organic matter) was

reduced by straw application at soil pH of 8.0. Several other workers (Yoon

et al., 1975; Dikshit et al., 1976; Raj and Gupta, 1986; Nagarajah et al.,

1989) have also reported that application of rice or wheat straw decreased

the Zn concentration in both flooded and upland soils. As a consequence, Zn

uptake and dry matter production were reduced compared to that in un-

treated control. Saviozzi et al. (1997), however, observed no significant eVect

of wheat straw (applied at 2% by weight) on the content and distribution of

Zn and Cu in soils. Even so, rice straw application has been found to

increase the Zn content of rice plants, possibly through its amelioritic

eVect on soil pH and ESP.

In calcareous soils, application of crop residues decreased the capacity

factor due to organic acids converting solid-phase labile Zn to soluble Zn

complexes (Prasad and Sinha, 1995a). The diVusion coeYcient of Zn was

increased with the addition of crop residues due to the presence of chelat-

ing agents released during their decomposition and thereby increasing the


concentration of total diVusible Zn. The diVusion coeYcient and Zn uptake

by rice in calcareous soils are related linearly.

VI. EFFECT OF CROP RESIDUES ON SOIL PROPERTIES

A. SOIL FERTILITY

Crop residues are an important constituent in nutrient cycling. The straw

of most cereal crops contains about 35, 10, and 80% of the total N, P, and

K taken up by the crop (Barnard and Kristoferson, 1985). Apart from the

straw is plant root material, which in most crops adds a substantial amount

of C to the soils. Long-term straw incorporation improves the fertility and

productivity of soils (Ponnamperuma, 1984). Soil organic matter has been

identified by many workers as a key factor in maintaining soil fertility and

crop production. Its maintenance is an essential requirement for increasing

and maintaining productivity. In most of Asia, rice straw incorporated into

the soil is the main source of SOM in the rice-based cropping systems. Since

the maintenance of soil nutrient status is an important aspect of sustain-

ability, the management of crop residues and fertilizer to maintain soil

fertility is necessary.

1. Soil Organic Matter

In tropical soils, SOM plays a major role in soil productivity because

it represents the dominant reservoir and source of plant nutrients. It

also influences pH, cation exchange capacity, anion exchange capacity,

and soil structure. Its level in soil was used as a general indicator of soil

productivity. A major factor contributing to the level of SOM is annual

input of plant residues. Residue managment impacts on SOM and long-term

fertility are becoming more relevant in the context of soil quality in tropical

environments.

The prominent means of maintaining SOM in irrigated rice-based crop-

ping systems in tropical countries have historically been the incorporation of

green manures, animal waste, or crop residues. In recent years, though, the

significance of green manures and animal wastes has been dramatically

altered by the increased use of mineral N fertilizers and other economic

considerations. More recently, crop residues including roots have become a

more common source of organic material added to the soil in many countries

in the tropics, where the use of combine harvesters is increasing (Flinn and

Marciano, 1984). For a given climatic region and soil type, the rate of


addition of carbon inputs is an important factor determining the amount of

organic matter that can be maintained in the soil. The soils tend to reach

equilibrium provided farming techniques and crop residue management

practices stay the same over a long enough period. Under conditions of

warm temperatures and increased water availability, organic matter

accumulation from residues is reduced.

Soil organisms use residues as a source of energy and nutrients, thereby

releasing CO2, inorganic compounds, and recalcitrant molecules, which

contribute to the formation of soil humus. Decomposition of crop residues

releases about 55–70% of the C to the atmosphere as CO2, 5–15% is

incorporated into microbial biomass, and the remaining C (15–40%) is

partially stabilized in soil as new humus (Stott and Martin, 1989). Because

the amount of carbon in soils is large and changes rather slowly, the

implications of a particular management system on the soil carbon may be

apparent only after several years to decades. Numerous calculations have

been made of the amount of residues needed to maintain organic matter at a

particular level (Paustian et al., 1997).

There exist only limited studies on the long-term eVect of crop residue

management on organic matter and N content of soils under rice-based

cropping systems in tropical and sub-tropical countries (Tables IX and X).

In these studies, increases in organic matter content due to crop residue

recycling are relatively small compared to those reported from temperate

regions (Prasad and Power, 1991). Incorporation of both residues increased

organic C and total N compared to removal or burning of straw (Dhiman

et al., 2000). When only rice or wheat straw was incorporated, organic C

content did not diVer significantly from removal or burning of straw. Rice

straw was more eVective in increasing total N content of soil than wheat

straw. Raju and Reddy (2000) reported that in rice–rice rotation, incorpora-

tion of rice straw to supply 25% of the recommended N fertilizer dose for

rainy season crop for 6 years significantly increased organic C content from

Table IX

EVect of Straw Management on the Nutrient Status of Mahaas Clay and Grain Yield Averaged for

Five Cultivars after the 16th Cropa

Straw

treatment

Organic C

(%)

Total N

(%)

Olsen P

(mg kg�1)

Exchangeable

K (mg kg�1)

Grain yield

(t ha�1)

Removed 1.81b 0.167b 9a 10.5b 3.2b

Burned 1.94b 0.173ab 11a 12.5a 3.4b

Incorporated 2.17a 0.182a 12a 11.6ab 4.1a

aIn a column, figures followed by a common letter are not significantly diVerent.

From Ponnamperuma (1984).


0.98% in straw removal treatment to 1.29%. Sharma (2001) reported that

organic C content increased from 0.56% in straw removal to 0.66% when

both the residues were incorporated for 2 years in rice–wheat rotation.

Burning and removal of crop residues were at par for their eVect on organic

C content.

Yadvinder-Singh et al. (2004b) reported that rice residue incorporation

increased organic carbon content of the sandy loam soil more significantly

than straw burning or removal after 7 years (Table X). Carbon sequestration

derived from changes in soil C content in the soil from rice residue applied at

7.1 t ha�1 annually for 7 years averaged 14.6%. In another long-term study,

Yadvinder-Singh et al. (2004a) reported that wheat straw incorporation in

rice increased organic C content from 0.40% in straw removal treatment to

0.53% in straw incorporation treatment after 12 years of experimentation on

a loamy sand soil. The values after 6 years were 0.38 and 0.49%, respectively,

suggesting smaller increases in organic C between 6 and 12 years than during

0–6 years. Carbon sequestration derived from changes in soil C content in

the soil from wheat straw incorporation for 12 years represented 10% of

the added carbon. The rate of increase in organic C with straw incorporation

is generally smaller in coarse-textured soils than in fine-textured soils.

For example, Verma and Bhagat (1992) and Dhiman et al. (2000) ob-

served marked increases in organic C in sandy clay loam soils with residue

incorporation after 4–5 years.

Naklang et al. (1999) observed no significant eVect of rice straw incor-

poration for 3 years on total and labile C content of a sandy soil. In a

rice-barley rotation under dryland conditions in northern India, Kushwaha

et al. (2000) observed a significant increase (28%) in soil organic carbon and

33% increase in total N with the incorporation of crop residues compared to

their removal after one annual cycle. It was suggested that for soil fertility

enhancement in dryland agroecosystems, postharvest retention of crop resi-

dues (20–40% aboveground biomass) of previous crop and its incorporation

in soil through minimum tillage in the succeeding crop should be followed.

Application of rice straw at 10 t ha�1 to an upland sandy soil caused a net

increase in soil C by 0.31 t ha�1 over no rice straw treatment (Ono, 1989).

The increase in C represented 8% of the C applied in rice straw. At higher

rates of straw addition, the net increase in soil C was increased but the

percent C increase did not change significantly. The soil C buildup in the soil

was significantly positively correlated with %N and negatively correlated

with C:N ratio.

Using data from a 24-year long-term experiment at IRRI, Los Banos,

Alberto et al. (1996) showed that straw incorporation improved organic

C, total N, available P, and exchangeable K above that of the burned

straw and no straw treatments. There was an average increase of 0.4 t ha�1

in rice yield with straw incorporation, while burning the straw resulted in

Table X

EVect of Crop Residue Management on Organic Carbon and Total N Content of Soil

Reference and country Type of crop residue and soil

Duration

of study

(years)

Residue

management

Organic

C (%)

Total

N(%)

Beri et al. (1995), India Rice straw in wheat and wheat

straw in rice; sandy loam

10 Removed 0.38 0.051

Burned 0.43 0.055

Incorporated 0.47 0.056

Sharma et al. (1987), India Rice straw in wheat and wheat

straw in rice, silty clay loam

6 Removed 1.15 0.144


IRRI (1986), Philippines Rice straw in rice–rice rotation;

clayey

12 Removed 1.67 0.173

Burned 1.74 0.179


Liu and Shen (1992), China Milk vetch green manure or

milk vetch þ rice straw in

rice–rice rotation

9 Removed 1.91 0.176

Green manure 2.06 0.190

Green manure

þ rice straw

2.21 0.194

Zia et al. (1992), Pakistan Rice straw in rice in rice–wheat

rotation; loam

3 Removed 0.53 —

Incorporated 0.63 —

Yadvinder-Singh et al.

(2004a), India

Wheat straw, green manure and

wheat straw þ green manure

in rice in rice–wheat rotation;

loamy sand

12 Straw removed 0.41 —

Straw incorporated 0.53 —

Wheat straw þgreen manure

0.59 —

318Y

AD

VIN

DE

R-S

ING

HET

AL

.

Dhiman et al. (2000), India Rice straw in wheat and wheat

straw in rice in rice–wheat

rotation; clay loam

3 Removed 0.51 0.062

Burned 0.51 0.063


Kumar et al. (2000), India Mustard straw in rice in rice–

mustard rotation, acidic

sandy clay loam

Removed 0.36 —


Prasad et al. (1999), India Wheat straw in rice in rice–

wheat rotation; sandy clay

loam

2 Removed 0.53 —


Rice straw in wheat in rice–

wheat rotation; sandy clay

loam

2 Removed 0.68 —


Verma and Bhagat (1992),

India


wheat rotation; silty clay

loam

5 Removed 1.09 —


Yadvinder-Singh et al.

(2004a), India


wheat rotation; sandy loam

7 Removed 0.38 —

Burned 0.39 —


Ponnamperuma (1984),

Philippines

Rice straw in rice in rice–rice

rotation; clayey

19 crops Removed — 0.181

Burned — 0.183

Incorporated — 0.202

CR

OP

RE

SID

UE

MA

NA

GE

ME

NT

319


negligible improvements. This reported increase occurred over a 14-year

period and highlights the time frame over which SOM increases occur.

A small increase in SOM associated with improved residue management

demonstrates how diYcult it is to improve SOM levels and, consequently,

nutrient levels in coarse-textured soils of the tropics. Incorporation of rice

straw at 5 t ha�1 year�1 for 12 years showed only a small increase in organic

C and total N content of soil with 2% initial organic C level (IRRI, 1986).

Straw removal or burning caused a decline in organic matter content during

the first 3 years of the study, while the straw incorporation maintained the

original level.

Field experiments on a rice–wheat cropping system in India showed that

incorporation of crop residues as compared to burning or removal increased

organic carbon and total N contents (Table X). Adiningsih (1984) reported

that incorporation of rice straw into the soil for 4 years increased the soil

organic matter content from 2.4 to 3.9% and total N content from 0.25 to

0.33% over straw removal in Indonesia. In China, Liu and Weng (1991)

found that returning rice straw to rice fields for 2 years usually increased soil

organic matter content by 0.03 to 0.05%. From a long-term field experiment

in Japan, Gotoh et al. (1984) estimated that 13 to 25% of the organic matter

returned to soil through rice straw was incorporated into the soil organic

matter in a slowly permeable grey lowland soil. In a 3-year study on a barley-

early rice-late rice cropping sequence in China, He and Liu (1992) reported

that addition of organic materials (green manure, crop residues, and FYM)

resulted in a mean increase (average of six experiments) of 0.053% organic

C compared to loss of 0.04% under inorganic fertilizer treatment. They

calculated that supply of 3.2 to 4.6 t ha�1 (mean of 3.8 t ha�1) of crop

residues ha�1 year�1 would be needed to maintain the soil health and to

improve productivity.

In a long-term study on a rice–wheat cropping system in northwestern

India, the incorporation of crop residues along with green manure in rice

increased soil organic carbon and total N contents as compared to straw

removal, but the increase was almost similar to that when crop residues were

applied alone. These data suggested little eVect of green manure on soil

organic matter content in semi-arid climates, particularly in coarse-textured

soils (Table IX). In a long-term study (1981–1990) in China, Liu and

Shen (1992) studied the eVect of milk vetch green manure in early rice

and milk vetch plus rice straw in late rice in a rice–rice cropping system.

The increases in organic matter and total N concentrations in soil were in

the decreasing order: green manure plus rice straw, green manure, and

inorganic fertilizers. Further, mixed application of green manure and crop

residues improved the quality of soil organic matter (Table IX). Vityakon

et al. (2000) reported that application of rice straw at 10 t ha�1 increased


organic C content of upland soil by 0.31 t ha�1 year�1 over no straw in loam

soil in Thailand.

Naklang et al. (1999) used the two indices to calculate a carbon manage-

ment index (CMI). They measured two fractions of organic carbon in soil.

The more labile fraction (CL) was measured by oxidation with 333 mM

KMnO4, and the nonlabile C (CNL) plus the C not oxidized by 333 mM

KMnO4, (i.e., CT-CL). The total C (CT) was measured by combustion. On

the basis of changes in CT between a reference site and the cropped site, a

carbon pool index (CPI) was calculated:

CPI ¼ CTcropped=CTreference ð6Þ

On the basis of changes in the proportion of CL in the soil (labiality ¼ L ¼CL/CNL), a labile index (4) was determined.

CMI ¼ CPI � LI � 100 ð7Þ

Incorporation of leaf litters increased the CMI from 9 in 1992 (initial) to

about 20 after 3 years in 1996 and CMI in no-litter treatment increased to 13.

Straw incorporation did not significantly aVect the CT (4.44 versus 4.11 mg

g�1) and CL (0.78 versus 0.79) compared to straw removal treatments. The

measurement of CL is a more sensitive indicator of SOM dynamics. Total

C measurement is still required to estimate bulk soil C change; however, CL

more accurately and quickly detects the impact of management on soil

C. Calculation of the CMI takes into account the change in CT pool size

and its lability and gives a more definitive picture of soil C dynamics than

when only a single parameter is used.

The studies on soil organic matter dynamics suggest that soil texture,

C inputs, and climatic conditions are the primary factors controlling stabili-

zation of soil C. Simulation models allow us to account for such interacting

factors and thus can be profitably used to understand the dynamics of soil

organic matter in crop residue-amended soils on a long-term basis. Most of

these models predictions have not been tested using observed data, and there

is a need to revalidate these models for rice-based cropping systems. There is

no single fixed quantity of SOM that can be considered as optimal for all

soils. All other factors held constant, an increase of 1% in SOM content

will have greater eVects for a sandy soil than for a clay-loam soil on the

overall productivity level. Benefits of increased SOM will also depend on

land use. For example, improved physical properties in clay soils might be

more useful for upland crops than lowland rice, as the common practice of

puddling rice soils is intended to destroy soil structure.

Soil organic matter levels tend to be stable or increase under irrigated

rice double cropping (Cheng, 1984; Nambiar, 1994; Witt et al., 2000).

Organic matter content is generally lower in rice-upland crop rotations


such as rice–wheat or rice-maize (Cheng, 1984; Nambiar, 1994; Witt et al.,

2000). The reduced soil C sequestration in the rice-upland rotation re-

sulted primarily from an increased amount of microbially mediated

C mineralization compared to the C mineralization rate in the rice–rice

system (Witt et al., 2000). Carbon sequestration with continuous rice crop-

ping would also be favored by the accumulation of phenolic end products

that appears to occur when crop residues decompose under anoxic condi-

tions in lowland rice systems (Olk et al., 1996). When crop residues are not

regularly incorporated in the lowland-upland crop rotations, the amounts of

labile SOM can decrease to the point of reducing the continuous supply of

available N through mineralization–immobilization turnover (Stevenson and

Kelley, 1985), which could lead to lower grain yield. In the light textured soils,

nutrients and soluble C compounds may move down the profile, thus resulting

in very slow, or no, long-term increase in soil fertility when residues are added

(Naklang et al., 1999).

Management of crop residues might also carry longer term impacts on the

chemical nature of SOM. The eVects of crop management on SOM quantity

in lowland rice soils have received more attention than have their eVects on

SOM quality. Few or no studies have examined the eVects of agronomic

practices on the quantity or quality of SOM and nutrient supply in intensive

continuous rice or rice–wheat rotations.

The quantity of SOM is not the sole factor that should be considered

when devising management practices to optimize the agronomic benefits of

SOM. A higher quantity of SOM does not automatically lead to a higher

quality of SOM. If most SOM-bound nutrients are in SOM fractions that

have low turnover rates, that is, high residence times, their roles in nutrient

supply will remain marginal. If the soil in question is a sandy soil, for

example, and if the crop obtains the bulk of its nutrients through decompo-

sition of the various SOM pools rather than through the exchange of

nutrients present on CEC complexes, the nutrient supply power of the soil

will remain low. Ultimately, it remains the quality rather than the quantity

of SOM that will lead to improved soil quality, and hence a more sustainable

cropping system, in particular for those agrosystems that are prone to land

degradation.

It remains a diYcult task to identify and quantify the intrinsic quality of

an SOM pool in terms of nutrient supply power, microbial activity, or

physical or chemical indices. Labile SOM pools are key suppliers of nutri-

ents to the crop, whereas other SOM pools are more recalcitrant in nature

and will provide fewer nutrients, but their chemical and physical properties

provide stability to the soil. The relative sizes of the labile versus more

recalcitrant pools that make up total SOM might have pronounced eVects

on the indigenous nutrient supply and perhaps even yield (Biederbeck et al.,


1984; McGill et al., 1986), illustrating the complexities of managing SOM

quality. The chemical nature of humic acid fractions changed with an

increasing number of annual irrigated rice crops (Olk et al., 1996). The

increasingly phenolic nature of the humic acid was speculated to be a

contributing factor to an apparent decline in soil N supply and grain yield

in continuously cropped lowland rice soils, as phenols are known to stabilize

nitrogenous compounds under controlled conditions (Haider et al., 1965).

The eVect of rotating upland crops with rice on SOM quality indicated that

the phelonic nature of labile SOM extracted from rice–wheat soils is more

similar to that of labile SOM from lowland rice–rice soils than that from

upland rice soils (Olk et al., 2000). Again, the agronomic significance of this

finding is not clear.

Bird et al. (2002) examined the five soil organic matter fractions from soil

samples obtained after 4 to 6 years of rice residue management treatments

using 15N-labeled urea. After 4 years of straw management treatments, soil

incorporation of straw increased mobile humic acid (MHA) and light

fraction (LF) carbon and N compared with burned straw. Immobilization

of fertilizer N peaked in all soil organic matter fractions after one growing

season (120 days) and was greater in the MHA over the 2-year study.

Nitrogen fertilizer sequestration was in MHA and LF and was greatest

with straw incorporation compared with straw burned. Turnover of immo-

bilized 15N fertilizer was fastest in the labile MHA and MFA (mobile fulvic

acid) fractions (7–9 years half-life) compared with a half-life of the moder-

ately resistant MAHA (metal-associated humic acid) fraction (53 years) and

most stable humic (HUM) fraction (153 years). The MHA and LF fractions

represented the primary active sink and source of sequestered N, aVecting

both short- and long-term soil fertility. A study by Devevre and Howarth

(2000) suggested that it is not primarily the accumulation of degradation

byproducts that may sequester N in SOM, as suggested by Olk et al. (1996).

