ARTICLE

Soil organic carbon sequestration as affected by tillage,crop residue, and nitrogen application in rice–wheatrotation system

Rajan Ghimire • Keshav Raj Adhikari •

Zueng-Sang Chen • Shree Chandra Shah •

Khem Raj Dahal

Received: 30 November 2010 / Revised: 16 March 2011 / Accepted: 20 March 2011 / Published online: 7 April 2011

� Springer-Verlag 2011

Abstract Despite being a major domain of global food

supply, rice–wheat cropping system is questioned for its

contribution to carbon flux. Enhancing the organic carbon

pool in this system is therefore necessary to reduce envi-

ronmental degradation and maintain agricultural produc-

tivity. A field experiment (November 2002–March 2006)

evaluated the effects of soil management practices such as

tillage, crop residue, and timing of nitrogen (N) application

on soil organic carbon (SOC) sequestration in the lowland

of Chitwan Valley of Nepal. Rice (Oryza sativa L.) and

wheat (Triticum aestivum L.) were grown in rotation add-

ing 12 Mg ha-1 y-1 of field-dried residue. Mung-bean

(Vigna radiata L.) was grown as a cover crop between the

wheat and the rice. Timing of N application based on leaf

color chart method was compared with recommended

method of N application. At the end of the experiment SOC

sequestration was quantified for five depths within 50 cm

of soil profile. The difference in SOC sequestration

between methods of N application was not apparent.

However, soils sequestered significantly higher amount of

SOC in the whole profile (0–50 cm soil depth) with more

pronounced effect seen at 0–15 cm soil depth under

no-tillage as compared with the SOC under conventional

tillage. Crop residues added to no-tillage soils outper-

formed other treatment interactions. It is concluded that a

rice–wheat system would serve as a greater sink of organic

carbon with residue application under no-tillage system

than with or without residue application when compared to

the conventional tillage system in this condition.

Keywords Conventional tillage � Crop residue �No-tillage � Organic carbon � Rice–wheat crop rotation �Soil depth

Introduction

Maintenance of SOC in rice–wheat cropping system is

important not only for improving agricultural productivity

but also for reducing carbon emission. Soil management

practices such as tillage operations are conventionally used

for loosening soils to grow these crops. But long-term soil

disturbance by tillage is believed to be one of the major

factors reducing SOC in agriculture (Baker et al. 2007).

Conventional tillage facilitates microbial oxidation of

macro-aggregate protected carbon (Janzen et al. 1998) and

frequent tillage inverting soil promotes losses of SOC

through physical breakdown of the residues (Six et al.

2000). The mineralization of SOC compounds increases

under oxidative conditions leading to higher CO2 flux to

the atmosphere (Reicosky et al. 1995).

One of the major problems of South Asian agriculture,

including that of Nepal Terai is the removal and/or burning

of crop residues to facilitate good seedbed preparation and

to avoid possible yield reduction through diseases and

insect pest (Aulakh et al. 2001a). Such practices along with

repeated tilling of land cause soil health and environmental

problems leading to low productivity of the cropping sys-

tems. It is likely that losses of carbon through such prac-

tices would also have been huge in rice–wheat cropping

system under conventional tillage practices. This cropping

R. Ghimire � S. C. Shah � K. R. Dahal

Institute of Agriculture and Animal Sciences (IAAS),

Tribhuvan University, Rampur Campus, Chitwan, Nepal

K. R. Adhikari (&) � Z.-S. Chen

Department of Agricultural Chemistry, National Taiwan

University, 1, Section 4th, Roosevelt Rd., Taipei 10617, Taiwan

e-mail: keshav.adhikari2008@gmail.com

123

Paddy Water Environ (2012) 10:95–102

DOI 10.1007/s10333-011-0268-0

system is practiced in a large tract of Indo-Gangetic Plains

of South Asia (13.5 million ha including 0.5 million in

Nepal) (Wassmann et al. 2004).

