dynamics of organic carbon and microbial biomass in ...nicra.iari.res.in/bidisha...

Environmental Monitoring and Assessment (2006) 119: 173–189

DOI: 10.1007/s10661-005-9021-8 c© Springer 2006

DYNAMICS OF ORGANIC CARBON AND MICROBIAL BIOMASSIN ALLUVIAL SOIL WITH TILLAGE AND AMENDMENTS

IN RICE-WHEAT SYSTEMS

B. BANERJEE1, P. K. AGGARWAL1, H. PATHAK2,∗, A. K. SINGH3

and A. CHAUDHARY1

1Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi 110 012,India; 2Unit of Simulation and Informatics, Indian Agricultural Research Institute,

New Delhi 110 012, India; 3Water Technology Center, Indian Agricultural Research Institute,New Delhi 110 012, India

(∗author for correspondence, Present address: Institut fur Meteorology und Klimaforschung,Kreuzeckbahnstr. 19 82467, Garmisch-Partenkirchen, Germany, e-mail: hpathak [email protected])

(Received 11 March 2005; accepted 22 August 2005)

Abstract. Rice-wheat cropping systems of the Indo-Gangetic plains (IGP) occupying 12 million ha

of productive land are important for the food security of South Asia. There are, however, concerns

that yield and factor productivity trends in these systems are declining/stagnating in recent years.

Decrease in soil organic carbon is often suggested as a reason for such trends. A field experiment was

conducted to study the soil organic carbon (SOC) and soil microbial biomass carbon (MBC) dynamics

in the rice-wheat systems. Use of organic amendments and puddling of soil before rice transplanting

increased SOC and MBC contents. Microbial biomass carbon showed a seasonal pattern. It was low

initially, reached its peak during the flowering stages in both rice and wheat and declined thereafter.

Microbial biomass carbon was linearly related to SOC in both rice and wheat indicating that SOC

could be used as a proxy for MBC.

Keywords: crop residue, farmyard manure, green manure, long-term experiment, reduced tillage, soil

fertility

1. Introduction

Rice-wheat cropping systems occupying about 12 million ha of productive land inthe Indo-Gangetic plain (IGP) is the backbone of food security of South Asia. In theIGP, the increased adoption of rice-wheat during the last four decades has resultedin a heavy usage of irrigation and inorganic fertilizer. In a recent analysis of 33 long-term experiments (LTE), Ladha et al. (2003a) observed that in the treatments whererecommended rates of N, P and K were applied, yields of rice and wheat stagnatedin 72 and 85% of the LTEs, respectively, while 22 and 6% of the LTEs showeda significant (P < 0.05) declining trend for rice and wheat yields, respectively.Concerns, therefore, are raised about the sustainability of this important productionsystem (Sinha et al., 1998; Aggarwal et al., 2000; Duxbury et al., 2000). Depletionof soil organic carbon (SOC) has been proposed to be one of the potential causes ofsuch yield decline (Ladha et al., 2003b; Pathak et al., 2003). In the major rice-wheat

174 B. BANERJEE ET AL.

regions of northwestern India the SOC has decreased from 0.5% in sixties to 0.2%at present (Sinha et al., 1998). No recycling of organic matter is the principal causeof such decline. In areas where rice crop is harvested manually, majority of theplant biomass is removed from the field and the straw is used as forage, fuel orbuilding material. In areas where harvesting is done with combine, north-westernIndia for example, rice straw is generally burnt before planting the next wheat crop(Samra et al., 2003; Pathak et al., 2004).

Soil organic matter, though usually comprising less than 5% of a soil’s weight,is one of the most important components of a field ecosystem. It serves as soilconditioner, nutrient source, substrate for microbial activity, preserver of the envi-ronment and major determinant for sustaining agricultural productivity (Schnitzer,1991). Changes in SOC may be difficult to monitor in the short term because of(1) low magnitude of change, (2) high background carbon levels and (3) high nat-ural variability of soils. The living and the most active part of SOC i.e., microbialbiomass carbon (MBC), rather than total amounts of SOC, therefore, has been sug-gested as a useful and more sensitive measure of a change in SOC status (Powlsonet al., 1987; Friedel et al., 1996). With a comparatively rapid rate of turnover of1–2 years, it is possible to detect changes in microbial fraction long before they aredetectable in the total organic matter (Jenkinson and Ladd, 1981). The MBC nor-mally comprises 1–3% of the total SOC. However, this percentage, the ‘microbialquotient’, has been reported to change in a consistent way and to provide a usefulindicator of the soil processes (Anderson and Domsch, 1989).