The larger and sustained microbial biomass found under flooded compared

to aerobic conditions may act to immobilize more N and make it less

available for plant uptake.

The composition and dynamics of SOM are generally the same in tem-

perate and tropical soils, except that turnover rates in tropical soils usually

are higher than in colder climates. Therefore, many results from temperate

soils can be used to explain SOM dynamics and control in tropical soils. The

main transformations occurring during residue decomposition and humifi-

cation are the loss of polysaccharides and phenolic moieties, modification of

lignin structures, and enrichment in recalcitrant, non-lignin aromatic struc-

tures (Zech et al., 1997). The rates of these transformations are controlled

primarily by climatic factors and only to a lesser extent by chemical factors

such as pH, C:N ratio, or litter quality. Soil organic matter stabilization by


interaction with minerals probably is more important in tropical than

temperate soils because of the more favorable climatic conditions for decom-

position of organic matter. The protective eVect of minerals is most

pronounced for labile constituents such as polysaccharides or proteins.

Studies on the contribution of labile fraction to SOM dynamics in tropical

ecosystems are very scarce.

Crop residue management can aVect N immobilization and stabilization

processes important to eYcient utilization of N from fertilizers, crop resi-

dues, and soil organic matter. Bird et al. (2002) reported that a consistently

larger soil microbial biomass N and C pool was observed when straw was

incorporated than when it was burned. Because soil microbial biomass is a

prime source of available N for the crop, the incorporation of straw led to an

increase in the crop-available soil N. Although total soil N content had not

changed after 5 years of straw incorporation or burning, a significant in-

crease had taken place in the more labile soil N pools (humic substances).

The more labile soil N pools remain key sources of readily available N for

crop utilization.

2. Total N

About 70% of the rice lands in south and south-east Asia contain

<0.2% N and are considered N deficient (Ponnamperuma, 1984). Incorpora-

tion of crop residues enhances the N content of several wetland rice

fields (Tables IX and X). Within 3 years of incorporating the rice straw at

6–7 t ha�1, total N content in soil increased by 0.021% over the straw

removal treatment. At IRRI, in situ incorporating the straw twice a year

caused an increase of 48 kg N ha�1 per season, averaged for two experiments

lasting 7 years (Ponnamperuma, 1984). In another study with three soils over

a 5-year period, the increase due to straw incorporation was computed to be

40 kg N ha�1 per season, about 10 kg N ha�1 per season more than the straw

N content. The extra N probably came from N fixation stimulated by straw

acting as an energy source for heterotrophs and as a CO2 supplement to

surface phototrophs (Ponnamperuma, 1984).

3. Available P and K

A number of studies (Alberto et al., 1996; Gangaiah et al., 1999;

Ponnamperuma, 1984; Prasad et al., 1999; Sharma, 2001; Singh and Sharma,

2000) (Tables IX and XI) showed a slight or no increase in the available P in

the soil and P uptake of rice and wheat in soils amended with rice or wheat

straw. In an 11-year field experiment on a loamy sand in Punjab (India),

incorporation of residues of both crops in rice–wheat rotation increased the

Table XI

EVect of Crop Residue Management on the Available P, K, and S Contents in Soil

Reference

and country

Type of crop

residue and soil

Duration

of study

(years)

Residue

management

Available nutrients

(mg kg�1)

Olsen-P NH4OAc-K S

Beri et al.

(1995),

India

Rice straw in

wheat and

wheat straw on

rice, sandy loam

11 Removed 17 45 55

Burned 14 58 34

Incorporated 21 52 61

Sharma et al.

(2001),

India

Wheat straw in

rice, sandy clay

loam

2 Removed 14.0b 151c —

Burned 14.5b 169ab —

Incorporated 16.1a 175a —

Verma and

Bhagat

(1992),

India

Rice straw in

wheat, sandy

clay loam

5 Removed 12.6b 49.7b —

Incorporated 15.4a 73.0a —

Prasad et al.

(1999),

India

Rice straw in

wheat, sandy

clay loam

2 Removed 63 —

Incorporated 70 —

Wheat straw in

rice, sandy clay

loam

2 Removed 113 —

Incorporate 120 —

Yadvinder-

Singh et al.

(2004a),

India

Wheat straw in

rice, loamy

sand

12 Removed 6.4b 32b

Incorporated 6.2b 38a

Wheat straw

þ GM

7.4a 40a

Yadvinder-

Singh et al.

(2004a),

India

Rice straw in

wheat, sandy

loam

7 Removed — 35c —

Burned — 41b —

Incorporated — 45a —


total and available P and K contents in soil over removal of residues

(Table XII). Similarly, in a long-term study in Bihar (India), Misra et al.

(1996) observed increases in availability of P and K in soil with the incor-

poration of crop residues in rice–wheat rotation. Application of wheat straw

continuously for 12 years in a loamy sand soil caused only a small increase in

available K while taking into consideration the total amount of total

K recycled through the straw (Yadvinder-Singh et al., 2003a). In another

study on a sandy loam soil, incorporation of rice residue caused a smaller

but more significant increase in available K content in the soil than did the

residue removal treatments (Table XI). On average, rice residue added about

175 kg K ha�1 annually in treatments. Despite such large additions, the

increase in K availability in the soil was small. One possible reason for the

small increases observed in residue-amended plots may be the loss through

Table XII

EVect of Crop Residue Management on Soil Fertility of a Loamy Sand Soil over 11 Years of

Rice–Wheat Cropping System

Soil property

Crop residue management

Burned Removed Incorporated

Total P (mg kg�1) 390 420 612

Total K (%) 1.71 1.54 1.81

Olsen P (mg kg�1) 14.4 17.2 20.5

Available K (mg kg�1) 58 45 52

Available S (mg kg�1) 34 55 61

From Beri et al. (1995).


leaching of a significant proportion of residue K during rice cultivation on

highly permeable coarse-textured soils. Beaton et al. (1992) reported no

significant eVect of crop residues on the available K concentration in soil.

The K added through crop residues was possibly leached from the soil,

converted to unavailable forms, or taken up by the subsequent crops. Prasad

et al. (1999) reported an increase of 7 mg K kg�1 soil with straw incorpora-

tion compared to straw removal after 2 years of study in a sandy clay loam

soil. In a 6-year study by Raju and Reddy (2000) in rice–rice rotation,

application of rice straw to supply 25% of the recommended N fertilizer

(60 kg N ha�1) in rainy season rice caused a small increase in Olsen-P

compared to straw removal, but available K increased from 113 mg K kg�1

in straw removal to 143 mg kg�1 in straw-incorporated treatment.

The benefits of straw incorporation are reflected not only in the increase

in soil K but also in plant uptake (Beye, 1977; Ponnamperuma, 1984). In a

field experiment on rice-potato-mungbean rotation, Chatterjee and Mondal

(1996) observed that crop residues applied at a rate to supply 25% of the

recommended N requirement (75% supplied through fertilizer) increased

the available K in soil and lowered the depletion of non-exchangeable

K compared to the inorganic fertilizer alone treatment. Gill and Adiningsih

(1986) observed a marked response to K application in two crops of rice and

one soybean crop when crop residues were removed at harvest on Orthoxic

Troppudult soil high in Al and low in K. Recycling of crop residues dramat-

ically improved the yields at low rate of K application and decreased crop

response to applied K. Magbanua et al. (1988) reported that there was a

strong K depletion when residues of previous crops were removed. Rice

straw incorporation improved nutrient (P, K, S) balance over straw removal.

Whitbread et al. (1999) also reported that removal of rice stubble resulted in

negative K and S balances in rice.


4. Available Micronutrients and Silicon

Incorporation of residues decreased the availability of Zn but had no

significant eVect on Cu and Mn availability in soil (Bijay-Singh et al., 1992).

In another study over a 5-year period on a silt loam soil in Himachal Pradesh

(India), incorporation of rice straw in wheat, however, caused a slight

increase in the availability of P, Mn, and Zn and a marked increase in the

availability of K (Verma and Bhagat, 1992). Likewise, incorporation of

crop residues on a long-term basis has been found to increase the DTPA

extractable Zn, Cu, Fe, and Mn contents in soil (Meelu et al., 1994).

Rice straw contains large amounts of Si that on incorporation can in-

crease its availability in soils. According to Sumida and Ohyama (1991),

application of rice straw increased the Si content of rice plants, which helped

increase the lodging resistance in rice.

B. CHEMICAL PROPERTIES

1. Redox Potential and Electrical Conductivity

Incorporation of crop residues is unlikely to have a significant eVect on

the changes in Eh of aerobic soils due to fast O2 diVusion into soil, except in

large aggregates. Fermentation of crop residues in waterlogged soils results

in low Eh (Beye et al., 1978) and high pCO2, which leads to initial increase in

the concentrations of Fe2þ and Mn2þ in soil solution followed by precipita-

tion of carbonates. In rice straw-amended flooded soils, not only did the

peaks of CO2 occur earlier, but the concentration also increased as com-

pared to control (Katyal, 1977). Murty and Singh (1976) observed an

increase of 0.04 atm of pCO2 due to the addition of wheat straw to flooded

rice. Yoo et al. (1990) observed that surface application of rice straw sup-

pressed the formation of an oxidized layer and increased reducing condi-

tions. Rice straw also reduced O2 concentration followed by an increase in

the accumulation of CO2.

Straw incorporation hastened and intensified soil reduction and also

increased pH and electrical conductivity (EC) of acid soils (Beye et al.,

1978) and decreased pH and EC of alkaline soils (Yodkeaw and De Datta,

1989). Katyal (1977) noted the acceleration and intensification of Eh and pH

changes and the achievement of peak concentrations of water soluble Fe,

Mn, and CO2 in three soils amended with crop residues. Several researchers

(Murty and Singh, 1976; Wu, 1996) observed increases in EC of soil with the

application of crop residues under anaerobic conditions.


2. Soil pH

Crop residues can influence soil pH through accumulation of CO2 and

organic acids during their decomposition in the soils. The reduction in Eh of

flooded soils after the incorporation of crop residues may increase soil pH

due to consumption of protons during the reduction of Fe and Mn oxides.

A sharp decrease in soil pH of flooded soils due to application of rice or

wheat straw has been recorded by Wu (1996) and Murty and Singh (1976).

Under controlled Eh levels, Atta et al. (1996) observed a slight decrease in

pH of soil suspension during 5 days of incubation. A slight decrease in

soil pH with the application of wheat or rice straw was also reported by

Saviozzi et al. (1997), Gangaiah et al. (1999), and Kushwaha et al. (2000).

However, Verma and Bhagat (1992) noted no significant eVect of rice straw

management on pH of an acidic sandy clay loam soils after 5 years of

experimentation.

Seki et al. (1989) observed that successive applications of rice straw and

wheat straw decreased the pH of the surface layer in coarse-textured grey

lowland and fine-textured yellow soils with a sharp decline in the first year.

In a silty clay loam soil (pH 7.3) amended with rice straw, Sharma et al.

(1989) recorded lower pH values of soil solution between 2 and 6 weeks after

transplanting compared to those from untreated soil. Retention of crop

residues on the soil surface can result in a decline in soil pH to levels that

may adversely aVect crop production (Schomberg et al., 1994a). In long-

term field experiments (over 10 years), Beri et al. (1995) and Bellakki et al.

(1998), however, observed no significant eVect of incorporation of crop

residue on soil pH. In contrast, Yoo et al. (1990) recorded a significant

increase in the pH of floodwater with the application of rice straw.

The eVect of application of plant materials on soil pH depends on the

composition of plant material used (Yan et al., 1996). The mineralization of

N-rich compounds followed by nitrification produces protons, resulting in

the acidification of the soil. The influence of residues on soil pH appears to be

rather small, but the method of residue management may greatly influence

the soil reaction. The potential for pH changes with surface-managed residues

is greatest in response to application of fertilizers or high-N residues to the

surface and the absence of mixing soil amendments through the soil profile.

C. PHYSICAL PROPERTIES

Crop residues play an important role in improving soil physical charac-

teristics, but the degree of improvement depends on particle size distribu-

tion. Sandy soils with low SOM contents lack substantial structure and are

prone to severe erosion. Adding crop residues or manure will increase


microbial activity, which in some studies has led to the buildup of SOM and

formation of macro- and micro aggregates (Sparling et al., 1992; Angers

et al., 1993). Di Verences in aggregate stability also depend on the sources of

the organic materials, such as fungal hyphae versus microbial polysacchar-

ides (Tisdall, 1991). On the other end of the particle spectrum, heavy clay

soils are often characterized by poor structure and aeration, but they can be

improved through the addition of organic amendments. Therefore, the

positive e Vect of SOM on soil structure will be more pronounced for a

clay soil than for a silty soil.

In most climates, removal or burning of crop residues leads to deteriora-

tion of soil physical properties (Kladivko, 1994; Prasad and Power, 1991). In

rice, puddling of soil by cultivation in standing water could adversely a Vect

soil structure through destruction of aggregates and peds (Sharma and De

Datta, 1985) and leads to formation of a pan of low permeability immediately

below the cultivated layer, particularly on fine-textured soils. This hard pan

could be detrimental for the productivity of the upland crop (say wheat) after

rice (Moorman and Van Breeman, 1978; Sur et al., 1981). A recent review has,

however, shown that puddling may or may not be detrimental to the suc-

ceeding non-rice crops and soil (Connor et al., 2003). Recycling of crop

residues influences soil structure, crusting, bulk density, moisture retention,

and water infiltration rate and may help reduce adverse e Vects of hard pan

formation in rice-based cropping systems, which may play an important role

in the upland crop (such as wheat or maize) after rice than the rice crop.

1. Aggregation

The role of soil organic matter in aggregate stability is summarized in Fig. 5.

Straw incorporation helps the formation and stability of aggregates through

increase in microbial cells, and excrets microbial products and decomposition

products released during the death of the microorganisms (Lynch and Elliott,

1983). The soil organic matter in turn is protected within aggregates for

decomposition (Dalal and Bridge, 1996). The amount and chemical composi-

tion of organic residues, temperature, and moisture conditions are the major

factors determining aggregation in soil (Prasad and Power, 1991). Thus, easily

decomposable plant residues such as green manure and grain legume residues

provide transient and temporary aggregate stabilizing agents, while cereal crop

residues provide persistent aggregate stabilizing agents (Elliott and Lynch,

1984). Chaudhary and Ghildhyal (1969) obtained a close relation (r ¼ 0.76)

between organic C increased by organic materials addition and aggregate

stability of soil under wetland rice. Likewise, Elliott and Lynch (1984) found

that the eVect of straw on aggregation in a silt loam soil decreased with

increasing straw N content in the range of 0.25 to 1.09%.

Figure 5 A generalized summary of soil aggregates stabilization by various sources of

organic matter (Dalal and Bridge, 1996).


Several researchers have reported an improvement in soil aggregation

after incorporation of crop residues into the soil under rice-based cropping

systems (Bhagat et al., 2003; Liu and Shen, 1992; Liu et al., 1990; Meelu

et al., 1994; Oh, 1984). In a 10-year study on a rice–rice cropping system on a

vertisol, application of rice straw incorporated to meet either 25 or 50% of

recommended fertilizer N requirement increased the water stable aggregates

(Table XIII). In a rice–wheat cropping system on a loamy sand soil, incor-

poration of wheat straw over a 5-year period in rice promoted formation of

soil aggregates, particularly 1–2 mm size, and mean weight diameter (Table

XIV). A mixed application of green manure and crop residues was more

eVective compared to their separate applications. Similarly, in a long-term

experiment (1981–1990) on rice–rice rotation in China, Liu and Shen (1992)

noted that application of crop residues promoted aggregation. The contents

of micro-aggregates (0.25–1.0 mm) were increased from 10.9% in inorganic

fertilizer treatment to 12.1% in milk vetch green manure and to 13.6% in

green manure plus rice straw treatment.

In a 4-year barley-early rice-late rice crop rotation in China, Rixon

et al. (1991) found that addition of 3 t ha�1 of crop residues in late rice did

not significantly aVect distribution and stability of aggregates and moisture

retention characteristics of a gleyed paddy soil. However, after 5 years of the

above study, a continuous improvement in soil structure, volume weight,

porosity, aggregation, and plasticity was observed (Zhu and Yao, 1996).

The eVect of crop residues on aggregation also depends on the aggrega-

tion potential of the soil. Datta et al. (1989) have shown that when clay

Table XIII

EVect of Rice Straw Application on Soil Physical Properties in Rice–Rice Cropping System over a

10-Year Period on a Clayey Soil

Treatment to

summer rice

Bulk

density

(Mg m�3)

HC

(cm h�1)a

Water

stable

aggregates

(%)

Porosity

(%)

Water

retention

(kg kg�1)

Maximum

water

retention

capacity

(kg kg�1)33 kPa 1.5 MPa

Inorganic

fertilizers

1.43 1.18 37.6 46 0.35 0.21 0.49

Rice straw to

meet 50% N

1.26 1.93 51.3 52 0.43 0.28 0.58

Rice straw to

meet 25% N

1.27 1.78 49.6 52 0.41 0.26 0.56

Green leaf

manure to

meet 50% N

1.29 1.80 50.1 50 0.42 0.26 0.57

aHC, hydraulic conductivity.

From Bellakki et al. (1998).

Table XIV

EVect of Green Manure and Crop Residues on Soil Aggregation and Bulk Density in a Rice–Wheat

Cropping System on a Loamy Sand Soil after 5 Years

Treatment

Water stable

aggregates (%) Mean

weight

diameter

(mm)

Bulk density

(Mg m�3)

>2

mm

1–2

mm

0.5–1

mm

0.1–0.5

mm 0–10 cm 10–20 cm

Residue removed 9.8 10.0 5.6 11.3 1.42 1.59 1.72

Residue incorporated 11.7 15.0 5.5 11.3 1.56 1.49 1.72

Green manure (GM) 11.1 15.5 6.1 12.0 1.58 1.51 1.71

Crop residue þ GM 17.1 11.1 6.9 9.1 1.68 1.48 1.68

From Meelu et al. (1994).


content in soil was low, burying of straw had a more favorable eVect on the

stability of aggregates, especially of crumbs 3–5 mm in diameter, than in soil

with 27% clay content. Likewise, Verma and Singh (1974) observed that

wheat straw caused a marked influence on soil aggregation in four diVerent

soils varying in texture. Maximum aggregation occurred in the sandy loam,

with minimum aggregation in alkali soil. Application of rice straw to alkali


clayey soil significantly increased water stable aggregates >0.25 mm. Total

organic C also increased, which resulted in a marked increase of macropores

as well as the aggregate size in the 2.0–0.84 mm size fractions (El Samanoudy

et al., 1993). In a friable self-mulching clay of the vertisol group, 34 years of

either stubble burning or incorporation had, however, little eVect on soil

structure (Dexter et al., 1982). The nature of plant material also plays an

important role in the development of soil structure. For example, Dhoot

et al. (1974) recorded the highest percentage of water-stable aggregates in

pearl millet-amended soil followed by rice straw or wheat straw and sesbania

green manure.