Rice is grown in Nepal mainly during monsoon season

(June/July–October/November). Wheat follows monsoon

rice and is harvested in March/April. Farmers strive to

increase productivity of these crops through the use of

improved technologies but in the last decade or so, per unit

productivity growth of rice–wheat cropping system is

declining in Nepal (Duxbury 2002), often resulting in food

deficits in the country. This decline in productivity is

attributed to the loss of soil organic matter, mineral nutri-

ents, soil aggregates, and structural stability (Hobbs and

Morris 1996), which results in low soil fertility and pro-

ductivity of the system.

Several field researches in this region (Gami et al. 2001;

Regmi et al. 2002; Shrestha et al. 2006) urged the need of

alternative ways for rebuilding soil organic matter on Indo-

Gangetic Plains. No-tillage and crop residue management

are suggested as suitable practices to decrease soil bulk

density and increase infiltration capacity (Regmi et al.

2002; Shaver et al. 2002) as well as to promote aggregate

stability and soil organic carbon (Six et al. 2002). Several

other benefits of no-tillage and crop residue application on

SOC sequestration are documented elsewhere (Duiker and

Lal 1999; West and Post 2002; Lal 2004a), which are

mostly limited to the temperate ecosystem thus are less

documented for tropical and sub-tropical regions and val-

ley agriculture of Nepal. Furthermore, much of the tillage

experiment results suggest that SOC is influenced by typ-

ical research conditions in each locality. For example, no-

tillage did not increase SOC at lower soil depths (Luo et al.

2010), whereas conventional tillage maintained it (Baker

et al. 2007). SOC is regulated mostly by the types and

frequencies of crops and cover crops grown and less

affected by weather and tillage functions (Luo et al. 2010).

These diverse research findings in different agro-ecosys-

tems motivated us to contribute to tillage and residue lit-

erature pertaining to SOC sequestration in a relatively less

explored rice-growing pocket area of Nepal. The existing

literature related to the rational N management for opti-

mum yield with the benefit of no-tillage and crop residue

application is also not clear. Regmi et al. (2002) reported

the supportive role of rational N management on SOC

enrichment in the soil.

Results from this field-scale study are the first estimates

providing a site-specific SOC stock data for the dominant

rice–wheat cropping system useful for national level car-

bon database development program, which has not yet

begun in Nepal. On this background, the objectives of this

study were to evaluate the effects of tillage, crop residue,

and N management on SOC sequestration under rice–wheat

crop rotation system in Chitwan Valley of Nepal.

Materials and methods

Experimental area and soil characteristics

The experimental area is located at 27�3804900 N, 84�2004500

E and 228 m above mean sea level (Fig. 1). It has sub-

tropical climate, i.e., cool dry winter, hot, and humid

summer, average annual rainfall of 2000 mm. Soil is

slightly acid (pH 5.4–6.0, 1:1 soil to water ratio), low

cation exchange capacity (\10 cmol(?) kg-1), medium

organic carbon (13.4–15.5 g kg-1), moderate to rapid

permeability and sandy clay loam soil texture by

hydrometer method (Day 1965). The soil texture below

30 cm changes to sandy loam (Table 1). The soil is clas-

sified as coarse loamy, hyperthermic, micaceous, Typic

Haplustoll in USDA Soil Taxonomy (Soil Survey Staff

2006). The study area was being used for similar cropping

system research even before this experiment (Table 2). The

data presented herein (November 2002–March 2006) is,

therefore, part of a longer-term field research at the Insti-

tute of Agriculture and Animal Sciences (IAAS), Tribhu-

van University in Chitwan Valley (inner Terai) of Nepal.

Treatments details and cultural operations

The experiment consisted of eight treatment combinations,

i.e., two levels of tillage, two levels of crop residue, and

two timings of N application. With net plot size of

3 m 9 5 m, three replications of eight treatment combi-

nations were arranged in a factorial Randomized Complete

Block Design (RCBD). The field layout is shown in Fig. 2.