High temperature in the tropics and repeated tillage in the intensive croppingsystems such as rice-wheat are the major constraints in increasing or even main-taining the SOC. Research needs to be carried out to understand the SOC dynamics,particularly the dynamics of MBC to optimize the management practices so thatSOC is either enhanced or maintained. Agronomic management practices like useof organic amendments and reduced tillage can play important roles in increas-ing SOC and MBC compared to conventional tillage without affecting crop yields(Duxbury et al., 2000; Tullberg et al., 2001). The objectives of the present studywere to 1) evaluate the dynamics of SOC in rice-wheat cropping systems, 2) assessthe influence of tillage and organic amendments on SOC and MBC, and 3) deter-mine the relationship between SOC and MBC in rice-wheat cropping systems inan alluvial soil.

2. Experimental Details

2.1. EXPERIMENTAL SITE AND SOIL

A field experiment was conducted in rice-wheat cropping systems for two years(2001–2003) in Indian Agricultural Research Institute, New Delhi research farmfor monitoring SOC and MBC. New Delhi is located at 28◦40′N and 77◦12′E at an

DYNAMICS OF SOIL ORGANIC C IN RICE-WHEAT SYSTEM 175

altitude of 228 m above mean sea level. The climate is subtropical semiarid withhot and dry summers and cold winters. The area receives 750 mm annual rainfall,about 80% of which occurs from June to September. The mean annual maximumtemperature is 35 ◦C while the mean minimum temperature is 18 ◦C.

The alluvial soil of experimental site was Typic Haplustept, well drained with pH8.0 and silty clay loam in texture. The soil had electrical conductivity 0.78 d S m−1,bulk density 1.4 g cm−3, hydraulic conductivity 4.8 cm day−1, cation exchange ca-pacity 7.3 cmol kg−1, organic carbon 0.62%, and available N, P and K 250, 56 and406 kg ha−1, respectively. The groundwater table at the site was 6.6 and 10 m deepduring the rainy and summer seasons, respectively.

2.2. TREATMENTS AND CROP MANAGEMENT

The experiment was laid out in a split-plot design. The main plots comprised of twotillage treatments viz. puddled and non-puddled in rice crop. The subplots included7 treatments with 3 replications in plots of 7.50 m long and 5.25 m wide (Table I).Rice (cultivars Pusa 44 and Pusa Sugandh 2 in 2001 and 2002, respectively) wastransplanted in the puddled plots at 20 by 15 cm spacing. In the non-puddled plotsrice was directly sown. Nitrogen as urea was applied at 120 kg N ha−1 in three splits

TABLE I

Treatments

Treatment Details

N source

Control (no N) No N but P and K at 26 and 40 kg ha−1, respectively

100% NPK N, P and K at 120, 26 and 40 kg ha−1 through urea, SSP

and KCl, respectively,

100% NPK (25% substituted by

FYM)

N, P and K at 90, 26 and 40 kg ha−1 through urea, SSP and

KCl, respectively, plus 30 kg N ha−1 through FYM

100% NPK + Green manure N, P and K at 120, 26 and 40 kg ha−1 through urea, SSP and

KCl, respectively, plus green manure at 2 Mg ha−1

100% NPK + Crop residues N, P and K at 120, 26 and 40 kg ha−1 through urea, SSP and

KCl, respectively, plus residue of the previous crop

100% organic source (50% FYM

+ 25% Biofertilizer + 25%

Green manure)