2. Porosity

In a long-term field study in China, rice straw incorporation increased the

porosity and formation of large micro-aggregates and decreased the bulk

density of paddy soils (Li et al., 1986; Xu and Yao, 1988). Rice straw and

rape straw were more eVective in increasing porosity of soils than sesbania

green manure or pig manure (Li et al., 1986). Bellakki et al. (1998) and

Bhagat et al. (2003) noted a significant increase in the porosity of fine-

textured soils after the application of rice straw and lantana residues. He

and Liu (1992) observed that in rice straw-amended soil, porosity (>200 mm)

increased quickly after drying, which is favorable for land preparation and

sowing of upland crop in time after rice harvest. Beaton et al. (1992)

reported that addition of rice straw (6 t ha�1) over a 68-year period com-

pared to inorganic fertilizers reduced the volume weight and increased the

porosity of paddy soils in Japan.

3. Hydraulic Conductivity and Infiltration Rate

Crop residues aVect hydraulic conductivity and infiltration by modifying

soil structure, proportion of macropores, and aggregate stability. Marked

increases in hydraulic conductivity and infiltration have been reported in

treatments where crop residues were retained on the surface or incorporated

by conventional tillage over the treatments where residues were either

burned or removed (Murphy et al., 1993; Valzano et al., 1997). In a 6-year

rice–wheat cropping system on a clay loam soil in India, Sharma et al. (1987)

noted increased cumulative infiltration of 7.39 cm h�1 under residue incor-

poration over 5.70 cm h�1 under residue removal. Similarly, in long-term

experiments on rice–wheat cropping system, incorporation of both rice and

wheat straw, as compared to their burning or removal, increased both

Figure 6 EVect of crop residue and green manure application on infiltration characteristics

of a loamy sand soil (Meelu et al., 1994).


infiltration rate and cumulative infiltration in sandy loam soils (Singh et al.,

1996; Walia et al., 1995). In another 5-year study on a rice–wheat cropping

system on a loamy sand soil, Meelu et al. (1994) observed increased rates of

infiltration on soil amended with green manure and crop residues (Fig. 6).

A mixed application of green manure and crop residues was more eVective in

increasing infiltration compared to their separate applications. On an alkali

clayey soil application, rice straw significantly increased hydraulic conduc-

tivity and total and quick drainage pores (El Samanoudy et al., 1993). In a

long-term rice–rice cropping system on a vertisol, Bellakki et al. (1998)

also noticed a significant increase in hydraulic conductivity of soil from

incorporation of rice straw (Table XIII).

4. Bulk Density, Compaction, and Penetration Resistance

In general, incorporation of crop residues into the paddy soils reduced

bulk density, penetration resistance, and compaction of soils under both

rice–rice and rice–wheat cropping systems (Bellakki et al., 1998; Meelu et al.,

1994; Singh et al., 1996; Walia et al., 1995). Xie et al. (1987) also reported

that continuous return of rice straw to a paddy field for 7 years resulted in a

soil bulk density decrease of 0.17 Mg m�3. In another long-term field

experiment over 25 years, incorporation of crop residues improved the

porosity and decreased penetration resistance of a gleyed soil (Roppongi


et al., 1993). Likewise, combined application of cereal crop residues and

green manure has proved to be more e Ycient in reducing bulk density,

penetration resistance, and crusting of surface soil layers over their separate

applications (Liu and Shen, 1992; Meelu et al., 1994; Verma and Singh,

1974).

Bhushan and Sharma (2002) reported that with the application of lantana

residues to a silty loam soil continuously for 10 years in rice–wheat rota-

tion, clods <2 cm in diameter increased while those 2–4 cm and 4–6 cm in

diameter decreased with straw additions. The mean weight diameter of clods

varied between 2.15 and 2.34 cm in lantana-treated soil versus 2.83 cm in the

control. The bulk density and breaking strength of soil clods were lower in

lantana-treated soil by 4–9% and 29–42% than in control, respectively.

About 23–47% less energy was required to prepare seed bed in lantana-

treated soil than in control soil. The long-term addition of residues caused a

fundamental change in soil structural processes. A significant change in soil

consistency and the related physical properties such as surface cracking and

clod formation occurred after the addition of residues continuously for 10

years. Lantana-treated soil would become friable relatively soon, thereby

decreasing the turnaround time after rice harvest.

5. Soil Moisture Characteristics

In a long-term (1920–1988) study in Japan, Beaton et al. (1992) found

that addition of rice straw increased the water retention in paddy soil at 0 to

3.2 pF moisture tension, while no e Vect was observed at pF 4.2. Rice straw,

thus, improved the supply of readily available water. In a 10-year field study

on a vertisol, the application of rice straw significantly improved the water

retention characteristics of paddy soil (Bellakki et al., 1998) (Table XIII).

Likewise, Lanjewar et al. (1992), Kushwaha et al. (2000), and Bhagat et al.

(2003) reported significant increase in the water holding capacity of soil after

straw incorporation compared with removal or burning. Pandey et al. (1985)

also observed that incorporation of rice and wheat straw for 5 years

increased soil water retention at �33 k Pa over straw removal.

Application of lantana residue for 10 years on a silty clay soil in a rice–

wheat cropping system in northwest India significantly increased the liquid

limit, plastic limit, shrinkage limit, and plasticity index (Bhushan and

Sharma, 2002). The friability limit of residue-treated soil decreased from

8.9 to 7.8–8.2% gravimetric moisture content of soil, but soil became friable

at a relatively higher moisture content. Soil cracking changed from wide,

deep cracks in a hexagonal pattern to a close-spaced network of cracks. The

cracks of sizes <5 mm increased and those of 10–20 mm and wider decreased

with residue additions.


D. BIOLOGICAL PROPERTIES

The role of soil organisms as primary agents of decomposition, energy

flow, and nutrient cycling has become the subject of increased interest. Crop

residues provide energy for growth and activities of microbes and substrate

for microbial biomass, and provide conditions for a source-sink of nutrients.

Crop residue management alters soil environment, thereby influencing

microbiological populations and activity in soil and subsequent nutrient

transformations.

1. Microbial Biomass

Microbial biomass, a small (1–5% by weight) but active fraction of soil

organic matter, is of particular concern in soil fertility considerations be-

cause it is more susceptible to management practices than the bulk organic

matter (Janzen, 1987). Soil microbial biomass (SMB) acts as a reservoir of

plant nutrients and is a major determinant for governing the nutrient (like N,

P, and S) availability in soils. Although SMB values are only a small portion

of total C and N in soils, this living portion of soil contains a substantial

amount of nutrients needed for crop growth. The amount of microbial

biomass and microbial activity depends on the supply of organic substrates

in soil. Therefore, regular addition of a suYcient amount of organic materi-

als such as crop residue is important in the maintenance of microbial

biomass and improvement of soil fertility.

Several researchers (Azmal et al., 1996; Sridevi et al., 2003) have reported

a marked increase in microbial biomass following incorporation of crop

residues. After straw incorporation, microbial biomass-C (MBC) increased

by two- to fivefold in 10 days and reached the highest value by 30 days. For

example, Ocio and Brooks (1990) observed that straw addition, compared to

control, increased the microbial biomass by 87.5% in a sandy loam soil and

by about 50% in a clay soil. Malik et al. (1998) found that application of

wheat straw and green manure in a rice–wheat cropping system caused a

large increase in microbial biomass during the initial phases of rice crop. An

increase in microbial biomass was sustained throughout the growing season

of rice and resulted in synchronization between N release and N uptake.

Patra et al. (1992) found more biomass C in wheat straw than in cowpea

residue-amended soil, but the amount of microbial biomass N (MBN) was

significantly higher in the latter. Azmal et al. (1997) reported that the

amount of microbial biomass C and N increased immediately after rice

straw incorporation into a clay loam soil incubated under aerobic condi-

tions, reached maximum values after 1 week of each application (2 g C as

rice straw kg�1 soil after every 6 weeks), and decreased thereafter. The level


of maximum biomass formation reached a ceiling after the second applica-

tion, suggesting that soil has a certain capacity to hold biomass.

Singh (1991) reported that microbial C was maximum in the wheat straw

(10 t ha�1) plus fertilizer treatment (408–420 mg g�1) followed by straw (360–

392 mg g�1) and fertilizer treatments (238–246 mg g�1) in rice-lentil (Lens

esculenta) crop rotation under dryland conditions. With time, straw plus

fertilizer treatment accumulated 77% more microbial biomass C over con-

trol. The initial flush of microbial activity probably results from rapid

catabolism of simple soluble C compounds initially present in crop residues.

Biomass N is in a constant state of turnover and represents a significant

proportion of the total N, which is relatively constant throughout the year.

Application of labeled wheat straw to a clay loam soil increased the biomass

N from 46 mg N in control to 80 mg N g�1 soil by day 5 and remained at this

level by day 20 (Ocio et al., 1991). Bremer and Van Kessel (1992) studied the

dynamics of microbial C and N following the addition of 14C- and 15N-

labeled lentil and wheat straw in a sandy loam soil under field conditions.

Microbial 15N accounted for 65 to 81% of the added residue 15N. The results

suggested that microbial biomass may reduce losses of N and other nutrients

during the periods of low crop demand and may act as a source of nutrients

during the active crop growth.

Kushwaha et al. (2000) reported that straw incorporation increased the

SMB carbon during crop growth from 214–264 mg g�1 in straw removal to

368–503 mg g�1 in straw incorporation treatment after two annual rice-

barley crop cycles. The MBC was increased by 48% and N by 60% in residue

retention over residue removal plots. Along with residue retention, tillage

reduction from conventional to zero increased the levels of MBC and MBN

over control. Addition of plant residues with a high C:N ratio may facilitate

transformation of fertilizer or soil N into a slowly available N and thus may

improve N use eYciency. Microbial biomass may act as slow-release fertiliz-

er. It has been reported that the larger and sustained microbial biomass

found under flooded compared to aerobic conditions may act to immobilize

more N and make it less available for plant uptake, as seen in the some areas

of the tropics (Cassman et al., 1995).

Bird et al. (2001) observed that soil microbial biomass was always signifi-

cantly greater when straw was incorporated than when it was burned.

Because soil microbial biomass is a prime source of available N for the

crop, the incorporation of straw led to an increase in the crop-available

soil N. Although the total N content did not change after 5 years of straw

incorporation or burning, a significant increase was noted in the more labile

soil N pools (humic substances).

Witt et al. (1998), however, observed no significant eVect of residue

incorporation on microbial biomass C and N, suggesting that microbial

biomasses are not sensitive indicators of processes governing net


N mineralization. Instead, incorporation of crop residues may have led to

enhanced microbial activity rather than microbial growth.

2. Microbial Population and Activity

Residue incorporation into the soil leads to increased bacterial and

fungal activities (Beare et al., 1996; Doran, 1980). For example, protein-

decomposing microorganisms increased during the early stages of incuba-

tion of rice straw under waterlogged conditions (Fujii et al., 1972), which

was followed by an increase in the population of cellulose-decomposing

microorganisms. Sulphate-reducing microorganisms then increased after a

lag phase. Nugroho and Kawatskka (1992a) observed that application of

rice straw (C:N ¼ 52:1) increased all the microbial populations. In that

study, simultaneous application of rice straw and NHþ4 –N to soil under

upland conditions increased the number of denitrifiers but significantly

depressed the N2 fixation activity. Beri et al. (1992) also observed that soil

treated with crop residues inhabited 5–10 times more aerobic bacteria and

1.5–11 times more fungi than the soil for which residues were either burned

or removed. Fujii et al. (1970), in contrast, found that with a short-term

incubation period (10 or 20 days) in an aerobic soil, the population of

nitrifying bacteria was higher in the absence of rice straw, but the reverse

was true with longer incubation periods (60 or 90 days). Ladatko and

Emtsev (1984) observed marked increase in the growth of Clostridium spp.

in a soil amended with rice straw. The increase in the growth of anaerobic

microorganisms was due to the formation of artificial anaerobic microsites

around the straw particles. Last but not least, residue quality may aVect the

microbial population, as smaller bacteria and fungal populations are greater

on cereal residues compared to those on legumes. As compared to bacteria,

fungi are more influenced by residue quality (Wardle, 1995).

3. Macroorganisms

Earthworms and micro-arthropods play a dominant role in organic mat-

ter decomposition and nutrient cycling associated with diVerent crop residue

management systems (Prasad and Power, 1991; Tian et al., 1993). Although

enough information is not available from rice-based cropping systems,

residue quality greatly influences macroorganism populations in the soil.

For example, the earthworm population was negatively correlated with the

C:N ratio, lignin:N ratio, and polyphenol concentration of the plant materi-

al (Tian et al., 1992), and the population of ants was significantly correlated

to N concentration of plant residues (Tian et al., 1993).


4. Enzyme Activities

Barreto and Westerman (1989) and Gill et al. (1998b) observed a signifi-

cant increase in urease activity in surface soils after incorporation of wheat

straw. Likewise, Guan (1989) reported that application of wheat straw

increased the invertase activity of soil by 40–90 times compared to the

control treatment in both laboratory and field experiments and that the

activities of urease and alkaline phosphatase were also increased by wheat

straw additions. Gialhe et al. (1976) observed that dehydrogenase activity

increased with rice and wheat straw incorporation and was further increased

by N application. Goyal and Chander (1998) also reported an increase in the

microbial biomass and dehydrogenase and alkaline phosphates activities

with the addition of wheat straw to a sandy loam soil.

E. CROP RESIDUES FOR RECLAMATION OF SALT-AFFECTED SOILS

Organic materials have been used as amendments for reclaiming saline

and sodic soils. Incorporation of crop residues can bring about favorable

changes in the physico-chemical properties of such soils. Puttaswamygowda

and Pratt (1973) reported that addition of straw to sodic soil prior to

submergence for 130 days substantially lowered the pH and exchangeable

sodium percentage and increased the Na, Ca, Mg, and Fe2þ ions and

electrical conductivity. Xie et al. (1987) observed that incorporation of

wheat straw with and without sesbania green manure (GM) to a silt loam

saline-alkali soil decreased salt concentration of the top soil in the first 3

years. Similarly, Swarup (1992) found that incorporation of wheat straw and

rice husk alone or in combination with GM into a sodic soil significantly

reduced the pH and exchangeable sodium percentage, and increased PCO2

and exchangeable Ca þ Mg and extractable Fe, Zn, Mn, and P. The degree

of improvement was in the order GM þ rice husk > GM þ wheat straw >GM > rice husk > wheat straw > control. The increased availability of

nutrients resulted in improved yields and nutrient uptake by rice. Marked

decreases in pH, exchangeable sodium percentage, and electrical conductivi-

ty of salt-aVected soils amended with crop residues have also been reported

by many other workers (Abdul-Wahid et al., 1998; Hussain et al., 1996;

Illayas et al., 1997; More, 1994).

In a lysimeter study using calcareous sandy loam soil under a rice–

wheat–maize fodder system, Sekhon and Bajwa (1993) reported that

irrigation with sodic water caused precipitation of Ca and increased the

accumulation of Na in the soil and adversely aVected the crop yields.

Incorporation of rice straw decreased the precipitation of Ca and carbonates

increased the removal of Na in drainage water, decreased pH and electrical


conductivity of the soil, and improved crop yields. The release of organic

acids during decomposition of residues possibly mobilized the soil Ca. The

quantity of gypsum required for controlling the harmful eVect of sodic

irrigation water on soil properties can be considerably reduced in the

presence of crop residues.

Incorporation of wheat straw into a saline soil at 7 t ha�1 for 3 years

improved soil physical properties such as bulk density, pore volume, and soil

water retention and improved soil productivity (Wang et al., 1988). Im-

provement in soil physical properties (bulk density, porosity, and hydraulic

conductivity) due to addition of crop residues was also reported by Hussain

et al. (1996). Thus, recycling of crop residues on salt-aVected soils is likely to

have greater benefits than on normal soils (Swarup, 1992; Abdul-Wahid

et al., 1998).

VII. BIOLOGICAL NITROGEN FIXATION

Naturally occurring heterotrophic and phototrophic bacteria use the

straw either directly by the use of hemicellulose and simple carbohydrates

or indirectly following the decomposition of cellulose by decomposer micro-

organisms. Asymbiotic N2-fixing bacteria can use crop residues for energy

through the use of some hemicellulose components (Halsall et al., 1985) or

products of straw decomposition (Roper and Halsall, 1986). The heterotro-

phic diazotrophs depend on carbon for energy. Since most N2-fixing bacteria

are unable to use cellulose directly as a substrate for N2 fixation, cellulose

must be degraded to simpler intermediates before being used by diazotrophs.

Adachi et al. (1989) showed the existence of linkage between anaerobic

cellulytic bacteria and anaerobic N2-fixing bacteria during the decomposi-

tion of straw. The role of crop residues in biological N2 fixation by hetero-

trophic and phototrophic bacteria has been reviewed in detail by Roper and

Ladha (1995).

Anaerobic conditions and a decrease in inorganic N content of soil

following incorporation of straw favor N2 fixation by heterotrophic and

phototrophic bacteria in waterlogged soils (Yoneyama et al., 1977). Under

laboratory conditions, a wide range of values of N2 fixation (0.8 to 7.07 mg

N fixed per g of straw in 14 to 56 days) have been obtained due to diVerences

in the form and amount of straw, time of incubation, and methods used for

quantification (Roper and Watanabe, 1986). Only a few quantitative data

on the amount of N2 fixed or N gained following straw application in

greenhouse or field conditions are available.

Enhanced N2 fixation in flooded soils amended with straw has been

reported by Rice and Paul (1972) and Charyulu and Rao (1981). Rao


(1980) estimated that N2 fixation in 30 days in flooded soil amended with

chopped straw at 5 or 10 t ha�1 and planted to rice was two to four times

that of the unamended control. Based on a per hectare furrow slice of 0.7 �106 kg dry soil ha�1, extrapolation of the values of 15N incorporation in

straw-amended soil in a 30-day experiment indicates N2 fixation of about

7 kg ha�1 in the unamended soil and 25 kg N ha�1 in straw-amended soil.

Santiago-Ventura et al. (1986) measured twice the N gain following straw

incorporation equivalent to 10 t ha�1 after the three consecutive rice crops

compared with control pots; N gain ranged from 2 to 4 mg N fixed g�1 straw

added. Nugroho and Kwatsuka (1992b) found maximum rates of N2 fixa-

tion as stimulated by rice straw amendment to be as high as 220 mg g�1

day�1 when the level of NHþ4 –N in the soil was below 7.8 mg N kg�1 soil.

High levels of NH4–N (98–298 mg kg�1 soil) inhibited the initial N2 fixation

activity. When denitrification occurred at high rates, N2 fixation was sup-

pressed and vice versa. Yoo et al. (1990) reported that surface application of

rice straw increased the pH of the floodwater to an optimum level for the

growth of N2-fixing microorganisms, and thereby increased the N2 fixation

by phototrophic bacteria and blue-green algae.