Two tillage levels were no-tillage and conventional

tillage. The no-tillage included surface seeding of all crops

in rotation, whereas the conventional practices included

farmers’ practices of tilling soil to the depth of 15–20 cm

from the surface. Farmers in Nepal Terai cultivate soil by

mold board plow or disc harrow as primary tillage opera-

tion, followed by cultivation again during seed sowing or

Fig. 1 Location of the experimental site, Rampur, Chitwan, in the

map of Nepal

96 Paddy Water Environ (2012) 10:95–102

123

planting. The conventional secondary tillage operation in

rice crop includes the puddling of rice soils to break soil

aggregates and churning soils to maintain water stagnation

during the growing period.

Two crop residue levels were (1) no-residue application

except the root biomass left in the field after crop harvest

and (2) crop residue application at 4 Mg ha-1 of rice straw

(C:N ratio 82:1) to wheat, 4 Mg ha-1 of rice straw to

mung-bean and 4 Mg ha-1 of wheat straw (C:N ratio 91:1)

to rice crop, making a total of 12 Mg ha-1 y-1 crop residue

application under field dried conditions. The purpose of

growing mung-bean cover crop after wheat harvest until

rice planting was to conserve moisture, fix atmospheric N,

and also to reduce N2O emission from the soil.

Table 1 Distribution of pH and

textural properties of soil by

depth in the study area

Soil depth (cm) pH Sand (%) Silt (%) Clay (%) Soil texture

0–5 5.4 57 19 24 22.5 m sandy clay loam

5–10 5.5 56 21 23 Sandy clay loam

10–15 5.5 56 21 23 Sandy clay loam

15–30 6.0 58 19 23 Sandy clay loam

30–50 5.9 65 15 20 Sandy loam

Table 2 Cropping history of

the study area, Rampur,

Chitwan Valley, Nepal

Period Cropping history

Before 1999 Monsoon rice—fallow annual rotation

November 1999–October

2001

N application treatments imposed with and without crop residue application

at 4 Mg ha-1 and four different times of N application imposed in no-

tillage cultivation under rice–wheat annual rotation system

November 2001–March

2002

No-tillage plot were split into conventional tillage and no-tillage plots for

growing wheat

November 2002–March

2006

Two tillage systems, two levels of crop residue and two timings of nitrogen

application evaluated

17

T0M0N2

18

T0M1N2

19

T1M1N1

20

T1M0N2

24

T1M0N1

21

T0M1N1

22

T0M0N2

23

T1M1N2

10

T1M1N2

9

T0M1N1

16

T0M1N2

15

T1M0N1

11

T1M0N2

12

T0M0N1

14

T0M0N2

13

T1M1N1

1

T0M0N2

8

T1M1N2

7

T0M1N2

2

T1M1N1

6

T0M1N1

3

T1M0N1

4

T0M0N1

5

T1M0N2

50 cm

50 cm

5 m

3 m

22.5 m

22.5 m

Fig. 2 Layout and

randomization of treatments in

experimental field of Rampur.

T0 no-tillage, T1 conventional

tillage, M0 no crop residue, M1

crop residue at 4 ton ha-1 for

each crop in rotation, N1

recommended practice of N

application management in rice

and wheat, N2 leaf-color-chart-

based N management in rice and

recommended practice in wheat.

Serial numbers shown in the

boxes (1–24) indicate total

number of experimental plots

under study

Paddy Water Environ (2012) 10:95–102 97

123

Wheat received 100 kg N application ha-1 in two splits,

i.e., the first-half at the Crown Root Initiation (CRI) stage

and the second-half 40 days after planting. Collectively,

these applications were called the first level of N applica-

tion in wheat and abbreviated as N1. In the second level of

N application (N2) in wheat, the first-half of 100 kg N was

applied at sprouting stage and the second-half at CRI stage.

Similarly, the N1 level of N application (100 kg N ha-1) in

rice was split into 50 kg N ha-1 as basal, 25 kg ha-1 at

maximum tillering stage, and 25 kg ha-1 at booting stage.

The N2 level in rice indicated N application at 25 kg ha-1

as basal and the rest applied at 20 kg N ha-1 based on leaf

color chart (LCC) reading. The procedure followed in

using LCC method is reported elsewhere (Singh et al.