N, P and K at 120, 26 and 40 kg ha−1, respectively, through

FYM, biofertilizer and green manure

Blank plot No crop no fertilizer

Tillage

Puddled rice Rice transplanted after conventional puddling the field

Non-puddled rice Rice was directly seeded after ploughing the field

Tilled wheat Wheat was sown after conventional ploughing

No-tilled wheat Wheat was sown with zero tillage


of 60, 30 and 30 kg N ha−1 at 11, 30 and 60 days after transplanting (DAT) of rice.Farmyard manure (FYM) was incorporated in soil 2 weeks before transplanting at6 Mg ha−1. On dry weight basis it contained 0.52% N, 0.38% P and 0.42% K. Greenmanure Subabool (Leucaena leucocephala) was incorporated in soil at 2 Mg ha−1

before transplanting rice. A carrier based culture of Azospirillum spp. CD strainwas used as biofertilizer at the time of sowing. Crop residues of the previous cropwere incorporated before sowing in the treatment with crop residues. Soil moistureregime was maintained at saturation throughout the cropping period and irrigationwas provided as and when required.

After rice each subplot was divided into two parts for wheat. In one part tillagewas done and in another with no tillage. So the design changed from split-plot inrice to split-split-plot in wheat (Table I). Wheat (cultivar HD 2687) was sown inrows 22.5 cm apart. Urea was applied at 120 kg N ha−1 in three splits of 60, 30 and30 kg N ha−1 at 0, 25 and 65 days after sowing (DAS) of wheat. Five irrigationsof ±6 cm were applied at 24, 64, 80, 97 and 117 DAS. Green manure, farmyardManure (FYM), crop residue and biofertilizer were applied in the same way asin rice.

2.3. SOIL SAMPLING AND ANALYSES

Soil samples from 0–15 cm layer were collected from 3 locations in each plot atdifferent crop growth stages (transplanting/sowing, tillering, flowering and harvest-ing). The entire volume of soil was mixed thoroughly and subsamples were usedfor analyses. Soil samples were air-dried for 7 days, sieved through 0.2 mm screen,mixed and stored in sealed plastic jars for further analysis. Representative subsam-ples were drawn to determine various physico-chemical properties using standardprocedures (Page et al., 1982).

2.4. ESTIMATION OF SOIL MBC

Fresh soil was used for determining the MBC using a modified chloroform fumi-gation extraction method (Witt et al., 2000). Soils were fumigated with chloroformand kept in dark for 24 h at 25 ◦C. Organic carbon in fumigated and unfumigatedsoil was extracted with 0.5 M K2SO4 solutions. Carbon in soil extracts was deter-mined using a dichromate method. The extract was digested at 150 ◦C for 30 minalong with 0.07N K2Cr2O7, 98% H2SO4 and 88% H3PO4. After cooling the soilextract was titrated with 0.01 N Fe(NH4)2(SO4)2 in 0.4 M H2SO4. Soil MBC wascalculated using the following equation:

MBC = (Ecf − Ecu)/Kc (1)

where Ecf is organic carbon extracted from fumigated soil and Ecu is organic carbonextracted from unfumigated soil. The value of Kc was taken as 0.45 (Witt et al.,


2000).

2.5. DATA ANALYSIS

Statistical analysis of the data were performed using SAS (ver. 8) statistical packagedeveloped by SAS Institute Inc.

3. Results and Discussion

3.1. EFFECT OF ORGANIC AMENDMENTS AND TILLAGE ON SOC IN RICE

During both the years, application of fertilizer or organic amendments significantlyaffected the organic carbon content of soil (Table II). Better crop growth and moreroot biomass production contributed to larger SOC in the fertilized as well as organ-ically amended plots. Soil amendment with FYM recorded the largest SOC content(0.7% and 0.75% in 2001 and 2002, respectively) in the puddled plots. Halvorsonet al. (1999) observed that increase in SOC with N application reflected the re-sponse of crop biomass to added N. More biomass production also led to greateramount of root exudation, which also increased the SOC content. Treatments withFYM and green manure recorded higher SOC as compared to other treatments dueto addition of 1286 and 972 kg C ha−1, respectively. It has been reported that or-ganic sources like FYM, green manure, crop residues and biofertilizers decomposeslowly resulting in organic carbon accumulation in soil (Sharma et al., 2001). Ex-periments conducted in Punjab, India in the rice-wheat cropping systems showed

TABLE II

Status of soil organic carbon at rice harvest with tillage and organic amendments