In aerobic soils, intese microbial activity during the decomposition of

crop residues results in the development of anaerobic and microaerobic

microsites in soils, including surface soils (Hill et al., 1990). These sites can

support N2 fixation by a wide range of free-living, diazotrophic bacteria,

including anaerobic bacteria. In situ measurements of N2 fixation associated

with wheat straw indicated amounts fixed (based on the acetylene reduction

technique) ranging from 1 kg N ha�1 in 31 days to 12.3 kg N ha�1 in 22 days

(Roper, 1983). The amount of wheat straw added to soil ranged from 4.3 to

7.2 t ha�1 under conditions where moisture was not limiting (i.e., field

capacity). In a laboratory incubation study, Saha et al. (1995) observed

that berseem (Trifolium alexandrinum) and rice straw significantly increased

aerobic nonsymbiotic N2-fixing bacteria, phosphate-solubilizing bacteria,

and S-oxidizing microorganisms, resulting in greater availability of N, P,

and S in the soil.

Crop residue-associated N2 fixation is modified by mineral N, tempera-

ture, moisture, oxygen concentration, soil characteristics, and straw man-

agement techniques (Roper and Ladha, 1995). In fact, straw decomposition

is also directly aVected by these factors. In a field experiment, Roper (1983)

observed a positive correlation (r ¼ þ0.98) between nitrogenase activity and

wheat straw decomposition. As already discussed, the N2 fixation rates in

straw-amended soils are higher under waterlogged conditions than under

upland conditions (Rao, 1976). Roper et al. (1994) found that nitrogenase

activity under field conditions was the highest with straw incorporation and

the activity decreased in the order straw incorporation > straw mulched >no tillage. The depth of straw incorporation into soil also aVected the


nitrogenase activity. Straw mixed lightly with the soil near the surface

produced significantly higher nitrogenase activity than soil in which straw

was incorporated throughout the plough layer (Roper et al., 1989). Kanungo

et al. (1997) recorded higher nitrogenase activity in the top 1–2 cm soil layer

after the placement of organic residues, while residue placement in 2–6 cm

layers significantly reduced nitrogenase activity, irrespective of soil type. The

high nitrogenase activity in the topsoil was associated with larger popula-

tions of Azospirillum, Azotobacter, and anaerobic N2 fixers and favorable

redox potential supporting growth of N2 fixers.

VIII. PHYTOTOXICITY ASSOCIATED WITH CROPRESIDUE INCORPORATION INTO THE SOIL

The adverse e Vects of substances originating from decomposing crop

residues have long been considered as a cause of poor growth and yield of

many crops (Patrick et al., 1963). Since breakdown of cellulose occurs

readily, many of the adverse e Vects of residues occur within a relatively

short time after the incorporation of residues and the sowing of the follow-

ing crop. Warmer climates further accelerate the breakdown of crop resi-

dues. Thus, incorporation of crop residues can have adverse e Vects on

subsequent crops other than rice if anaerobic conditions develop (Cannell

and Lynch, 1984). However, anaerobic decomposition of crop residues with

no-tillage may have adverse e Vects on seedling establishment of rice. Lynch

(1977) reported that under certain conditions, substances toxic to cereal

seedlings are produced by cereal residues that decay near the seedlings.

These findings assume greater importance when crops are grown immediately

after cereals and with minimal cultivation. When seed drills operate in soils

where crop residues are placed on the soil surface or are only shallowly

incorporated, seed and residue can be placed in close contact, particularly in

fine-textured soils.Wet conditions that lead to anaerobic decomposition of the

residues can adversely aVect seedling growth (Elliott et al., 1978; Kimbler,

1973). Kimbler (1973) reported that the degree of inhibition of growth of

wheat by wheat straw depended on the length of decomposition period and

was greatest when the period was only 2–6 days. Surface retention leads to

slow decomposition, and incorporation is recommended as soon as possible.

Phytotoxic substances (e.g., phenolic acid and acetic acid) are produced

from degrading crop residues preferentially under anaerobic soil conditions

(at least in localized zones) and seldom accumulate in aerobic soil because of

rapid metabolization by microorganisms. Gaur and Pareek (1974), however,

detected a larger number of phenolic and aliphatic acids under aerobic

than under anaerobic conditions. In a laboratory incubation study, the


addition of rice or wheat straw produced large amounts of acetic acid

under anaerobic conditions 4 to 8 days after the incorporation of straw

(Bhat, 1991). Tanaka et al. (1990) reported that straw incorporation resulted

in accumulation of reducing substances and various aliphatic aromatic

acids in soil, which can inhibit rice root growth. Low temperature and

acidity further favor the production and persistence of fatty acids (Cho

and Ponnamperuma, 1971). At temperatures over 30 8C, these acids disap-

pear within 2–3 weeks of straw incorporation. The organic acids are phyto-

toxic in the millimolar concentration range and can cause significant crop

losses, which can be between 13 and 29% in heavy clay soils when seed is

direct drilled in the presence of wheat straw in winter (Graham et al., 1986).

Studies on homogenous slurries of a soil in a chemostat showed that the

formation of organic acids from plant residues is primarily linked to Eh; the

critical Eh being about zero (Lynch and Gunn, 1978).

Goodlass and Smith (1978) observed that evolution of C2H4 from soils

under anaerobic conditions was stimulated by amending soils with barley or

wheat straw. Temporary anaerobic conditions resulted in large increases in

C3 and C4 hydrocarbons. The association between degradation products and

C2H4 suggests that both may be implicated when root growth is adversely

aVected by the anaerobic decomposition of plant residues. Wu et al. (1997)

observed that application of rice straw increased the level of reducing

substances in soil at 20 days after application and reduced rice plant weight

at 30 and 70 days after planting. In a greenhouse study, Sharma et al. (1989)

found that total water soluble organic acids extracted from the root zone of

rice plants (100 mm soil depth) increased with increasing amounts of rice

straw (Fig. 7), but the acid production decreased with increasing rate of

percolation. Highest acid concentration (364 m mol L�1) was obtained with

the addition of 20 t rice straw ha�1 and a percolation rate of 15 mm day�1.

The organic acids formed at 2 weeks after transplanting did not persist in

soil solution; rather, they disappeared rapidly and the rice yields were same

under all the treatments.

The toxic eVects of aliphatic acids on rice growth have been widely

studied. Most investigations have been of short term and on young

plants. Nevertheless, in several instances, quite low concentrations of acetic

acid, propionic acid, and butyric acid have killed rice seedlings (Rao and

Mikkelsen, 1977). The injury caused by monobasic aliphatic acids depends

on the type of acid present and its concentration. The inhibitory eVect on

rice seedlings generally increases with increasing molecular weight, increas-

ing with order formic, acetic, propionic, and butyric acid (Chandrasekaran

and Yoshida, 1973). Tanaka et al. (1990) observed that rice root elongation

was markedly inhibited by the solution extracted from flooded soil

with incorporated wheat straw; the extract contained aliphatic and pheno-

lic acids under acidic conditions. Huang and Lu (1996) reported that

Figure 7 Temporal changes in volatile organic acid concentrations in the soil solution

collected at 100 mm soil depth as aVected by added rice straw (mean of three percolation rates)

(Sharma et al., 1989).


pre-flooding after rice straw incorporation for 2 weeks is suYcient for

oVsetting any adverse eVect due to phytotoxicity and N deficiency in rice.

Wallace and Whitehead (1980) have reported that volatile fatty acids are

more toxic than nonvolatile aliphatic acids between 0.5 and 1.0 mM con-

centrations and that the organic acids produced at one site do not diVuse

very far onto the soil. Therefore, the establishing crop roots must not come

into close contact with decomposing residues.

Adverse eVects of decomposing residues on crops under aerobic condi-

tions have been widely reported (Bhowmik and Doll, 1982). Phenolic acids

such as ferulic, p-coumaric, and p-hydroxybenzaldehyde released from living

or dead tissues of variety of plant species caused adverse eVects on the

growth of crops (Nelson, 1996). Elliott et al. (1981) could not demonstrate

the phytotoxicity to winter wheat on plots when wheat straw was mixed into

the soil. N immobilization during straw decomposition rather than phyto-

toxicity appeared to be the primary factor adversely aVecting yield because

yield decline was largely overcome by high rate of N application. Chung

(2001) identified p-hydroxy benzoic acid (6.34–6.87 mg kg�1), p-coumaric

acid (0.34 mg g�1) and ferulic acid (0.05 mg g�1) during the decomposition

of rice straw. The nature and composition of allelopathic compounds

depended on the type of crop residue or variety. P-hydroxy benzoic acid


(10�3 M) showed the greatest inhibitory eVect on barnyard grass seed

germination, seedling growth, and dry weight.

In the rice field, the concentration of phytotoxins varies during the growth

period of the crop and is probably greater in the early stages of flooding. It

may also vary spatially within the rhizosphere. The rice crop can also show

considerable compensatory growth from adverse eVects on early growth after

rice straw has been ploughed into the soil (Gotoh and Onikura, 1971).

Organic acids accumulated around straw only in the early stages of decompo-

sition, and hence if straw decomposition could be accelerated by any means,

the danger period for seedling could be reduced. The concentration of organic

acids in flooded soils in the tropics receiving 5–10 t ha�1 of straw is not toxic

to rice (Ponnamperuma, 1984). Witt et al. (2000) noted no evidence that

late residue incorporation caused phytotoxic eVects as a result of reduced

organic compounds or toxins produced during residue decomposition.

It is recommended to plough crop residues shortly after harvest is com-

pleted, because the decomposition of the straw occurs early after incorpora-

tion, the phytotoxicity occurring in the initial period of growth of the rice

plant can be alleviated, and stable yields can be obtained.

IX. WEED CONTROL AND HERBICIDE EFFICIENCY

Weeds are a major problem in the productivity of rice-based cropping

systems. Depending upon their type and intensity, 20 to 50% or even greater

losses in grain yields of rice and wheat are common due to competition from

the weeds (Walia and Brar, 2003). Most studies in weed control in rice and

other crops have been confined to evaluating the eVects of herbicide, tillage,

water, and their interactions (Bhagat et al., 1999; Gajri et al., 1999). Few

studies have been conducted on the dynamics of weed population and

herbicide eYciency under residue management in rice-based cropping sys-

tems. Such information is needed in weed control strategies for rice-based

cropping systems to improve their productivity. Kumar and Goh (2000)

reported that crop residues can suppress weeds in many ways, for example,

(1) through their physical presence on the soil surface as mulch and by

restricting solar radiation reaching below the mulch layer, (2) by direct

suppression caused by allelopathy, and (3) by controlling N availability.

Burning of residues can help in eVective removal of weed seeds and weeds.

The major disadvantage of incorporation of rice straw compared to burning

is the increase in weed and possible pest pressure. Roeder et al. (1998)

reported that compared with farmers’ traditional burning of crop and

weed residues, mulching reduced rice yield by 43% in one out of four

comparisons and increased weed biomass by 19–100%.


In addition to influencing the weed growth and population, crop residue

management and tillage practices also influence the eYciency of soil-applied

pre-emergence herbicides (Kumar and Goh, 2000). Because pre-emergence

herbicides are applied to the soil, the amount and quality of residues and ash

content left behind after residue burning might aVect their activity. Contin-

uous burning or incorporation of residues of both crops over years results in

buildup of ash or organic matter in the soil. The eYciency of soil-applied

herbicides may decline because of increased absorption capacity of soil. Brar

et al. (1998), however, observed that there was no significant eVect of

burning or incorporation of crop residues on the eYcacy of butachlor

applied to rice and isoproturon applied to wheat in a rice–wheat cropping

system. Mt. Pleasant et al. (1992) observed that mulching residues had little

eVect on weed control and crop yields were always higher when residues

were incorporated in a rice-based cropping system. The reports on the eVect

of crop residue management practices on weed growth and herbicide

eYciency are not conclusive and need further investigation to improve the

productivity of rice-based cropping systems.

The eVects of crop residue management on the pests and diseases in

rice-based cropping systems in the tropics have not received much attention.

X. EMISSION OF GREENHOUSE GASES

Methane (CH4) and nitrous oxide (N2O) are important greenhouse gases,

N2O being about 300 and CH4 being 15 times more radiatively active than

CO2 (mass basis, considering residence time in the atmosphere) (Rodhe,

1990). Flooded rice soils are a major source of atmospheric CH4, contribut-

ing about 10% of the total global emissions of CH4 (Mitra et al., 1999; Neue

and Sass, 1996; Rennenberg et al., 1992; Sass et al., 1990; Wassmann et al.,

1998). Global methane emission from flooded rice fields has been estimated

at 20–100 Tg year�1 (Neue, 1993). In comparison, the total agricultural

sources of N2O are quite small, ranging from 0.03 to 3.0 Tg N year�1

(IPCC, 1996).

Incorporation of organic materials (crop residues, green manures, com-

post) to regenerate depleted soil resources and promote sustainable food

productions in the tropics should significantly increase CH4 emissions. Thus,

residue management strategies may create conflicts between the goals of

sustainable agriculture and mitigation of greenhouse gases when used in

flooded rice-based systems. Soil properties, water management, organic

amendment, and temperature have been reported as the major factors

controlling the amount of CH4 emitted from rice fields (Sass et al., 1991;

Schutz et al., 1989). It has been estimated that CH4 emissions from rice


cultivation in India (45 million ha) should not exceed 2.5 t year�1. The main

reason for low CH4 emissions from rice fields in India is that the soils have

very low organic C or receive very little organic amendments (Jain et al.,

2000). The burning of crop residues also contributes to the global CH4

budget. For each ton of crop residue burned, 2.3 kg CH4 is emitted (Grace

et al., 2003). In rice–wheat cropping system, 0.14 t year�1 will be emitted, if

one-half of the 12 million ha under rice–wheat cropping system is burned.

A. METHANE

Organic C from added crop residues, organic manures, soil organic

matter, or rice plant roots is the major driving force for CH4 production in

rice-based agriculture systems (Wang et al., 1992; Yagi and Minami, 1990).

Numerous studies from all over the world have demonstrated that added

crop residues, composts, and green manures enhance CH4 fluxes relative to

unamended controls (Bossio et al., 1999; Chen et al., 1993; Chidthaisong

et al., 1996; Glissmann and Conard, 1999; Neue et al., 1994; Rath et al.,

1999; Wassmann et al., 1993). The seasonal emissions from paddy rice with

organic additions ranged from 1.1 to 148 g CH4 m�2 and increased methane

emissions 1.2- to 32-fold over unamended control soils. Crop residues serve

as a substrate for a complex microbial community, including methanogenic

microorganisms. Most studies on the microbiological aspect of CH4 produc-

tion in flooded rice soil have focused on methanogens (Asakawa and

Hayano, 1995; Asakawa et al., 1998). In addition to methanogens, the

degradation of organic matter to its most reduced status (CH4), however,

involves at least two other kinds of nonmethanogens: the zymogenic bacteria

and the acetic acid- and hydrogen-producing bacteria. Thus, from the point

of view of microbiological ecology, diVerent eVects of various organic ferti-

lizers on CH4 production potential might be closely related to the amount of

easily decomposable organic matter.

In principle, the degradation pattern in soils with and without amended

straw is similar, with acetate, propionate, and H2 as the main intermediates

of anaerobic degradation and CH4 being formed from H2/CO2 (11–27%)

and acetate (84–89%). However, the early phase of straw degradation diVers,

as a large variety of fatty acids accumulate transiently (Glissmann and

Conard, 1999). A study by Weber et al. (2001) indicated that the methano-

gens colonizing rice straw are less diverse than those inhabiting the soil.

Polysaccharolytic bacteria in rice soils constitute the first step in the degra-

dation process and eventually produce substrates needed for the production

of CH4. Distinct trends of multiple rate patterns for CH4 emission from

waterlogged soils have been shown in laboratory and field studies (Hou et al.,

2000). The first peak, between 20 and 40 days at 25 8C, probably originated

Figure 8 EVect of rice straw application on methane production in a sandy soil (Hou et al.,

2000).


from the decomposition of easily decomposable forms of C in the rice straw,

such as microbial products and polysaccharides (Watanabe et al., 1995). The

second change in rate of CH4 emission observed may have been associated

with the decomposition of structural components of the rice straw, such as

cellulose and lignin.

The e Vect of rice straw application on CH4 production potential is shown

in Fig. 8. Methane production in the treatment without rice straw supple-

ment occurred at a much lower rate during the whole period of incubation, in

which the highest production rate was less than 40 mg CH4 kg�1 soil day �1.

After the application of rice straw, the CH4 production rate increased

substantially. Both the quantity and the quality of added organic materials

influence CH4 emission from soils. Yagi and Minami (1991) showed that

while rice straw increased CH4 emission by a factor of 3.3, addition of rice

straw compost increased CH4 emission only slightly compared to the appli-

cation of mineral fertilizers. The extent and variability of observed methane

enhancements by organic additions are governed by several factors, the most

obvious being quantity. Schutz et al. (1989) established that CH4 emissions

from paddy rice progressively increased with increasing rice straw additions

from 3 to 12 t ha�1. Straw levels over 12 t ha�1 did not increase CH4 fluxes

further. Likewise, Wang et al. (1992) found increasing CH4 flux to be

proportional to rice straw input levels. A field study (Yagi and Minami,

1990) also showed that rice straw applied at rates of 6–9 t ha�1 enhanced

CH4 emission rates by 1.8–3.5 times. As reported by Sass et al. (1991) and


Watanabe et al. (1995), CH4 production was enhanced by the addition of

straw in flooded soil only.

Watanabe et al. (1995) proposed a simple straw rate response model to

predict cumulative CH4 emissions from a known rice straw application to

any soil:

Y ¼ k½aðEÞ=ð1 þ bðEÞe�cðEÞxÞ� þ Yð0Þ; ð8Þ

where Y is the fractional increase in CH4 emission relative to a chemical

fertilizer control, and x is the level of incorporated organic matter (t ha�1).

Adjustments to the coeYcients a, b, and c were added to account for

responses to temperature (E) and diVerences of soil type (k). Such modifica-

tions reflect observations that daily and seasonal CH4 fluxes are temperature

dependent (Parashar et al., 1991; Schutz et al., 1989; Yagi and Minami,

1991). Incubation studies have shown that large diVerences in CH4

production potential of soils are related to organic C content (Majumdar

et al., 1998).

The extent and rapidity with which added organic materials are decom-

posed depend greatly on chemical composition, including C:N ratio, lignin

and polyphenol content, and other critical compounds. Yadvinder-Singh

and van Cleemput (1998) reported that maximum methane (9980 mg g�1)

was emitted from soil amended with sugar beet leaves, and emissions of CH4

from wheat and rice straw were 4953 and 5030 mg g�1 in 40 days in a silty

clay soil under flooded conditions. The emissions of CH4 from composted

farmyard manure and poultry manure-amended soils were very low.

From an incubation experiment in a Chinese flooded rice soil, Hou et al.

(2000) reported that organic matter, added as rice straw and organic manure

(pig, chicken, and cattle manure), increased CH4 production rate significant-

ly. The results showed that organic manures had diVerent promoting eVects,

with pig manure increasing the CH4 production rate most, followed by rice

straw, chicken, and cattle manure. The CH4 production potential caused by

organic manures was closely related neither to the total C added to the

system nor to the C:N ratio of the materials. A significant linear relationship

between CH4 production and the logarithm of the number of zymogenic

bacteria was found, with an r value of 0.96. This finding suggests that the

number of zymogenic bacteria may be used as an index to predict CH4

production potential in flooded rice fields.