2002). The basal dose of N was applied at the time of

puddling in conventional tillage, whereas it was applied on

the 12th day after sowing of rice in no-tillage plots. For

both treatments N was applied in the form of Urea

(NH2CONH2).

In this study, data were used from three cycles of rice

(variety Sabitri) grown from June to October and four

cycles of wheat (variety BL 1473) grown from November

to March during the study years. In no-tillage plots, rice

and wheat seeds were dipped in moist cow-dung before

sowing to avoid birds picking. Seed rates of wheat and

mong-bean were 120 and 20 kg ha-1, respectively. Rice

was planted at 20 cm row to row and 15 cm plant to plant

distances with five seedlings per hill. Overnight soaked and

shortly dried in shade wheat seeds were planted at a row

spacing of 20 cm apart in a continuous furrow. Mung-bean

was broadcast and no fertilizer was applied to it. Amounts

of NPK fertilizers applied to rice (100:60:40 kg ha-1) and

wheat (100:40:40 kg ha-1) were based on recommenda-

tions of National Rice and Wheat Research Stations in the

country. Fertilizers in rice and wheat were side-dressed at

basal, whereas applications that followed basal were

broadcast. Availability of irrigation during wheat and

mung-bean seasons was not dependable but it was also not

deficit for normal crop growth; however, unusual heavy

rain during 2004 rice season probably muddled the treat-

ment effects to a certain extent (Fig. 3).

Soil sampling and data analysis

After wheat harvest in March 2006, soil samples were

collected with a tube auger (2.5 cm diameter). Five random

locations were chosen in each of 24 observational plots and

samples taken from each location for five soil depths (0–5,

5–10, 10–15, 15–30, and 30–50 cm) separately. Depth

interval of 0–50 cm was also used to enable a comparison

with the SOC stock results of other studies. The first 50 cm

soil depth is critically important because 58–81% of the

total organic carbon is held in this layer (Batjes 1996).

Samples were then air-dried in the shed and prepared

composite samples to represent a plot for each depth. The

collected samples were ground and sieved through a 10

mesh screen and analyzed for SOC using the Graham

Colorimetric method (Graham 1948). Bulk density was

determined by Core Ring method (Black and Hartge 1986)

and the value used to convert organic carbon percentage by

weight to content by volume. The bulk density and SOC

values obtained from laboratory analysis were subject to

analysis of variance (ANOVA) for 3-factor factorial RCBD

using MSTAT-C. ANOVA was performed for each soil

depth and Duncan’s Multiple Range Test (DMRT) used to

compare treatment means at 0.05 probability level. While

soil bulk density was not significantly different among

treatments and depths, a single mean bulk density value

averaged over all treatments and soil depths was used in

this study (1.07 Mg m-3 with standard deviation of

±0.11 Mg m-3). Soil organic carbon stock was calculated

according to Shofiyati et al. (2010), which is given below:

Cstock ¼ BD� Corg � D� A ð1Þ

where Cstock is the carbon stock (Mg ha-1), BD is soil bulk

density (Mg m-3), Corg is organic carbon (wt%), D is the

thickness of soil sampling layer (m), and A is the area (ha).

Multiply C (%) by 10 = soil organic carbon (g kg-1).

The annual sequestration rates of added SOC through

tillage, crop residue, and N treatments were calculated

using the following formula:

Rate of carbon sequestration Mg C m�3y�1� �

¼ SOCtreatment � SOCcontrol=time yearsð Þ ð2Þ

where SOCcontrol = soil organic carbon in the reference

(untreated) plot, SOCtreatment = soil organic carbon in the

treatment, and time = 3.5 years.