Soil organic carbon (%)

Rice 2001 Rice 2002 Mean

Non- Non- Non-

Treatments Puddled puddled Puddled puddled Puddled puddled

Control (no N) 0.60 0.55 0.64 0.59 0.62 0.57

100% NPK 0.67 0.57 0.7 0.60 0.69 0.59

75% NPK + 25% N by FYM 0.70 0.58 0.75 0.62 0.73 0.6

100% NPK + Green manure 0.70 0.58 0.70 0.6 0.7 0.59

100% NPK + Crop residues 0.66 0.56 0.69 0.62 0.68 0.59

100% organic source 0.68 0.56 0.68 0.61 0.68 0.59

Blank plot 0.68 0.56 0.66 0.58 0.67 0.57

LSD (P < 0.05) Fertilizer – 0.03 Fertilizer – 0.03

Puddling – 0.04 Puddling – 0.01


that the incorporation of crop residues increased SOC compared to their removalfrom field (Yadvinder-Singh et al., 2000).

The treatments with puddling had higher SOC than the non-puddled treatmentsin both the years of the experiment. Puddling i.e., the repeated tillage of saturated(submerged) soil, has considerable effects on the physical, chemical and biolog-ical properties of soil that influences microbial growth. It softens soil, promotesroot growth and reduces water and nutrient losses by leaching, thereby increas-ing nutrient and organic carbon availability (Sharma et al., 2003). Availability oforganic carbon may also be increased by the mechanical break down of organicmatter due to repeated tillage of the inundated soil. Moreover, spontaneous growthof photo-dependent and free-living blue green algae (BGA) is a basic feature inthe puddled wetland rice fields. Growth of these microbes and subsequent decom-position of their biomass contribute to larger SOC in puddled rice soil comparedto the non-puddled soil. A favourable temperature in the puddled soil compared tothe aerobic soils might also have contributed towards enhanced microbial activityand larger SOC (Gajri and Majumdar, 2002). Moreover, slower decomposition oforganic matter in puddled soil under anaerobic condition reduced loss of carbon asCO2 resulting in larger organic carbon accumulation.

3.2. EFFECT OF ORGANIC AMENDMENTS AND TILLAGE ON SOC IN WHEAT

Like rice, impact of fertilizer and manure application on SOC was observed inwheat also (Table III). Unfertilized plots recorded less SOC compared to the fertil-ized plots. Application of FYM and green manure increased SOC by 7.8%, 10.9%,4.7% and 6.3% than the unfertilized plot at the time of harvesting of wheat inthe year 2001–2002. Rasmussen and Rohde (1988) reported that N fertilizationin winter wheat increased SOC. Tillage treatments (i.e., tilled or no tilled) beforesowing wheat, however, did not affect SOC content. This is contrary to the re-ports that zero tillage improves SOC (Dick et al., 1998; Allmaras et al., 1999; Lal,1999). We argue that tillage practices will have no significant effect on SOC in theshort term i.e., 2–3 years. This is more so in the tropical regions where tempera-ture is high. The increase in SOC might take place after a long period of time ifapplication of organic amendments is continued. As our experiment is still con-tinuing the future results will show the impact of tillage on SOC in a long-termbasis.

3.3. TRENDS OF SOC IN RICE-WHEAT CROPPING SYSTEM

In both puddled-tilled and puddled-no-tilled site SOC increased after transplantingand high values were observed during 364–475 days (Figures 1a and 1b). Thisincrease in SOC was clearly evident in the FYM and green manure treatments.It is also evident that organic carbon content of surface soil slightly increased


TABLE III

Status of soil organic carbon at wheat harvest with tillage and organic amendments

Soil organic carbon (%)