Bronson et al. (1997a) observed that organic matter additions as rice

straw (5.5 t ha�1, dry) or green manure (Sesbania rostrata, 12 t ha�1, wet)

stimulated methane flux several-fold. Rice straw resulted in higher CH4

emissions than GM. The GM plots showed highest CH4 fluxes in the first

2 weeks, but thereafter straw–amended emitted the most CH4. Green ma-

nure has more easily decomposable C than straw, although more C was

added as straw. Sesbania green manure, being easily degradable material,


required the lower activation energy by methanogens to use the substrate as

C source than wheat straw (Bhat and Beri, 2002). Rice straw applied before

the winter fallow period reduced CH4 emission by 11% compared with that

obtained from fields to which the same amount of rice straw was applied

during field preparation. Surface mulching of straw instead of incorporation

into the soil showed 12% less emission.

Composts consistently produced lower CH4 emissions than fresh green

manures or straws. Aerobic composting reduces readily decomposable car-

bon to CO2 instead of CH4 (Inoko, 1984) and also modifies the original

organic constituents to forms more resistant to subsequent degradation

(Watanabe et al., 1995). Consequently, when compost is incorporated into

anaerobic soils, less available carbon is present for methanogenesis. Howev-

er, the agricultural benefit derived from compost is maintained, especially if

composts are applied year after year (Inoko, 1984). Thus, composting pro-

vides a compatible option for adding organic materials to flooded soils

without substantially enhancing methane emissions. Following the same

principal, Miura (1995) found that fall rice straw incorporation or winter

mulching combined with spring incorporation significantly reduced CH4

emissions during the subsequent summer rice season.

Jain et al. (2000) reported that additions of organic manures and crop

residues enhanced CH4 emissions from rice fields. There were wide varia-

tions in CH4 emissions because of the variety of organic amendments.

Rice fields amended with biogas slurry emitted significantly less CH4 than

those amended with other organic amendments. They further reported that

CH4 emission rates were very low (between 16 and 40 kg CH4 ha�1 season�1)

when the field was flooded permanently. Application of organic manure

(FYM plus wheat straw) in combination with urea (1:1 N basis) enhanced

CH4 emission by 12–20% compared with fields treated with urea only. The

site in New Delhi represents one example of very low CH4 emissions from

rice fields. Emissions from other sites in northern India may be higher than

those in New Delhi, but they are still lower than in other rice growing

regions in India. Jain et al. (2000) reported that organic amendment inputs

promoted CH4 emissions, but total emission remained less than 25 kg CH4

ha�1. This finding contrasts with results from other network stations with

irrigated rice where total emissions generally exceeded 100 kg CH4 ha�1 after

manure application (Wassmann et al., 2000a). The low impact of organic

manure in the experiment in New Delhi could be related to high percolation

rates. Constant inflow of oxygen into the soil and downward discharge of

methanogenic substrate resulted in low CH4 production (Inubushi et al.,

1992; Yagi et al., 1994). Thus, emissions were very low even when organic

matter was applied. In other stations of the network, organic amendments

stimulated emissions during the first half of the season (Wassmann et al.,

2000b).


Ishibashi et al. (2001) studied the eVect of surface application of rice straw

in no-till rice on methane emission in three soils during rice growing season.

It was found that CH4 emissions from the no-tilled direct-seeded field on the

average were 21, 47, and 91% of that from the tilled transplanted field in

high-percolating site, low-percolating site, and extremely low-percolating

(4.4 mm day�1) site, respectively. Straw incorporation leads to significantly

more methane production than burning or removal. Over the long term,

however, incorporation may provide benefits through the accumulation of

C as soil organic matter.

B. NITROUS OXIDE

The biologically mediated reduction processes of nitrification and denitri-

fication are dominant sources of N2O generation in soils (Paul and Clark,

1989). Nitrous oxide is also produced to a much lesser extent by the abiotic

process of chemodenitrification (Bremner, 1997). Denitrification processes

can terminate with N2O, or, more commonly, N2O is further reduced to N2

gas. Conditions that promote N2O emissions over N2 are high NO�3 levels

and/or increasing O�2 , while increasing organic carbon levels tend to favor

N2 production (Firestone, 1982). Nitrous oxide emissions from rice fields

occur as a result of nitrification–denitrification during periods of alternating

wetting and drying. Emissions are usually small in irrigated rice systems with

good water control and small to moderate inputs of fresh organic material

(OM) (Bronson et al., 1997a,b).

Bronson et al. (1997a) reported that organic amendments, particularly

rice straw, helped in reducing N2O emissions. In the flooded rice soil, straw

addition possibly stimulates O2 consumption in the aerobic soil layer and in

the rhizosphere, resulting in smaller zones in which nitrification can occur.

Enhanced immobilization of fertilizer N with straw would result in less NH4

available for nitrification–denitrification. Additionally, the high CH4 con-

centration in straw-amended soil could inhibit nitrification (McCarty and

Bremner, 1991). Methane emissions ranged from 3 to 557 kg CH4 ha�1 with

an average of 182 kg CH4 ha�1.

Few measurements have been published for N2O emissions from flooded

rice soils amended with organic materials. The existing information indicates

that N2O emissions from flooded soils with organic additions are similar to

or less than soils receiving chemical fertilizers, indicating that organic

amendments do not appear to influence N2O emissions very much.

Most information on N2O emissions from rice soils focuses on water

management and nitrogen fertilizers as controlling variables (Cai et al.,

1997, 1999). A trade-oV relationship between CH4 and N2O, i.e., conditions

that favor CH4 production suppress N2O and vice versa, is also well


recognized (Mosier et al., 1998a). So, while organic amendments seemingly

have no impact on N2O emissions from flooded soils, management practices

before or after rice may produce a significant e Vect.

Aulakh et al. (2001) showed that denitrification is a significant N loss

process under wetland rice amounting to 33% of the recommended dose of

120 kg N ha�1 on a permeable sandy loam soil. Integrated management

of wheat straw (6 t ha �1) and GM (20 t ha�1 supplying 88 kg N ha�1) and

32 kg N ha�1 as urea fertilizer N significantly reduced cumulative gaseous

N losses to 51.6 kg N ha�1 as compared to 58.2 kg N ha �1 for 120 kg N ha�1

alone. The gaseous losses under wheat were 0.6–2% of the applied fertilizer

N. Interplay between the availability of NO3 and organic C largely con-

trolled denitrification and N2O fluxes in flooded summer-grown rice, where-

as temperature and soil aeration status were the primary regulators of the

nitrification–denitrification processes and gaseous N losses during winter

grown upland wheat. The irrigated rice–wheat system is a significant source

of N2O, as it emits around 15 kg N2O–N ha�1 year �1.

The quantity of organic additions may also aVect N2O emissions. In one

of the few studies looking at the impact of organic materials on N2O,

Bronson et al. (1997a) suggested that organic additions to flooded soils

stimulated oxygen depletion to the point of inhibiting nitrification and

thereby N2O emissions. From this, one could hypothesize that increased

oxygen depletion with more organic material and consequently N2O emis-

sions would decline even more. Burning of crop residues also contributes to

the global N2O budget. For each ton of crop residue that is burned, 40 g

N2O is emitted (Grace et al., 2003).

C. MITIGATION STRATEGIES

The objective of reducing CH4 emissions must be combined with

improvements associated with increased yields and straw recycling; adhering

to CH4 emission quotas might increasingly aVect rice production practices.

Possible mitigation options for reducing methane emission from rice fields

include reduced length of flooding, temporary drainage (Wassmann et al.,

2000b), rice cultivar selection, kind and application mode of mineral fertili-

zers, and soil and crop management strategies to achieve a high acceptance

(Mosier et al., 1998a,b; Neue, 1993; Yagi et al., 1994). CH4 emission was

reduced significantly by early incorporation of rice straw during the fallow

period, adding to the agronomic benefit of this practice.

Bronson et al. (1997b) recorded seasonal N2O emissions during a fallow

period as high as 172 and 183 mg N m�2, where rice straw and a green

manure had been incorporated the previous season, respectively. Such emis-

sions might be considered maximums because assimilation of nitrogen


mineralized from organic additions by fallow weed species or upland crops

helps to retain N within the system and minimize N2O emissions (Buresh

et al., 1993). Given the influence of soil type, climate, and organic additions

on CH4 and N2O emissions, comprehensive studies are needed to quantify

more thoroughly the trade-oV eVects between CH4 and N2O during an

annual cycle within rice-based cropping systems.

Water management is an important management factor when trying to

minimize CH4 or N2O emissions from rice-based cropping systems. Mid-

season drainage, which originally was developed in Japan as a means to

supply oxygen to rice roots, is also very eVective in reducing seasonal CH4

emissions from rice (Jain et al., 2000; Yagi and Minami, 1990). Despite

projected decreases in CH4 emission by such methods, aerobic soil condi-

tions during fallow and upland cropping intervals between rice crops

enhance N2O emissions generated by nitrification of mineralized organic

N and subsequent denitrification of NO3 when flooding is reestablished

(Bronson and Mosier, 1993). Unintentional mid-season drainage is possible

in many rice cropping systems of South and Southeast Asia where light

textured soils or water distribution and management problems influence

the ability of farmers to keep their soils flooded (Jain et al., 2000). Site-

specific adaptations will be required for an optimum eVect, considering rice

yields, water consumption, and CH4 emissions.

In summary, methane emissions can be reduced significantly by adopting

the following mitigation practices: water management through intermittent

irrigation or drainage, the use of composted organic manures instead of

fresh manure, allowing pre-decomposition of crop residues under aerobic

conditions before rice planting, and the selection of suitable cultivars that

emit less CH4. It appears that composted organic additions are the best way

to meet sustainable agriculture goals while minimizing greenhouse gas emis-

sions from paddy rice. Adding crop residues or green manures in suYcient

quantities to increase soil organic matter levels or replenish deficient nutri-

ents for flooded rice exacerbates N2O emissions to unacceptable levels. Of

course, it is important to establish that CH4 and N2O emissions arising from

the composting process do not exceed emissions during rice cultivation.

Direct dry seeding of rice as well as other crops following rice into surface

residues will reduce N2O and CH4 emissions. Grace et al. (2003) suggested

three feasible, cost-eVective agronomic interventions that would have an

immediate eVect by reducing greenhouse gases production in the rice–

wheat cropping systems and that will no doubt be applicable to other rice-

based cropping systems in the tropics: (1) a reduction in residue burning, (2) a

reduction in flood irrigation frequency for rice, and (3) the use of minimum

or no tillage for upland crops following rice (e.g., wheat or maize). It was

estimated that Adopting these measures would result in total savings in CO2

equivalent emissions of 1680 kg ha�1 year�1.


XI. AGRONOMIC RESPONSES TO CROPRESIDUE MANAGEMENT

Recycling of crop residues is an essential component in achieving sustain-

ability in crop production systems. Since crops respond diVerently to the

application of diVerent organic materials to soil, evaluation of crop residues

in terms of fertilizer eVect is complicated by the variable nutrient contents of

the materials and the host of other eVects (as already discussed) these may

have on crops and soils. In some cases, straw incorporation can actually

lead to a reduction in crop yields due to release of phytotoxic compounds

during decomposition and immobilization of soil and fertilizer N, causing

N deficiency in the crop planted immediately after straw incorporation. In

many studies in which crop residues proved to be superior to inorganic

fertilizers, the eVect may be due only to better supply of nutrients from

organic matter. It is, in fact, impossible to monitor all the eVects of organic

matter on nutrient availability. Evaluation of the fertilizer eVects of an

organic resource (e.g., for N) requires that the material be assessed both at

an equal N application and on equal mass (or carbon) application, prefera-

bly in each case over a range of application rates. The eVect of residue

incorporation on succeeding crops depends on the amount of residues and

the time and method of incorporation. Though the long-term eVects of crop

residue incorporation are generally expected to be beneficial in terms of

increasing soil organic matter content, availability of nutrients, cation ex-

change capacity, and microbial, the time scale for these improvements is

generally long (e.g., >5 years). However, improvements in soil conditions do

not always flow to yields. Thus, despite the very large body of literature on

the recycling of crop residues, there exists very little information that enables

proper evaluation of organic residues for their fertilizer value.

A. RICE–WHEAT CROPPING SYSTEM

1. Effect on Crop Yields

Crop residue management as practiced in the rice–wheat cropping system

is of three types (1) wheat straw management in rice and its residual eVect in

following wheat, (2) rice straw management in wheat and its residual eVect

in following rice, and (3) wheat straw management in rice and rice straw

management in wheat (cumulative eVect). In several studies, incorporation

of wheat straw into the soil had pronounced but variable eVects on the

growth and yield of subsequent rice (Table XV). For example, in a field

experiment on clay loam soil, rice yields under removal or incorporation of

Table XV

E Vect of Wheat Straw Management on Grain Yield (t ha �1) of Rice and Its Residual EVect on the

Grain Yield of the Following Wheat in Rice–Wheat Cropping System in India

Experimental

details Crop

Wheat straw management in rice

Straw

removed

Straw

burned

Straw

incorporated

Straw þgreen

manure Reference

Haryana, 3-year

study, clay loam

soil

Rice 6.97 7.23 7.01 a7.17 Agrawal et al.

(1995)

Wheat 4.65 4.84 4.43 4.52

Punjab, 12-year

study, loamy

sand soil

Rice 5.74 — 5.37 5.99 Yadvinder-

Singh

et al.

(2004a)

Wheat 4.41 — 4.32 4.44

Madhya Pradesh,

2-year study

Rice 2.21 — 2.82 — Pandey et al.

(1985)

Wheat 4.48 — 5.59 —

West Bengal, acid

silty clay loam

soil, 2-year

study, Wheat

straw

incorporated 10

days before rice

planting

Rice 3.74 — 4.17 — Sharma and

Mittra

(1992)

Wheat 1.80 — 2.00 —

Uttar Pradesh, clay

loam soil (pH

8.6), wheat straw

(10 t ha�1)

incorporated 30

days before rice

transplanting

Rice

(a) 100%

NPK

4.10 — 4.45 Rajput

(1995)

(b) 50%

NPK

— — 4.08

Wheat 2.29 — 2.72 —

a30 kg ha�1 extra fertilizer N.


wheat straw were similar (Agrawal et al., 1995). On a loamy sand soil,

incorporation of wheat straw reduced rice yield by 7% (average for 12

years) compared to when it was removed (Yadvinder-Singh et al., 2004a).

Incorporation of wheat straw into an acidic clay loam soil significantly

increased the grain yield of rice, with significant residual e Vect in the suc-

ceeding wheat crop (Sharma and Mitra, 1992). Similar observations were

also made by Pandey et al. (1985) and Rajput (1995). A beneficial e Vect of

wheat straw on the grain yield of rice even in the first year of study has been

reported by many workers (Alam and Azmi, 1989; Zia et al., 1992).

Table XVI

EVect of Time of Rice Straw and Fertilizer N (120 kg N ha�1) Management in Wheat and Its

Residual EVect on the Following Rice in Rice–Wheat Cropping System in Indiaa

Experiment 1:

Bijay-Singh et al. (2001)

Experiment 2:

Yadvinder-Singh

et al. (2004b)

Grain yield (t ha�1)15N

recovery

(%)

15N

losses

(%)

Grain yield (t ha�1)

Treatment Wheat Rice

Wheat

(1993–2000)

Rice

(1994–1999)

Straw removed 5.06a 4.90a 40.9a 31.7b 4.94a 6.19a

Straw burned 5.11a 5.13a 40.8a 31.3b 5.10a 6.25a

Straw incorporated

(40 DBSb)

4.89a 4.87a 36.1ab 36.6ab 5.17a 6.34a

Straw incorporated

(20 DBS)

5.00a 4.97a 34.4bc 34.0ab 5.22a 6.29a

Straw incorporated

(20 DBS) and

25% N applied at

incorporation

4.79a 5.02a 30.4c 45.2a 4.95a 6.33a

Straw incorporated

(10 DBS)

— — — — 4.97a 6.29a

aIn a column, figures followed by a common letter are not significantly different.bDBS, days before sowing of wheat.


In a long-term experiment (1984–94) in the Indo-Gangetic plains of India

(Rattan et al., 1996), both rice and wheat had a higher yield with inorganic

fertilizers than any of the crop residue management treatment in the first

year. After 2 to 3 years, the combination of wheat straw and inorganic

fertilizers produced yields similar to those with inorganic fertilizers. It was

after 3 to 4 years that the combined use of inorganic fertilizers and wheat

straw started giving higher yields than inorganic fertilizer treatment.

Interestingly, green manuring in conjunction with wheat straw helped to

mitigate the adverse eVect of wheat straw in rice (Yadvinder-Singh et al.,

2004a). Similarly, Aulakh et al. (2001) reported that compared to application

of 120 kg N ha�1 through urea alone, rice production was greater with wheat

straw incorporation when an average of 86 kg N ha�1 of a prescribed 120 kg

N ha�1 dose was applied as green manure and the balance as urea N. Green

manure and incorporation of wheat straw in rice–wheat cropping systems has

the potential to increase soil organic matter while maintaining high yields.

In a field experiment conducted using 15N-labeled urea, grain yields of

wheat and the following rice were not adversely aVected by incorporation

of rice straw at least 20 days before sowing of wheat (Table XVI). In

another 7-year study, compared with residue removal or residue burning,


incorporation of rice residue 10 to 40 days before seeding wheat did not

show any adverse eVect on wheat yield (Table XVI). The application of 25%

of fertilizer N as starter N at the time of residue incorporation showed some

depression in wheat yield in all years compared with no starter N under

20-day incorporation treatment, although the diVerences were not significant.

It was suggested that N applied concurrently with straw incorporation gets

immobilized and does not remineralize easily. In this study, annual additions

of 40–50 kg N ha�1 through rice residue for 7 years did not influence grain

yield of wheat, as the recommended split application of 120 kg N ha�1 (one

half drilled at sowing and the remaining half top dressed at 21–25 days after

sowing) was already applied to all the treatments in wheat.

There also exist several other reports showing similar rice and wheat

yields under diVerent residual management practices (burning, removal, or

incorporation) (Singh et al., 1996; Walia et al., 1995). Kavinandan et al.

(1987) reported that incorporation of wheat straw 10 days before rice

transplanting and rice straw 3 weeks before wheat sowing gave 0.25 and

0.42 t ha�1 higher yields of rice and wheat, respectively, over incorporation

of wheat straw at 3 days before rice transplanting and rice straw at 2 weeks

before wheat sowing; the diVerences, however, were not significant.

In a field experiment in Faislabad (Pakistan), incorporation of rice straw

into the soil produced significantly higher yields of wheat (3.51 t ha�1)

compared to when rice straw was removed (2.91 t ha�1) (Salim, 1995).

Singh et al. (1996) reported that in Pantnagar (India), incorporation of rice

straw 3 weeks before wheat sowing significantly increased wheat yields on a

clay loam but not on a sandy loam soil. In the Himachal Pradesh state of

India, however, incorporation of rice straw at 30 days before wheat sowing

produced significantly lower wheat yields than removal or burning of straw

in the first 2 years, remained at par in the third year, and produced a

significantly higher yield and N uptake from the fourth crop onward

(Table XVII) (Verma and Bhagat, 1992). The causes of lower yields with

straw incorporation, particularly in the initial period of study, were immo-

bilization of N and slow decomposition of rice straw at low temperatures

during wheat growth. However, with the advancement of time (fourth crop

and onward), the previously added rice straw might have decomposed,

resulting in significantly higher wheat yield and N uptake under this treat-

ment. Straw mulch increased the wheat yield and N uptake significantly over

straw incorporation during the 2 years, which might be due to more favor-

able soil moisture regime, regulation of soil temperature, control of weeds,

and an increase in the microbiological activity. Yield and N uptake of

following rice under straw burn treatment did not vary significantly from

the straw removal during the entire study period. Straw mulch produced the

lower yield and N uptake of rice as compared to other straw management

treatments without N application during the first two rice crops but had a

Table XVII

EVect of Rice Straw Management on Grain Yield (t ha�1) of Wheat and Its Residual EVect on

Grain Yield of the Following Rice in Rice–Wheat Cropping System in India

Crop residue management

Experimental

details Crop

Straw

removed

Straw

burned

Straw

incorporated Reference

Himachal

Pradesh, data

averaged for 4

years, acidic

clay loam soil,

rice straw

chopped and

incorporated 4

weeks before

wheat sowing

Wheat 2.76 — 2.79 Sharma et al.