Results and discussion

Effect of tillage

The soils under no-tillage sequestered consistently higher

amounts of organic carbon than under conventional tillage

in the upper 15 cm soil depths (Table 3). The rate of car-

bon sequestration was highest at 0–5 cm soil depth

(0.59 Mg C ha-1 y-1), which corresponded to 28% higher

organic carbon under no-tillage compared to conventional

tillage treatment. The sequestration reduced with depth but

it was still 13 and 12% higher at 5–10 and 10–15 cm soil

depths under no-tillage when compared to conventional

tillage treatment, respectively. The SOC sequestration was

not significantly different between tillage types below

15 cm soil depth probably due to the lowland environment

where carbon leaching processes are constrained primarily

98 Paddy Water Environ (2012) 10:95–102

123

by subsoil plow-sole induced by previous shallow tillage

operations (although not measured). Deep tillage such as

the chiseling is seldom practiced here which would other-

wise break the plow-sole or compaction due to repeated

wheels passing. Although the trend of higher SOC in the

surface layer and low SOC below agrees with Luo et al.

(2010), our results contradict in terms of overall soil profile

SOC (1.64 Mg C ha-1 y-1) which was significantly higher

in no-tillage (9.85%) than in conventional tillage down to

50 cm. These results also differ from Baker et al. (2007)

stating that long-term continuous gas-exchange measure-

ments have also been unable to detect C gain due to

reduced tillage.

Higher organic carbon in the upper soil layer could be

attributed to a greater contribution of root biomass

concentration (preferably 0–5 cm in case of rice and

wheat) under no-tillage soils (Blanco-Canqui and Lal

2007). The relatively near-surface higher water content and

the favorable temperature of no-tillage soils during the

growing season might have provided a favorable environ-

ment for SOC accumulation in the surface soil. Contrarily,

mechanisms of particulate loss of carbon through eluvia-

tion and accumulation below plow layer are suggested for

higher subsoil SOC under conventional tillage (Blanco-

Canqui and Lal 2008). But under the condition of soil

puddling for rice planting creating surface sealing, shallow

surface plowing of the field for wheat planting and

appearance of low SOC throughout the profile suggested

that the loss of carbon by oxidation would have been the

dominant process in this soil under conventional tillage.

For this experimental condition, we agree to the reports

that losses of SOC under conventional tillage might be due

to disruption of macro-aggregates making SOC more sus-

ceptible to mineralization (Camberdella and Elliott 1993)

and decomposition (Six et al. 2002; Wright and Hons 2005)

in plow layer. Buried crop residues during tillage operation

that are accumulated immediately below plow layer,

which decompose at slower rate than surface residues

could contribute to SOC under conventional tillage

(VandenBygaart et al. 2003).

Effect of crop residue

Crop residue applied soils had consistently higher amount of

SOC at all soil depths than soils without crop residue;

however, the effect was not significant (Table 3). Results are

in line with Reicosky et al. (2002) who indicated no mea-

surable difference in SOC between returning aboveground

Fig. 3 Monthly rainfall, temperature (T), and relative humidity (RH) during the experiment, November 2002–March 2006 recorded from a

nearby weather station located within 1 km radius (Meteorological Station of National Maize Research Project, Rampur, Chitwan, Nepal)