(1985, 1987)

Rice 2.37 — 2.47

Himachal

Pradesh, 5-

year study,

acidic clay

loam soil, rice

straw chopped

and

incorporated 4

weeks before

wheat sowing

Wheat

(1984–87)

2.6 2.6 2.2 Verma and

Bhagat

(1992)

Rice 3.7 3.6 3.7

Wheat

(1987–89)

2.4 2.4 2.4

Rice 3.8 3.7 4.0

Punjab, sandy

loam soil. Data

are reported

for the fourth

cropping cycle

Wheat — 4.88 5.18 V. Beri and B. S.

Sidhu (personal

communication)

Rice — 6.05 6.00


significant residual eVect from the third crop onward. Straw incorporation in

wheat increased rice yield by 38% in the third crop and 45% in the fourth

crop over straw removal.

Sharma et al. (1985, 1987) observed no significant eVect of straw incor-

poration on the grain yield of wheat and on the following rice (Table XVII).

Pathak and Sarkar (1997) observed that at recommended fertilizer N (120

kg ha�1), rice straw incorporation produced lower rice yields than straw

removal.

In a long-term field experiment in Ludhiana in northwestern India, Beri

et al. (1995) found that incorporation of rice and wheat residues into soil

resulted in significantly lower yields than removal or burning of residues

(Table XVIII). It was suggested that the depression in rice yield was not due

Table XVIII

EVect of Wheat Straw Incorporation in Rice and Rice Straw Incorporation in Wheat on Crop

Yields in Rice–Wheat Cropping System

Reference

and country

Type of

residue and

soil type

Duration

(years)

Residue

management

practice

Grain yield

(t ha�1)

Rice Wheat

Beri et al.

(1995),

India

Rice straw

in wheat and

wheat straw

in rice,

sandy loam

11 Removed 5.50a 4.14a

Burned 5.65a 4.26a

Incorporated 4.63b 3.97b

Brar et al.

(1998),

India

Rice straw

in wheat and

wheat straw

in rice,

loamy sand

3 Removed 3.19a 4.25a

Burned 3.66a 4.41a

Incorporated 3.40a 4.01a

Sarkar

(1997),

India

Rice straw

in wheat and

wheat straw

in rice,

sandy clay loam

2 Removed 4.68b 3.50b

Half residues

incorporated

5.75a 4.25a

Full residues

incorporated

4.80b 3.75b

Table XIX

EVect of Wheat Straw Incorporation in Rice and Rice Straw Incorporation in Wheat on Crop

Yields in Rice–Wheat Cropping System in India

Residue

management

Grain yield of

wheat (t ha�1)

Grain yield of

rice (t ha�1)

1993–94 1994–95 1995–96 1996–97 1994 1995 1996

Removed 4.28 3.80 3.93 4.30 7.02 6.20 7.28

Burned 4.37 4.28 3.44 4.47 7.05 6.10 7.23

Incorporated 3.58 3.65 3.75 4.92 7.66 6.74 7.67

LSD ( p ¼ 0.05) 0.56 ns 0.29 0.28 0.30 0.26 0.31

From Dhiman et al. (2000).


to N immobilization. Dhiman et al. (2000) reported that on a clay loam

soil, rice yields increased significantly with the incorporation of residues of

both rice and wheat as compared to burning or removal (Table XIX),

but wheat yields decreased with residue incorporation, particularly in the

initial 2 years of the study. In the fourth cropping season, wheat yield was

higher in the residue-incorporated treatment than residue burning or re-

moval treatments. The average productivity during the 4-year period was


11.5 t ha�1 year�1 when residues of both the crops were incorporated, and it

was higher by about 0.61 t ha�1 year�1 than from burning and removal of

residues. In a calcareous sandy loam soil in Bihar (India), Prasad and Sinha

(1995b) studied the eVect of incorporation of crop residues after chop-

ping (2 cm size), soaking in 2% urea solution, and then inoculating with

cellulytic culture (Aspergillus spp.) to hasten the decomposition. At recom-

mended fertilizer levels, incorporation of crop residues compared to removal

increased mean yields of wheat and rice by 7.2 and 8.5%, respectively.

Prasad et al. (1999) concluded that residues of both rice and wheat

can safely be incorporated without any detrimental eVects on the crops of

rice and wheat grown immediately after incorporation. Rice straw was in-

corporated 32–42 days before sowing of wheat, and wheat straw was

incorporated 65–76 days before rice planting. Wheat yields were slightly

reduced with rice straw incorporation in the first year of study (3.7 versus

4.1 t ha�1).

In microplot experiments with early rice-late rice–wheat rotation in

China, fertilizer utilization by rice was 82.6 and 47.7% on a sandy loam

soil and 75.5 and 51.8% on a light clay soil for rice straw (C:N ¼ 89:1) plus

N fertilizer and N fertilizer alone, respectively. The grain yield from the total

rotation was also higher under rice straw plus fertilizer N than under

fertilizer N alone treatment (Xu, 1984).

In a field experiment at Yanco (Australia), Bacon et al. (1989) observed

that increasing quantities of rice stubble retained on the soil surface

increased soil NO�3 –N concentrations by 46% and wheat on these plots

had a 37% increase in grain yield and 29% increase in N uptake. Bacon

and Cooper (1985) obtained higher yields from wheat direct-drilled into

undisturbed rice stubble plots over where stubble was incorporated at sow-

ing. The high yields were due to increased availability of both soil and

fertilizer N. Delayed stubble incorporation until wheat sowing caused great-

er yield depression due to N immobilization than when stubble was

incorporated early after rice harvest. It is also possible that the wide range

of phytotoxins released during stubble decomposition directly inhibited

plant growth.

2. Fertilizer Management in Straw-Amended Soils

Incorporation of cereal residues into the soil generally causes rapid

immobilization of soil and fertilizer N during the early stages of decomposi-

tion, resulting in N deficiency in the succeeding crop. Proper management

of fertilizer N may lead to reduced rates of N immobilization by crop

residues, thus increasing the eYciency of N usage. The improved fertilizer

management practices may include optimum method, time, and rate of


fertilizer N application, which may diVer from that when residues are

removed or burned. One obvious solution to the N immobilization problem

would be to place the fertilizer below the C-enriched surface soil layer for-

med due to surface placement of crop residues (Doran and Smith, 1987).

Yadvinder-Singh et al. (1994b) concluded that on soils amended with crop

residues, band placement of urea prills and deep placement of large urea

granules would lead to significantly lower amounts of fertilizer N immobili-

zation than mixed application of commercial urea granules. The limited

contact between fertilizer N and the decomposing microorganisms was

the main reason for the low rates of N immobilization with large urea

granules. The adverse eVect of N immobilization on crop growth can also

be avoided by applying additional fertilizer N or by delaying planting.

Another fertilizer management option may be to apply a part N fertilizer

at the time of straw incorporation to enhance decomposition of residues or

to allow suYcient time for the decomposition of crop residues before the

planting of next crop. Thakur and Pandya (1997) reported that precondi-

tioning urea with rice straw and soil in the ratio 1:3:1 (urea:straw:soil) was

significantly superior to urea alone in respect of grain yield and N uptake of

wheat.

a. Fertilizer N Rate. The target of eYcient nutrient management is to

maintain stable nutrient cycling in the long term while supplying suYcient

nutrients to crops in the short term. From a 3-year study on a rice–wheat

cropping system in Uttar Pradesh (India), Misra et al. (1996) reported that

total grain yields of rice and wheat increased due to the incorporation of

both the straws, with an extra dose of 20 kg N ha�1 applied at straw

incorporation over burning and straw incorporation without an extra

N dose. Singh and Sharma (2000) showed that application of 20 kg extra

fertilizer N as compared to recommended N levels of 120 kg N ha�1 to wheat

on straw removal plots gave a significantly higher grain yield and nutrient

uptake on straw-amended plots. In another study, the application of 30 kg

extra N ha�1 compared to the recommended fertilizer increased the rice yield

only slightly (Table XV) (Agrawal et al., 1995).

Sharma and Mitra (1992) found that incorporation of rice straw 15–20

days before wheat sowing decreased the grain yield, while incorporation

of wheat straw 15–20 days before transplanting increased rice yields. How-

ever, application of 15 kg N ha�1 as a starter dose with straw application

increased the yields of both rice and wheat crops. From a 2-year study, Brar

et al. (2000) reported that application of 40 kg N ha�1 at rice straw incor-

poration in addition to the recommended N fertilizer dose (120 kg ha�1) in

two equal splits at sowing and 3 weeks after sowing produced a significantly

higher wheat yield (4.94 versus 5.31 t ha�1) and N uptake (101 versus 116 kg

N ha�1) than application of recommended N fertilizer. Application of


irrigation at straw incorporation to enhance straw decomposition further

increased the wheat yield by 0.2 t ha�1 compared to no irrigation.

Narang et al. (1999) reported that wheat responded significantly to

the application of 160 kg N ha�1 during the first 2 years of straw incor-

poration (both rice and wheat) as compared to the recommended N rate of

120 kg N ha�1 when residues are removed. In the third year of the study, a

significant response to fertilizer N was observed up to 120 kg N ha�1 in

straw-amended wheat plots. Irrespective of the residue load, response of rice

to fertilizer N was also observed up to 160 kg N ha�1 in the first year of

study, but the grain yield increased significantly up to 120 kg N ha�1 in the

second year of study. Results from this study suggested the application of

25–30 kg ha�1 higher fertilizer N doses to rice in wheat on straw-amended

fields during initial 1–2 years after residue incorporation compared to the

rates recommended for straw removal fields. Later on, recommended ferti-

lizers may be needed to achieve higher yield productivity of rice–wheat

systems.

Singh and Sharma (2000) reported that incorporation of wheat residue

(40–50 days before rice transplanting) with no N or at low N rates resulted

in an adverse eVect on crop yields of rice and wheat. When adequate

N (180 kg N ha�1) was applied, residue incorporation increased produc-

tivity by 0.4–0.7 t ha�1 and nutrient uptake by 40–65 kg ha�1 over remo-

val or burning of residues. Residue incorporation increased eYciency of

applied fertilizer N in rice and had a significant residual eVect in following

wheat.

Thakur and Singh (1987) estimated optimum N rates of 115 and 140 kg

ha�1 for rice on fields without and with wheat straw (5 t ha�1) incorporation.

Thus, higher fertilizer N may be required for crops grown on soils amended

with crop residues to get maximum benefits. Jha et al. (1992) obtained the

highest grain yield of rice when rice straw and green manure (mungbean)

along with 60 kg N ha�1 was applied in three equal splits compared to rice

straw or green manure applied alone. Grain yield of following wheat was

also higher when rice straw and green manure were incorporated.

In a 2-year field experiment in Modipuram (northwestern India), incor-

poration of half of the crop residues along with recommended fertilizers

consistently produced higher yields of both rice and wheat than incorpora-

tion of full residues or removal of residues (Sarkar, 1997). The rice grain

yield with half residue was 5.80 t ha�1 and wheat yield was 4.38 t ha�1

compared to 4.70 t ha�1 of rice and 3.71 t ha�1 of wheat for no straw

treatment.

In a rice–wheat rotation on a calcareous soil, application of crop resi-

dues along with FYM gave the highest yield followed by FYM, crop

residues, and no amendment (Prasad and Sinha, 1995b). The grain yield

recorded with 50% NPK plus FYM plus crop residues was higher than that


with the 100% recommended dose of NPK alone, indicating that FYM

plus crop residues substituted 50% of NPK in each of wheat and rice

production.

Malik and Jaiswal (1993) reported that application of 58 kg N ha�1

as urea super granules plus 28 kg N ha�1 as wheat straw produced a

significantly higher rice yield than the recommended practice of applying

87 kg N ha�1 as commercial urea granules. The grain yield of the following

wheat was not aVected by the previous residue management practices in the

rice crop. Under dryland conditions in Uttar Pradesh (India), Singh and

Singh (1995) found that incorporating rice straw (10 t ha�1; C:N ¼ 75.5) 3

weeks before planting rice integrated with 50% of recommended NPK

fertilizers produced the highest rice yield and improved the profitability

of the system. Rajput (1995) found that incorporation of wheat straw

(10 t ha�1) resulted in up to 50% savings in the recommended NPK fertilizers

(60 kg N þ 13.1 kg P þ 25 kg K ha�1). A higher yield potential of rice was

achieved when wheat straw was applied along with recommended NPK

fertilizers (Table XV). The residual eVects of wheat straw on the following

wheat were also substantial. In another study (Kundu et al., 1994), how-

ever, wheat straw applied to rice had little eVect on the grain yield of the

succeeding wheat crop. In fact, several other long-term studies also showed

that it is not possible to substitute a part of fertilizer N requirement of rice

with N added through wheat straw (Table XX).

Katyal et al. (1998) reported results from long-term field experiments

conducted at five sites in India during 1983–1991. At Kanpur (U.P.,

India), 50% recommended NPK fertilizers plus 50% N through wheat straw

in rice followed by 100% recommended NPK fertilizers in wheat stabilized

the yields of rice and wheat. However, at the three other locations it was not

possible to substitute a part of fertilizer N (25% N) with crop residues

(Table XX). In West Bengal (India), the application of semi-decomposed

wheat straw (0.78%N, dry weight basis) at 3 t ha�1 along with 25 or 50% of

the recommended NPK fertilizers (80 kg N þ 26 kg P þ 33 kg K ha�1) 30

days before sowing of rainfed rice resulted in the highest yields. In a rice–

wheat cropping system, 5 t wheat straw ha�1 was incorporated 3–10 days

before rice transplanting and 5 t rice straw ha�1 was incorporated 2–3 weeks

before seeding wheat (Kavinandan et al., 1987). Most of the studies on rice

straw management were conducted at recommended N rates, and thus it is

diYcult to quantify the contribution of rice straw in supplying N to plants in

the cropping system due to the fact that the amount of N fertilizer applied

exceeds that for optimum yields.

b. Time and Method of Fertilizer Application. Sharma and Bali

(1998) showed that application of 30 kg N ha�1 at straw incorporation

and remaining 90 kg N ha�1 top dressed during wheat growth soil produced

Table XX

EVect of Wheat Straw and Fertilizer Management in Rice and Their Residual EVect on Wheat in Rice–Wheat Cropping System in India

Location Year/site

Crop Treatment

Reference

Rice 100% NPK

50% NPK þ50% N as WSa

75% NPK þ25% N as WS

LSD

( p ¼ 0.05)

Wheat 100% NPK 100% NPK 75% NPK

Uttar Pradesh (Faizabad) 1984–87

1987–93

Rice 3.9 3.5 3.3 0.28 Kumar and Yadav (1995)

Wheat 3.4 3.0 3.1 NS

Rice 4.5 4.3 4.4 NS

Wheat 3.4 3.4 3.1 0.29

Madhya Pradesh, 3-year study Jabalpur

Raipur

Rice 4.14 3.37 3.27 0.24 Dubey et al. (1997)

Wheat 2.94 2.75 2.57 0.13

Rice 4.65 4.20 4.19 0.18

Wheat 1.77 1.84 1.78 0.13

Punjab (Ludhiana) 8-year study Rice 6.37 5.25 5.97 — Katyal et al. (1998)

Wheat 4.55 4.33 3.93 —

Uttar Pradesh (Kanpur) 8-year study Rice 3.98 3.31 3.54 — Katyal et al. (1998)

Wheat 4.63 4.48 4.11 —

Madhya Pradesh (Jabalpur) 8-year study Rice 4.72 3.89 4.03 — Katyal et al. (1998)

Wheat 2.68 2.61 2.08 —

West Bengal (Kalyani) 8-year study Rice 3.50 3.48 3.45 — Katyal et al. (1998)

Wheat 2.65 3.06 2.55 —

aWS, wheat straw.

CR

OP

RE

SID

UE

MA

NA

GE

ME

NT

363


a significantly higher yield (2.0 versus 3.12 t ha�1) than that from applying

120 kg N ha�1 in two equal split doses (half drilled at sowing and half top

dressed at 1 month after sowing) on a silty clay loam. Incorporation of rice

straw reduced the wheat yield over straw removal at recommended N level

through its beneficial eVect on residue decomposition. In field experiments

using N-labeled urea, application of a part of the recommended N (25%) at

the time of straw incorporation (to hasten decomposition of straw) led to

large N losses and low wheat yield (Bijay-Singh et al., 2001). Recovery

of 15N by wheat was maximum (41.8%) when rice straw was removed or

burned and the minimum (30.4%) when 30 kg of 120 kg N ha�1 fertilizer was

applied along with straw incorporation at 20 days before wheat sowing

(Table XVI).

From long-term field experiments (1990–96) in three locations in India,

Yadav (1997) reported that when 20 kg N ha�1 was applied at the time of

incorporation and the remainder of the recommended N (100 kg or 120 kg

N ha�1 for rice and 120 kg N ha�1 for wheat) during the growth period,

grain yields of rice and wheat were significantly lower than those obtained

with other N scheduling practices included in the study. However, when an

extra 20 kg N ha�1 was applied at the time of residue incorporation over and

above the recommended N dose at one location (Jammu and Kashmir) or

when N levels were enhanced by 20 kg N ha�1 over the recommended rates

at the other site (Uttar Pradesh), the crop yields were the highest. On average

of six crop cycles, these practices produced an additional 150 and 510 kg

grain ha�1 at the first site and 570 and 810 kg grain ha�1 at the second site in

rice and wheat crops, respectively. Jiang et al. (1998) recommended the

application of 105 kg N ha�1 in three equal splits (sowing, tillering, and

stem elongation) on plots amended with wheat straw (3 t ha�1). Split

application of fertilizer N increased wheat yield by 43% compared to fertil-

izer N applied in single or two splits. Bacon and Cooper (1985) found that

application of N to wheat at tillering or stem elongation, compared to at

sowing, significantly increased soil mineral N content at least until anthesis.

Wheat on the stubble-incorporated plots did not respond significantly to

N application at sowing or stem elongation, while N application at any time

more than doubled wheat productivity on the stubble-retention plots. Delay-

ing N application until tillering significantly increased yields on stubble

incorporation, stubble retention and burned plus till plots. While only 70

kg N ha�1 was required for maximum yield at stem elongation, 140 kg

N ha�1 was necessary at sowing. It was concluded that stubble and fertilizer

management techniques could be manipulated in order to regulate soil

mineral N status, which in turn determined plant N uptake and yield of

wheat. In Kanto (Japan), with continuous application of rice and wheat

straw, rice yields were low during the initial 3–4 years, but the yield of rice

increased dramatically by continuous application of rice straw (Roppongi,


1987). The adverse eVects were alleviated when the basal application of

N was increased.

B. RICE–RICE CROPPING SYSTEM

1. Effect on Crop Yields

Rice–rice is a dominant cropping system in Bangladesh, China, Philippines,

Korea, Japan, Indonesia, and eastern and southern parts of India. Studies

with rice straw in rice–rice systems in a number of countries demonstrate

widely varying response to straw incorporation. In sharp contrast to rice–

wheat cropping systems, the majority of the studies on rice–rice cropping

systems show that incorporation of rice residues enhances rice yield and

N use eYciency. Ismunadji (1978) from Indonesia reported that incorpora-

tion of 10 t rice straw ha�1 increased grain yield of rice to 2.6 t ha�1 from

2.2 t ha�1 in the untreated control. Burning of rice straw produced a rice

yield almost similar to that obtained with its incorporation. Experiments in

the Philippines showed that straw incorporation for more than 6 years

increased the rice yield by 0.4 to 0.7 t ha�1 compared with fields that used

to receive chemical fertilizers and where rice straw was either removed or

burned (Table XXI) (Ponnamperuma, 1984). On the unfertilized plot, when

the soil was a P-deficient acid clay, the extra yield amounted to 23%

(Ponnamperuma, 1984). The beneficial eVects of rice straw incorporation

were, however, small during the initial 2–6 years of the study. Sharma et al.

(1989), however, observed no significant eVect of rice straw incorporation

(6 t ha�1) on grain yield of rice in a short-term study in the Philippines. In a

long-term study in Japan, Beaton et al. (1992) noted no beneficial eVects of

Table XXI

EVect of Long-Term Management of Rice Straw on Crop Yields (t ha�1) in Rice–Rice

Cropping System

Straw

treatment

Experiment 1 Experiment 2 Experiment 3

2–5

years

6–10

years Mean

2–6

years

7–10

years Mean

1966–

1980

1981–

1985

1986–

1989

Removed 9.7 7.1 8.3 9.0 7.3 8.2 7.53 8.03 7.74

Incorporated 9.9 7.7 8.7 9.3 8.0 8.7 7.54 8.17 8.44

Burned 9.9 7.1 8.3 — — — — — —

Experiment 1 and Experiment 2 from Ponnamperuma (1984); Experiment 3 from Beaton et al.

(1993).


incorporation of rice straw over straw removal in the initial 15 years of the

study (Table XXI), but in the final 4 years (20–23 years), an average increase

of 0.70 t ha�1 in rice grain yield was observed with rice straw incorporation

over removal.

The length of the period allowed for decomposition of crop residues

before the sowing/planting of the next crop aVects the agronomic res-

ponse to applied residues. Houng and Hwa (1975) found that when rice

straw was allowed to decompose for 4 or more weeks before sowing, there

was no adverse e Vect on germination of rice seeds. In many other studies,

crop residues were allowed to decompose for 2 or more weeks before

rice transplanting to avoid the adverse e Vects of phytotoxicity and

N immobilization on crop growth (Ali et al., 1995; Lanjewar et al., 1992;

Wu et al., 1997). Sharma and Mitra (1990) observed that rice yields were

increased significantly when rice straw was applied 30 days before trans-

planting, and rice straw also exhibited a favorable residual e Vect on the yield

of the second rice crop.

Witt et al. (2000) reported that early residue incorporation improved the

congruence between soil N supply and crop N demand by wet season rice,

especially during the vegetative stage of crop growth. This has resulted in

13–20% greater rice yields with early (60–63 days before transplanting)

compared to late (14–15 days prior to transplanting) residue incorporation

in rice–rice systems without applied N or with moderate rates of applied N.

From South Korea, Han et al. (1991) reported that the application of

7.5 t rice straw ha�1 along with the recommended dose of fertilizers pro-

duced a rice yield of 4.8 t ha�1, which was significantly higher than the

4.3 t ha�1 obtained with the application of recommended fertilizers alone.

Similarly, Sistani et al. (1998) from Malawi, Lee et al. (1995) from South

Korea, Beye (1977) from Senegal, Ali et al. (1995) from India, and Gotoh

et al. (1984) from Japan also observed beneficial e Vects of rice straw in-

corporation on rice yield. From a 6-year field experiment, Finassi (1976)

observed that incorporation of rice straw caused a significant increase in rice

yield at the highest rate of N (120 kg ha �1) application only.

In a field experiment over four cropping seasons in Andhra Pradesh

(India), Vamadevan et al. (1975) obtained the highest rice yield and

N uptake in all the seasons when the rice straw was incorporated into the

soil, but there was a small eVect at high levels of applied N (100 or 150 kg

N ha�1). Houng and Lin (1976) and Oh (1979) observed that incorporating

rice root residues into the soil increased rice yield on a poor soil rather than

on a fertile soil. In a pot study, incorporation of 10 t rice straw ha�1 into

0–6 cm soil layer without fertilizer increased rice grain yield by 21.2%, while

application of fertilizer N along with rice straw increased yield by 52.5%

(Rao, 1973). The residual eVect of rice straw in the following rice crop was

equal to 42% increase in grain yield. The beneficial eVect of rice straw


was explained by the increased biological N2 fixation by the free-living

microorganisms in the flooded soil amended with rice straw.

Adverse as well as no eVects of incorporation of crop residues into soil

on rice yield have also been documented. In poorly drained paddy fields,

incorporation of rice straw adversely aVects the rice growth due to the

presence of strong reducing conditions in the soil (Kuboto, 1984). On such

fields, it is recommended to incorporate the rice straw shortly after harvest.

Allowing decomposition of the straw over longer period alleviates the

injury occurring in the initial growth of the rice. In paddy fields with heavy

clay soils, rice yield decreased in the first and second year after straw

incorporation; however, after 3–4 years of continuous application, as the

amount of soil mineralized N increased and reduction in soil becomes less

pronounced, plant growth and the yield increased (Kuboto, 1984). Proper

water management, which includes drainage, is also important in such soils.

In a lysimeter experiment, Kondo et al. (1980) observed that rice straw

tended to decrease the rice yield in the presence of fertilizer N compared

with no straw. Corft et al. (1985), however, reported that 6 t rice straw ha�1

along with recommended N, P, K, and S fertilizers showed no eVect on rice

yield over N, P, K, and S fertilizers alone.

2. Integrated Management of Fertilizers and Crop Residues

Crop residues incorporated during fallow periods will cause immo-

bilization of soil N, but net N mineralization occurring during the

following cropping season would need to be accounted for when evalua-

ting N requirements of the following crop. To determine the amount

of N fertilizer that can be reduced with annual straw incorporation, a

N fertilizer response study was conducted by Eagle et al. (2001). As the

level of N fertilizer applied increased, grain yield increased when straw was

burned or incorporated. However, grain yields when straw was incorporated

continuously for 5 years were higher than when straw was burned. These

trials indicated that N fertilizer application can be decreased when straw is

incorporated, because no yield response was further observed when more

than 115 kg of N ha�1 was applied. It was recommended that N rates can be

decreased by at least 30 kg N ha�1 after 5 years of straw incorporation.

Clearly, an active, labile N pool was formed when straw was incorporated

that led to a reduction in fertilizer N dependency for rice.

In long-term experiments carried out at four locations in India, Hegde

(1996) observed that at three locations it was not possible to substitute a part

of N (25% of the recommended N) needs through rice straw without ad-

versely aVecting crop yields (Table XXII). In Kerala, located in deep South

of India, however, rice straw incorporation could substitute for 25% of

Table XXII

E Vect of Integrated Management of Inorganic Fertilizers and Rice Straw on Crop Yields (t ha�1)

in Rice–Rice Cropping System at Four DiVerent Locations in India

Treatment Rainy season rice Winter season rice

Rainy season Winter season Site 1a Site 2b Site 3c Site 4d Site 1a Site 2b Site 3c Site 4d

100% NPK 100% NPK 5.21 3.62 4.72 3.68 5.42 3.86 4.51 2.99

75% NPK 75% NPK 4.46 2.93 3.93 3.65 4.71 3.88 3.85 2.90

50% NPK 100% NPK 4.40 2.59 3.23 3.64 4.61 3.80 4.73 3.04

75% NPK þ25% N as RSe

100% NPK 5.03 3.21 4.07 3.66 5.00 3.47 4.00 2.88

50% NPK þ50% N as RS

100% NPK 4.73 2.96 3.53 3.68 5.05 3.84 4.79 2.99

LSD ( p ¼ 0.05) 0.19 0.04 0.18 0.16 0.21 0.05 0.20 0.12

aSite 1: Kharagpur (West Bengal), data averaged for 5 years, Udic Ustochrepts sandy clay loam

soil (pH 5.4).bSite 2: Bhubneswar (Orissa), data averaged for 10 years, Haplustalts sandy loam soil (pH 5.9).cSite 3: Maruteru (Andhra Pradesh), data averaged for 4 years, Chromustrets clayey soil

(pH 7.0).dSite 4: Karemane (Kerala), data averaged for 8 years, Typic Tropfluvent sandy loam soil

(pH 5.2).eRS, rice straw.

From Hegde (1996).


fertilizer N needs of kharif rice. The relatively high temperatures during both

kharif and winter seasons in Kerala might have helped for quick decomposi-

tion of rice straw. At low-fertility levels (50% N, P, K), application of rice

straw significantly increased the grain yield of rainy season rice over no

straw treatment at all the four sites. Raju et al. (1987) and Elankumaran

and Thengamuthu (1986) also concluded that it is not possible to substitute a

part of N through rice straw due to its high C:N ratio, which causes

immobilization of soil and fertilizer N. In a field trial in eastern India,

Bhattacharyya et al. (1996) recorded N substitution of up to 50% of the

recommended fertilizers from the incorporation of 5 t rice straw ha �1 in an

acidic red soil. Russo (1974) reported that incorporating 3–6 t chopped rice

straw ha�1 with 65 kg N plus 28 kg P ha�1 produced a slightly higher rice

yield than that obtained with the application of 120 kg N plus 43.1 kg P plus

110 kg K ha �1. Kamalan et al. (1989) obtained 0.3 t ha�1 of additional rice

yield with the application of urea super granules (USG) combined with rice

straw over USG alone. In another study in Malawi, Sistani et al. (1998)

observed that on rice straw-amended plots, application of urea in briquette

form compared with prilled urea significantly increased rice grain yield in

two of the three experiments.


Raju and Reddy (2000) conducted a 6-year study on a clay loam soil to

investigate the eVect of rice residue management on crop yields in rice–rice

system. Rice straw equivalent to supply 25 and 50% fertilizer N requirement

of the rainy season crop was incorporated before rice transplanting, and

the residual eVect of rice straw was studied in the succeeding winter rice.

The incorporation of rice straw along with recommended fertilizers proved

superior to inorganic fertilizers alone in increasing rice yield, soil organic

matter, and available K contents in soil. This study showed that it is possible

to reduce the total fertilizer needs of both rainy and winter season rice by

25% without any adverse eVect on system productivity. The N balance was

negative in all the treatments, but the P balance was positive. The K balance

was positive when 50% of the fertilizer N was applied as rice straw.

In a greenhouse experiment, Shen et al. (1993) obtained a higher rice grain

yield by adding 10 g wheat straw plus 350 mg urea N kg�1 soil, but the 10 g

straw plus 150 mg urea N treatment registered higher fertilizer use eYciency. It

was suggested that an adequate N supply to rice plants could be maintained by

applying suYcient N fertilizer with straw to have a C:N ratio equal to 20.

Huang and Lu (1996) observed no adverse eVect of rice straw on plant growth

and total 15N recoverywhen rice straw was incorporated alongwith (NH4)2SO4

at a C:N ratio of <25:1. The application of rice straw helped alleviate the

adverse eVects of excessive N rates and thus increased the rice growth and

yield response to high N rates. Patel et al. (1997) found that preconditioning

of urea with rice straw plus soil in the ratio of 1:3:1 was significantly superior

to urea alone with respect to grain yield and N uptake of rice.

In Maharashtra state of India, Bulbule et al. (1996) observed that man-

agement practices consisting of basal incorporation of rice straw (2 t ha�1)

integrated with deep placement of urea briquette (2.1 g per four hills) at

transplanting using a modified 20 � 20 cm spacing produced significantly

higher grain yield (average increase of 1.3 t ha�1) of rainfed transplanted rice

than did the rice straw incorporation integrated with two equal splits of

prilled urea at the same N rate. The additional yields obtained with deep

placement of urea briquettes were possibly due to reduction in immobiliza-

tion of fertilizer N over top dressing of prilled urea. Similarly, Elankumaran

and Thandamuthu (1986) reported that USG combined with rice straw

produced about 10% higher rice grain than USG alone at the same rate of

N application in south Indian state of Kerala.

Pandey and Tripathi (1992) reported that application of 33% of recom-

mended fertilizer N (87 kg ha�1) through rice straw plus 66% as prilled urea

produced a rice yield of 4.7 t ha�1, which was higher than that obtained with

the application of prilled urea in three equal splits (4.0 t ha�1). In South

India, Ramaswami (1979) observed that incorporation of 10 t rice straw

ha�1 along with 39 kg P ha�1 produced a rice yield that was similar to that

obtained with the application of 180 kg N plus 39 kg P plus 75 kg K ha�1.


The straw application increased biological N2 fixation and P availability in

soil. Rutu and Widjaja (1994) reported that application of crop residues

increased N and P use eYciency from <20 in control to 40 kg grain kg�1

N and from 30 kg in control to 60 kg grain kg�1 P. Similarly, Sudjadi et al.

(1989) observed a twofold increase in N use eYciency in rice after 4 years of

incorporation of rice straw in Indonesia. In another field experiment in

Indonesia, incorporation of 3 t rice straw ha�1 increased grain yields of 13

rice cultivars by up to 1.3 t ha�1 and significantly increased the K content of

shoot compared with untreated control (Ismunadji et al., 1973). It was

suggested that rice straw could be incorporated into soil as a substitute for

K fertilizer. Kwak et al. (1990) reported that rice straw application increased

rice yield in the no-P plots.

Becker et al. (1994a) reported that co-incorporation of green manure and

straw from previous rice depressed yield and N use eYciency in the second

rice dry season when yield potential is high. A residual eVect equivalent to

10% increase in grain yield however, was, observed in the third rice crop.

Synchronizing soil N supply with N demand by incorporating residues with

suitable chemical fertilizers may not immediately increase rice grain yield but

will improve long-term soil fertility.

In pot and laboratory studies using 15N-labeled rice straw and15N-labeled (NH4)2SO4, Hwang et al. (1993) found that 17.5 to 23.5% of

straw N was mineralized during rice growth, of which 30–50% was subse-

quently absorbed by rice plants. Rice straw inhibited plant growth in its

early stages, but application of fertilizer N with rice straw stimulated its

decomposition, thereby increasing the mineralization of straw N and

subsequent recovery by rice plants and reducing levels of residual N in

soils. He et al. (1994) measured N recovery by rice grain from rice straw

N incorporated in the field from 9.8 to 14.5%. At IRRI, Philippines, Broad-

bent and Reyes (1972) obtained a high recovery of fertilizer N (>72%) by

rice in two soils even when chopped rice straw was incorporated under

greenhouse conditions. In a pot study in Japan, Shiota et al. (1984) found

that recoveries of fertilizer 15N and rice straw compost N were 58–61% and

13–15%, respectively. Total N uptake at harvest was the highest when rice

straw compost was applied along with fertilizer. Li et al. (1981) from China

reported that rice straw incorpotaton increased N uptake by rice compared

to straw removal.

In Bangladesh, continuous cropping with rice–rice–rice has led to destruc-

tion of soil clay through the process known as ferrolysis This is the major

degradation process in the fine-textured soils, which may be one of the causes

of yield decline/stagnation of rice, including organic matter depletion (Farid

et al., 1998). Rice straw incorporation after each rice harvest continuously for

12 years resulted in average yield of 10.99 t ha�1 (for three rice crops in a year)

compared with 6.48 t ha�1 in the traditional practice of growing rice. The


sustainability of rice yields was also achieved with straw incorporation, as rice

grain yields declined from 7.15 t ha�1 in 1984–85 to 6.15 t ha�1 in 1994–95

under the farmers’ practice, but grain yields increased from 9.50 t ha�1 in

1984–85 to 12.20 t ha�1 in 1994–95 under straw incorporation treatment. The

yield increase and sustainability in the straw-incorporated treatment was

ascribed to improvement in the physical and chemical properties of soil

and the release of several plant nutrients such as K, S, and micronutrients

from the decomposing straw, thereby increasing soil fertility. On average, 12

t ha�1 of rice straw were incorporated each year, which recycled 70 kg N, 12

kg P, and 166 kg K ha�1. The nutrient balance for N, P, and K was positive

in straw-incorporated plots, but large negative nutrient balances were

recorded when rice residues were removed from the fields.

C. RICE–LEGUME CROPPING SYSTEM

Green gram (Vigna radiata), black gram (Vigna mungo), cowpea

(V. unguicuata), soybean (Glycine max), and groundnut (Arachis hypogae)

are the important summer food legumes that fit into the rice-based multiple

cropping systems; they leave substantial amount of residues. Legume resi-

dues provide biological fixed N to the next crop in addition to the benefits

oVered by non-legume crops. Food legumes are capable of fixing large

amounts of N, but removal of seed or green pods can constitute an export

of considerable N. Accumulation of N by legumes in tropical rice-based

cropping systems is influenced by water regimes, inoculation, soil fertility,

nutrient supply, and soil and crop management (Buresh and De Datta, 1991;

Yadvinder-Singh et al., 1994a).

The quantity of N in above-ground residues remaining after grain harvest

ranges from 17 kg to 101 kg N ha�1, and N fertilizer equivalent ranges from

37 to 100 kg N ha�1, with a mean value of 40–45 kg N ha�1 (Buresh and De

Datta, 1991). In addition to above-ground residues, roots of food legumes

contain up to 40 kg N ha�1 at final harvest, which is progressively released

during crop growth. This may explain in part the reported considerable

benefits of legumes to a subsequent rice crop despite a modest return of

N in residues (Kulkarni and Pandey, 1988) or even when all the above-

ground residues had been removed prior to cultivation (De et al., 1983).

Residues of grain legumes after harvesting pods have a lower N content than

those of green manure but rapidly release N in tropical flooded soils. The

potential benefits of legume residues and legume green manures in rice other

than fixed N are described by Buresh and De Datta (1991) and Yadvinder-

Singh et al. (1991). Studies with 15N-labeled legume residues indicate that

N recovered by subsequent rice ranges from 25 to 45% of the N originally

contained within the legume residues (Yadvinder-Singh et al., 1991).


In a rice–wheat-mungbean system, Rekhi and Meelu (1983) incorporated

straw of mungbean grown after wheat just before rice planting and observed

an increase in rice yield equal to the application of 60 kg fertilizer N ha�1.

Similar results were also obtained by Bhandari et al. (1992). Incorporation

of legume residues after harvesting grains exhibited high agronomic

eYciency and apparent N recovery compared to cowpea used as green

manure under both lowland and upland conditions in Philippines (John

et al., 1989). Prasad and Palaniappan (1987) reported than incorporation

of mungbean and soybean residues along with recommended fertilizers

produced the highest rice yield. In a 2-year field experiment on lentil-rice

crop rotation, Prasad et al. (1990) found that incorporation of lentil residues

(2.7 to 5.6 t ha�1) in rice exhibited no additional benefit over lentil root

biomass. Incorporation of lentil residues, however, resulted in recycling of

about 50–60% of the total N, P, and K removed by the lentil crop.