Table 3 Effects of tillage, crop residue, and nitrogen management on

soil organic carbon sequestration by soil depth, Rampur, Chitwan,

Nepal, November 2002–March 2006

Soil depth (cm) Soil organic carbon stock (Mg C ha-1) SEM

T0 T1 M0 M1 N1 N2

0–5 11.8a 9.20b 10.2a 10.8a 10.3a 10.7a 0.29

5–10 11.2a 9.94b 10.4a 10.8a 10.4a 10.8a 0.28

10–15 11.8a 10.5b 11.1a 11.2a 11.1a 11.2a 0.29

15–30 21.6a 23.7a 22.2a 23.1a 21.8a 23.5a 0.80

30–50 23.6a 23.2a 22.9a 23.8a 22.6a 24.2a 1.21

0–50 82.2a 74.8b 77.1a 79.9a 76.5a 80.5a 2.03

Means followed by the same letter for a soil depth are not signifi-

cantly different at 5% level of significance by DMRT

T0 no-tillage, T1 conventional tillage, M0 no crop residue, M1 crop

residue at 4 Mg ha-1 for each crop in rotation, N1 recommended

practice of N management in rice and wheat, N2 leaf color chart based

N management in rice and recommended practice in wheat

Paddy Water Environ (2012) 10:95–102 99

123

corn biomass and removing silage over 30 years of study

period. Dexter et al. (2000) reported that incorporation or

removal of wheat straw had no effect on soil microbial

respiration from soil aggregates in rotation of wheat crop

with other small grains. Other researchers (Johnson and

Chamber 1996; Nicholson et al. 1997) also reported little or

no significant effect of cereal crop residue application on

SOC sequestration. The SOC in residue applied soil in this

study did not exceed 6% in any soil depths than in soils

without crop residue application. This corresponded to

0.14 Mg C ha-1 y-1 of SOC sequestration rate being the

highest at the 0–5 cm soil depth. From a similar study,

Aulakh et al. (2001b) attributed greater loss of carbon from

residue applied soils to increased soil microbial respiration

and carbon mineralization. They documented higher CO2

production during initial flooding after irrigation in rice and

wide C:N ratio in wheat supplied carbon for prolonged

period of time. Heavy rainfall (June–September 2004)

causing water stagnation and partial removal of wheat res-

idues (Fig. 3) also appears to have reduced the overall

effectiveness of residue application on SOC in this study.

Effect of N management

There was no statistical difference between the two N

management practices to sequester organic carbon in soils

(Table 3). Although not significant, a trend was apparent

suggesting that N management under LCC method could

result in higher SOC sequestration than under recom-

mended practice of N management for this cropping sys-

tem. The SOC content at 0–5, 5–10, 10–15, 15–30, and

30–50 cm soil depths were 3, 4, 1, 7, and 6%, respectively,

higher under LCC method than under recommended

method of N application in rice and wheat. The usefulness

of LCC method over recommended method of N applica-

tion is obvious from the fact that in the latter case N is

applied without considering the real need of the crop which

can reduce N use efficiency and increase losses from NH3

volatilization and/or microbial oxidation (Tiwari et al.

2000). This causes carbon mineralization resulting in low

organic carbon in the soil. Nitrogen application using LCC

method, on the other hand, might increase root and shoot

residue amounts added to soil and quality by supplying N

as per plant need, maintain higher equilibrium N and

thereby enhancing the total microbial biomass carbon in

the soil system (Duraisami et al. 2001).

Tillage and crop residue interaction effects

Interaction effect between tillage and crop residue was sig-

nificant at 0–5, 5–10, and 10–15 cm soil depths (Table 4).

No-tillage with crop residue application sequestered 34, 16,

and 21% higher SOC than under conventional tillage without

crop residue application at 0–5, 5–10, and 10–15 cm soil

depths, respectively. These corresponded to 0.73, 0.37, and

0.48 Mg C ha-1 y-1 of SOC sequestration rates at 0–5, 5–10,

and 10–15 cm soil depths, respectively. The SOC seques-

tered under no-tillage with crop residue application at

0–5 cm depth was 29% higher than under conventional

tillage with crop residue, and 17% higher than under

no-tillage without crop residue. These results suggest that

no-tillage with crop residue application would result in dis-

tinctly higher carbon sequestration at upper soil depths than

under other tillage and residue combinations considered in

this study. These SOC sequestration rates are in the range

(0.53 Mg C ha-1 y-1) reported by Franzluebbers (2005) for

no-tillage and cover cropping.

The results also partially corroborate with several pre-

vious studies (Six et al. 2002; West and Post 2002; Wright

and Hons 2005) that higher SOC sequestration might be

due to the role of crop residues, among others, in con-

serving soil moisture and protecting carbon from oxidation

and mineralization (Halvorson et al. 2002). The surface

applied resides provide opportunities for the buried root

residue to react with clay particles, organo-mineral com-

plexes and favor higher SOC sequestration (Blanco-Canqui

and Lal 2008). In a meta-analysis involving 69 paired-

experiments where sampling extended up to 60 cm, Luo

et al. (2010) noted 11% higher SOC by increasing cropping

frequencies and concluded that role of no-tillage is greatly

regulated by cropping system. In this study, single effect of

residue application was not significant but its significance

became apparent after its interaction with tillage system. It

means the effect of residue application was greatly modi-

fied by tillage system.