Sangakkau (1987) reported that rice yields after ploughing residues of

mungbean or Phaseolus vulgaris were 3.2 and 3.4 t ha�1, respectively, as

compared to the 3.0 t ha�1 after fallow. In that study, no residual eVect from

residue incorporation was detected in the second rice crop. In the

rice-soybean cropping system, Adisarwanto et al. (1996) reported that appli-

cation of rice straw increased soybean yield by 103% at two sites in Indone-

sia. In upland rice-soybean rotation, Ismunadji (1978) recorded soybean

yields of 410, 450, and 810 kg ha�1 under untreated control, 20 t rice straw

ha�1 incorporated, and 20 t rice straw ha�1 applied as mulch treatments,

respectively. In that study on upland rice-soybean cropping system on

orthoxic tropudult soil high in A1 and low in K, response to K application

was obtained when crop residues were removed at harvest. Recycling of crop

residues dramatically improved the yields at low rates of K application and

reduced the crop response to K application. In a greenhouse study with a

rice-lentil cropping system, Tamak et al. (1993) observed that incorporation

of rice straw increased lentil yields by 34% over control. In field trials on

groundnut-rice, groundnut-maize-rice, and maize-soybean-rice in Peru, Loli

and Chuguizuta (1993) reported that incorporation of legume residues in

rice significantly increased rice yields, while incorporation of maize residue

inhibited germination and reduced rice yields.

D. OTHER RICE-BASED CROPPING SYSTEMS

Sugar beet produces large quantities of crop residues. In most cases,

residues from sugar beet resulted in higher yields of the following crop

(Watanabe, 1989). In a rice-sugar beet cropping sequence in Punjab (India),

incorporation of sugar beet tops containing 90 kg N ha�1 increased rice

grain yield by 52% over untreated control. Rice yield increased significantly


up to 120 kg N ha �1 without and up to 80 kg N ha�1 with sugar beet tops

(Kapur and Kanwar, 1994). The estimated urea N equivalent of sugar beet

tops was 37 kg N ha�1 in rice and 19 kg N ha�1 in the following crop of

sugar beet. The apparent recovery of N from sugar beet tops by rice was

20–32% at di Verent levels of N application.

In rice-potato-sesame and rice-potato-mung bean cropping sequences,

incorporation of crop residues along with 75% of the recommended N, P,

and K fertilizers consistently increased the productivity of constituent crops

in the two sequences (Jayaram et al., 1990). In a 2-year field experiment on a

red yellow podzolic soil on a rice-maize-cowpea sequence in Indonesia, crop

residues supplied a significant amount of N to the following rice crop (Sisworo

et al., 1990). As expected, cereal residues were of lower value as a source of

N than were legume residues. In a rice-potato-groundnut rotation, Sanyal

et al. (1993) recorded similar yields of rice with 100% recommended NPK

fertilizer applied alone and incorporation of crop residue along with 75% of

recommended NPK fertilizers. Crop residue incorporation increased nutrient

uptake by 14%. Suyamto (1993) observed that in a rice-maize cropping

system, the application of 5 t rice straw ha �1 significantly increased the rice

yield by 26% over control. The beneficial e Vect of rice straw on crop yield was

equivalent to 73 kg K ha �1. The e Vect of rice straw mulch on maize, however,

was not significant. In a 3-year field experiment on rice-mustard rotation on a

deep clayey soil, application of rice straw mulch in mustard conserved more

water in the profile during the early stages of growth and resulted in low soil

mechanical resistance, leading to better root growth. Rice straw mulch signif-

icantly reduced the grain yield of mustard and chickpea in rice-mustard and

rice-chickpea rotations (Rathore et al., 1998). Cotton stalks after picking are

normally uprooted and used as fuel. In a rice-cotton-rice cropping system in

Tamil Nadu (South India), incorporating 22.6 t cotton sticks ha�1 (adding

270 kg N ha�1) on a clay loam soil significantly increased the rice yield by

22.2% over no stick incorporation (Budhar and Palaniappan, 1999). These

data suggest that cotton residues can be incorporated in the following rice for

higher yields and fertilizer N dose can be reduced.

XII. SUMMARY AND CONCLUSIONS

The intelligent management and utilization of crop residues is essential

for the improvement of soil quality and crop productivity under rice-based

cropping systems of the tropics. Crop residues, usually considered a prob-

lem, when managed correctly can improve soil organic matter dynamics and

nutrient cycling, thereby creating a rather favorable environment for plant

growth. Crop residues contain large quantities of nutrients, and thus the


return of crop residues to the soil can save a considerable quantity of

fertilizers. The most viable option is to retain residue in the field; burning

should be avoided. The major issue is adapting drills to sow into loose

residues. Strategies include chopping and spreading of straw during or

after combining or the use of disc-type trash drills.

The important conditions that influence crop residue decomposition

under field conditions are temperature, moisture, aeration, and N applica-

tion. Several other factors, such as residue quality, tillage, and soil proper-

ties, also aVect microbial decomposition of crop residues. Residues rich in

lignin and polyphenol contents experience the lowest decay. Decomposition

of crop residues occurs at a rapid rate (about 80% of crop residue C is lost in

the first year) under the warm and humid conditions of the tropics. Expo-

nential models have often described the process of C decomposition carried

out by soil microorganisms, and these have suggested the existence of at least

two carbon fractions—labile and resistant.

Factors that control C decomposition also aVect the N mineralization

from crop residues. Decomposition of poor-quality residues with low

N contents, high C:N ratios, and high lignin and polyphenol contents

generally results in microbial immobilization of soil and fertilizer N. The

period of N immobilization varied from 4 to 8 weeks depending on temper-

ature and mineral N content of the soil. The N immobilization potential of

cereal residues is very high (26–35 mg N g�1 added C) and is often higher

than available mineral N content in soils. Net rates of N mineralization will

occur when plant residues with C:N ratios <40 are incorporated. C:N ratios

have been criticized because they are species specific and are influenced by

soil N supply (site specific). Most of the studies on N mineralization–

immobilization have been carried out under laboratory conditions, and it

is not precisely known to what extent these can be extrapolated to field

conditions. The extent of N immobilization is less in anaerobic than

aerobic conditions. Nutrient immobilization caused by the addition of resi-

dues will last only a few years before the system adjusts to a new equilibrium,

and the rate of mineralization of nutrients in the whole system is increased.

The qualitative controls (factors) on the amounts and timing of N release

from crop residues are known, but quantification of mineralization–

immobilization over both a short- and long-term basis and understanding

of the relationship with diVerent types of residues, inputs, and management

are not adequate. Little is known about the eVect of tillage on mineralization

of N from crop residues and mechanisms controlling mineralization in

rice-based cropping systems under tropical conditions.

Application of crop residues with a high C:N ratio often leads to adverse

impacts on available N in soil and growth of crops planted immediately after

straw incorporation. A large number of organic compounds, particularly

phenolic acid and acetic acid, are released during the decomposition of crop


residues under anaerobic conditions. The accumulation of these organic

compounds can adversely aVect the seedling growth. The accumulation of

organic acids in residue-treated soils occurred during the initial 15–20 days

of the decomposition period. The accumulation of organic acids is likely to

be greater in soils with low percolation rates. The serious decrease in soil

available N content can be oVset by proper application of N fertilizer in

combination with rice straw, and the toxic eVects of organic acids and some

reducing substances resulting from decomposition of rice straw may be

alleviated or eliminated by allowing the rice or wheat straw to decompose

for some time (2–4 weeks) before planting the next crop.

Crop residue management may aVect N cycling and N use eYciency of

crops in several ways. Rice has been found to recover up to 25% of the rice

straw N. Although the total amount of N contributed by straw from a single

application will be relatively small, the long-term eVects should be substan-

tial. The eVect of crop residues on N losses by leaching and denitrification,

and on the availability of fertilizer N, particularly under surface placement

of residues, is not conclusive and needs further investigation under field

conditions. Both the positive and the negative eVects of residues on fertilizer

use eYciency have been reported. Incorporation of crop residues markedly

increases the activities of urease and many other enzymes in soil. Large NH3

volatilization losses from urea applications to soils amended with crop

residues both under flooded and upland conditions have been reported.

The application of crop residues can cause short-term immobilization of

both P and S, particularly in aerobic soils. Only a small fraction (5%) of the

residue P is available to the plants in the first year, and a major fraction is

immobilized as microbial biomass. The availability of P in the soil and

uptake by rice increased with straw incorporation in flooded soils. Crop

residue incorporation generally increased the P adsorption and P sorption

maxima in soils but markedly reduced the aYnity coeYcient or rate of

adsorption. Incorporation of crop residues in rice increased the eYciency

of P in rock phosphates. Crop residues contain large amounts of K, which

upon incorporation increased K availability in soil and helped to reduce

K depletion from nonexchangeable K fraction of soil.

Long-term application of crop residues increased the organic matter, total

N content, and availability of several nutrients (though to a small extent) in

soils. The rate of increase in soil organic matter is low due to high turnover

rates of C under tropical conditions. The increase in soil organic matter

levels due to crop residue recycling was determined by the duration of the

study, amount and quality of residue, soil type, climatic conditions, and

cropping system followed. Crop residues influence the chemical and

biological properties of the soil. In many situations, residue retention may

reduce nutrient availability, and additional fertilizer applications may be

required to attain yields equal to those previously achieved.


Crop residues exerted a favorable, though highly variable, influence on

diVerent soil physical parameters. Residue management alters soil proper-

ties, mainly by causing a gradual increase in soil organic matter content. The

eVects of residues on soil physical properties were dependent on soil type,

tillage, soil moisture conditions, duration of study, and cropping system

followed. The beneficial eVects of crop residues on soil physical properties

are likely to be greater under the rice–wheat than under the rice–rice crop-

ping system.

Crop residues caused marked increases in microbial populations and

microbial biomass in soils. The addition of crop residues to flooded soils

enhanced biological N fixation by phototrophic and heterotrophic bacteria.

The estimate of biological N fixation showed that 15–25 kg ha�1 more N may

be fixed per season by amending the soils with crop residues under field

conditions. The values of biological N fixation under upland conditions are

lower than under flooded conditions.

The many reports of investigations into crop residue management and

yield showed that results have been variable—no eVect, yield increase,

or yield decline. Yield decline associated with stubble retention may be due

to three main factors: short-term nitrogen immobilization, fungal diseases,

and phytotoxicity. The degree of stubble decomposition at the time of

planting has a great bearing on the likelihood of problems for the crop.

Furthermore, some of these problems are likely to decrease over time. In

cases in which rice yield increases occurred with stubble retention, these

increases took at least 3–5 years to be expressed.

In the rice–wheat cropping system, rice is likely to be more benefited than

wheat. The rice straw recycling in wheat will have small or even negative

eVects on wheat yields over the short term (1–3 years). These negative eVects

can, however, be eVectively mitigated by co-incorporation of cereal residues

and leguminous green manures. Long-term recycling of crop residues can have

substantial beneficial eVects, and high crop yields could be obtained from the

combined use of inorganic fertilizers and crop residues. The eVects of crop

residue recycling on growth and yields of crops are influenced by residue load,

time allowed for decomposition, temperature and moisture conditions during

decomposition and crop growth, nutrient supply, and soil type. The crop

residues are likely to have greater eVects on yields of crops grown in fine-

textured than in coarse-textured soils. The beneficial eVect of a starter dose

on crop yields in straw-amended soils has been reported in a few studies, and it

needs further evaluation. The time of fertilizer N application is important in

enhancing the yields of crops on residue-treated soils.A few studies showed that

a dose of fertilizer N higher than recommended for untreated soils may be

needed in the initial 1–2 years for straw-amendedfields. Information from long-

term studies indicated that it may not be possible to replace a part of fertilizer

N by wheat straw in the rice crop.


Under the rice–rice system, the yield advantage due to rice straw incor-

poration over straw burning or removal ranges from 0.2 to 0.7 t ha�1 per

season, and it increases with time and soil fertility buildup. Higher rice yield

responses to straw incorporation were obtained on acidic and sodic soils

than on normal soils. The length of the decomposition period for crop

residues before planting of the next crop influenced the agronomic response

to applied residues. In most of the studies carried out in India, it was not

possible to substitute a part of fertilizer N requirement through rice straw. In

addition to N, increased P use eYciency has been observed in some studies

conducted on straw-amended soils. As compared to non-legume residues,

legume residues showed greater potential in meeting the fertilizer N

requirement of the succeeding crop. The results from experiments on the

agronomic value of crop residues are variable because of a number of factors

involved, including residue quality, management factors, and edaphic

factors, and their complex interactions with various management factors

in determining the ultimate crop yield. This suggests that one residue

management system in unlikely to be suited to all situations.

As emphasis is placed on environmentally sound practices, the envi-

ronmental consequences of crop residue recycling in rice-based cropping

systems need more attention. In submerged soils, crop residues may encour-

age emission of CH4, and techniques may be developed to minimize CH4

production. On the other hand, the increase in soil organic matter levels with

crop residue incorporation will be useful in sequestering C so as to reduce

CO2 levels in the atmosphere.

The application of current knowledge on residue management will help

reduce the adverse eVects of crop residues on crop yields. It may be con-

cluded that nutrient cycling through crop residues holds great promise in

securing not only a high level of crop productivity but also improved soil

health. Greater knowledge in this area should improve our ability to manage

soil nutrients eYciently.

XIII. RESEARCH NEEDS

Regions with plummeting numbers of draft animals due to increasing

farm mechanization will simultaneously increase the straw available for

incorporation and create new straw disposal issues in continuous rice sys-

tems. The introduction of new farming implements will allow more diverse

options for straw incorporation. New issues will thus call for a new genera-

tion of studies to optimize straw management. Because economic constraints

often clash with agronomic objectives in straw management, future studies

should focus on strategies that are economically feasible for local farmers.


These studies should also integrate eVects of straw management on nutrient

cycling with the precise formulation of mineral fertilizer applications so that

farmers can attain desired yield targets and minimize costs while reducing

environmental degradation through excessive fertilizer application. The

participation of farmers at the research, development, and implementation

stages is of paramount importance because of the wide range of rice-based

cropping systems that must be considered and biophysical variations. Al-

though a great deal of information is available from temperate regions,

much needs to be done in tropical climates where rice is the major crop.

Some research needs are the following:

1. In order to interpret soil organic matter dynamics and nutrient cycling,

more reliable data on yields of both above- and below-ground crop

residues are needed. Evaluation of crops in terms of residue production,

decomposition, and nutrient retention with the objective of improving

organic inputs to soil is needed.

2. At present, proper technology for in situ incorporation of crop residues is

not available. Development of better technology for in situ incorporation

and rapid decomposition of crop residues by appropriate tillage, irriga-

tion, fertilizer management, and inoculation of suitable microorganisms,

particularly for intensive rice-based cropping systems commonly fol-

lowed in Asia is a need of the time.

3. Better characterization of crop residues for their chemical composition

(macro- and micronutrients, organic compounds) that will help in im-

proved prediction of release of C, N, S, and other nutrients is needed.

A database on residue qualities should be prepared.

4. More information needs to be collected on the eVects of management

(including residue quality, application rate and method, timing, inter-

actions with inorganic fertilizers, temperature, soil moisture, soil type,

and tillage) on decomposition and nutrient release rates and production

of phytotoxic compounds from crop residues. Better quantification of

mineralization–immobilization would help improve fertilizer N manage-

ment and minimize losses of N to the environment. Available informa-

tion is restricted to a limited number of situations, mainly for temperate

regions. Data based on decomposition and N mineralization rates will

be useful for model development and validation.

5. Several plant quality parameters and indices have been proposed for

predicting decomposition and N release. There is a need to identify

some robust plant indices that provide improved prediction of nutrient

release and soil organic matter formation. These indices, coupled

with decomposition models, should be able to assess crop residues for

decomposition, nutrient release, and soil organic matter formation. The


measurement of a few quality parameters then can replace the need for

detailed decomposition studies for each plant material in each location.

6. More research on N cycling and fate of nutrients added through crop

residues needs to be carried out in rice-based cropping systems. Detailed

experiments are required to measure gross and net mineralization–

immobilization in residue-amended soils for representative sites across

a range of rice-based cropping systems, climates, and soils. There is a

need to extrapolate short-term incubation results to field situations.

7. Evaluation of crop residue eVects on the interaction between inorganic,

organic, and microbial forms of P and S in the soil and subsequent P and

S availability is needed. Future studies should also focus on the miner-

alization and transformations of micronutrients contained in crop resi-

dues.

8. Encouragement of the use of multidisciplinary approaches to problems

associated with residue management is needed.

9. The fate of nutrients in residue-amended fields should be investigated.

10. Investigation of nutrient cycling within residue management and re-

duced tillage practices to provide a better understanding of changes in

nutrient availability to plants and SOM dynamics is needed. Such

studies will help further increase our understanding of how soil manage-

ment influences the quality and quantity of various organic matter pools

important in the cycling and release of nutrients or the quantification of

ecosystem C budget.

11. More research is needed on whether diVerent crop residues have diVer-

ent eVects on SOM quality. Specifically, contributions from crop straw

and crop roots to SOM formation could all be distinguished from each

other, and rice materials should be distinguished from wheat materials

in rice–wheat systems. These materials vary considerably in their lignin

content, which might have some significance to the phenol accumulation

reported by Olk et al. (1996).

12. Many of the changes in soil properties and nutrient cycling may become

apparent only after several years (10 years or more) of residue manage-

ment, and the long-term results may diVer from those obtained over

short-term. Therefore, there is a strong need to establish long-term

experiments at sites carefully selected for variations in temperature,

moisture regimes, soil mineralogy, and agricultural management cover-

ing diVerent rice-based cropping systems in tropical countries.

13. Direct and indirect eVects of crop residue incorporation on soil quality

and crop productivity in rice–rice and rice-upland crop systems should

be investigated.

14. Because of a number of variables and complexities of interactions

among soils, crops, and climate, an accurate prediction of the eVect of

crop residue management practices on nutrient cycling, crop growth and


yield, and soil properties will be possible only through the development

of realistic process-based computer simulation models. Research pro-

blems related to residue management should be carried out with consid-

eration of potential modeling uses of data. Collection of additional data

needed for model validation and exploration of alternate modeling

scenarios could then help in understanding research results. Several

models are available to simulate the impact of management practices

on soil organic matter and crop yield, but most of them have not been

evaluated and tested for tropical regions.

15. Investigation of largely unexplained benefits that arise from crop

residue recycling, such as microbiological, biological N2 fixation, pest

suppression, physical, etc., is needed.

16. Estimates of the relative costs of diVerent options must be developed, as

the most attractive choices might have significant impacts on environ-

mental quality through their eVect on microbial processes that deter-

mine the magnitude of C storage in the soil, methane emission into the

atmosphere, and long-term soil fertility.

Addressing these research needs will provide information to properly

integrate crop residues with inorganic fertilizers. The most critical output

in this regard will be the ability to accurately adjust the rate and timing of

fertilizer addition in line with crop residue management for diVerent crops,

soil types and agroclimatic situations.

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