Table 4 Interaction effects of tillage and crop residue management

practices on soil organic carbon sequestration by Rampur soils,

November 2002–March 2006

Soil depths Soil organic carbon stock (Mg C ha-1) SEM

T1M0 T1M1 T0M0 T0M1

0–5 9.50c 9.90c 10.9b 12.7a 0.41

5–10 10.0b 9.87b 10.8ab 11.6a 0.40

10–15 10.1b 10.8b 11.3ab 12.2a 0.42

15–30 21.3a 21.9a 23.0a 24.3a 1.13

30–50 23.9a 22.3a 21.9a 25.3a 1.71

0–50 76.1b 73.4b 77.9ab 86.4a 2.87

Means followed by the same letter for a soil depth are not signifi-

cantly different at 5% level of significance as determined by DMRT

T1 conventional tillage, T0 no-tillage, M0 no crop residue, M1 crop

residue at 4 Mg ha-1 for each crop in rotation, SEM standard error of

mean

100 Paddy Water Environ (2012) 10:95–102

123

Results of SOC stock and carbon sequestration

rates in regional context

Rice–wheat cropping system has higher potential for car-

bon sequestration relative to other tropical ecosystems due

to slower decomposition during anaerobic rice-phase and

the higher input of biomass carbon by rice and wheat crops

in comparison with legumes and other row crops (Sahrawat

2004; Kukal et al. 2009). For 0–50 cm soil depth, the

maximum SOC stock values observed in this study

(\82 Mg ha-1) is approximately equal to one-half of the

SOC stock values as reported in the tropical forest soils of

Taiwan (Tsai et al. 2010). However, our SOC results are

similar to those observed by Tan et al. (2004) in Mollisol

Soil Order in Ohio, USA (169 Mg ha-1 for 0–100 cm soil

depth) and also similar to SOC stock values in rice fields in

Japan (93 Mg ha-1 for 0–50 cm soil depth) (Takata 2010).

In comparison with average carbon sequestration rate in

eight South Asian countries (\200 kg ha-1 y-1) reported

by Lal (2004b), carbon sequestration rate from residue

application alone in this study was low (140 kg ha-1 y-1)

but the sequestration rates resulting from tillage and resi-

due interactions ([480 kg ha-1 y-1) were high. As men-

tioned previously, the lower rate of sequestration from

application of residue alone would have occurred largely

due to increased microbial respiration and carbon miner-

alization. It would have been the consequence of residues

exposed to high temperatures for prolonged time on the soil

surface after crop harvest, particularly during summer

months. This part of crop rotation which has not yet been

thoroughly examined in the literature might also be an

important source of carbon emission and hence warrants

further research.

Conclusions

The results from this study and the literature review sug-

gest that the rice–wheat cropping system in Chitwan Valley

of Nepal presents greater benefits of carbon sequestration

from residue application in no-tillage system relative to

that in conventional tillage system. Low SOC in conven-

tional tillage system is likely due to greater carbon losses

through mineralization (although not measured) of applied

residues under the prevailing hot and humid weather con-

dition. However, it is promising that the rate of SOC

sequestration is much higher in 0–50 cm soil depth than the

reported average value for the South Asian region.

The single effect of crop residue and the statistical dif-

ferences between methods N application on SOC seem to

be insignificant but not suppressed. However, the interac-

tion effect highlights that SOC sequestration from residue

application is greatly regulated by types of tillage

treatments. More information is needed on the effects of

tillage, crop rotation, residue application, and soil vari-

ability on carbon input and output to further our under-

standing of the potential C sequestration in this rice–wheat

cropping system of Nepal.

Acknowledgments The authors are grateful to professor Dr. John

M. Duxbury and senior research associate Dr. Julie G. Lauren of the

Soil Management and Collaborative Research Support Program (SM-

CRSP) of Cornell University, USA for providing financial support

and the necessary technical guidance for this study.

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