Wheat 2001 Wheat 2002 Mean

Treatments Tilled No-tilled Tilled No-tilled Tilled No-tilled

After puddled rice

Control (no N) 0.64 0.65 0.49 0.56 0.57 0.61

100% NPK 0.64 0.67 0.61 0.61 0.63 0.64

75% NPK + 25% N by FYM 0.71 0.67 0.64 0.66 0.68 0.67

100% NPK + Green manure 0.68 0.71 0.60 0.66 0.64 0.69


100% organic source 0.71 0.70 0.64 0.66 0.68 0.68

Blank plot 0.68 0.63 0.54 0.57 0.61 0.60

After non-puddled rice

Control (no N) 0.64 0.64 0.59 0.6 0.62 0.62

100% NPK 0.66 0.66 0.63 0.63 0.65 0.65

75% NPK + 25% N by FYM 0.76 0.67 0.64 0.66 0.70 0.67

100% NPK + Green manure 0.68 0.68 0.65 0.66 0.67 0.67


100% organic source 0.71 0.67 0.61 0.65 0.66 0.66

Blank plot 0.68 0.65 0.57 0.58 0.63 0.62

LSD (P < 0.05) Fertilizer – 0.02 Fertilizer – 0.04

Tillage – NS Tillage – NS

Puddling – NS Puddling – NS

during the growth of the second rice crop. The maximum value of SOC was 0.81%in the FYM treatment. Similar results were reported by Rekhi et al. (2000). Innon-puddled soil organic carbon was maximum (0.76%) at 292 DAT in the FYMtreatment (Figure 2a). This coincided with the harvesting of the first wheat crop.In this treatment SOC increased during the growth of the first wheat crop, then itslightly decreased and again it showed increasing trend during the second wheatcrop. Similar trend was also observed in non-puddled no-tilled soil (Figure 2b). Butin this case maximum value was 0.69% at 259 DAT in the green manure and 100%organic source treatments.

3.4. EFFECT OF ORGANIC AMENDMENTS AND TILLAGE ON MBC IN RICE

Fertilizer and tillage treatments significantly affected MBC content of soil in boththe years (Table IV). In the year 2001, MBC was maximum in FYM (202 mg kg−1)


Figure 1. Soil organic carbon (SOC) content in (a) puddled-transplanted rice and tilled wheat and (b)

puddled-transplanted rice and no-tilled wheat.

and green manure (177 mg kg−1) treated plots in puddled soils. Fertilizer appli-cation also enhanced MBC content of soil. However, organic amendments suchas FYM and green manure were superior to 100% NPK and crop residue treat-ments in terms of soil MBC. They increased MBC by 19% and 4%, respectively,compared to 100% NPK treatment in puddled soil while in non-puddled soil thecorresponding increases were 6% and 9% respectively (Table IV). Puddling ofsoil before rice transplanting significantly increased the MBC of soil. Blank plotrecorded significantly less MBC than fertilized plots. Maximum (176 mg kg−1) soilMBC was found in puddled no-tilled soil (Figure 3). In the year 2002 also MBCcontent was higher in the amended and puddled soils than the unamended and non-puddled soil. Addition of carbon to soil through FYM, green manure, crop residuesand biofertilizer stimulated microbial activity, which increased the MBC contentof soils in these amended plots. Gunapala and Scow (1998) observed that MBC


Figure 2. Soil organic carbon (SOC) content in (a) non-puddled direct seeded rice and tilled wheat

and (b) non-puddled direct seeded rice and no-tilled wheat.

of soil was higher in the organic farming systems compared to the conventionalfarming because of addition of organic carbon. Application of carbon through cropresidue resulted in higher microbial biomass than control but it was less comparedto other fertilizer treatments because of high C:N of residues, which slows downthe mineralization process. Microbes take longer time to decompose these residuesand bring the nutrients in available form. Besides most of the residues remain onthe soil surface where biological activity is less (Stewart, 1993). However, fertil-izer application lead to increased microbial activity in surface soil (Hossain et al.,1995).

Puddling had considerable effects on microbial growth. Submerged conditionand the subsequent puddling of soil in rice provide a favourable environment for thegrowth of microbes, particularly soil bacteria (Roger, 1996) resulting in higher MBC


TABLE IV

Status of soil microbial biomass carbon with tillage and organic amendments

Microbial biomass carbon (mg kg−1)

Rice 2001 Rice 2002 Mean

Non- Non- Non-

Treatments Puddled puddled Puddled puddled Puddled puddled

Control (no N) 160 137 172 161 166 149

100% NPK 170 144 182 171 176 157

75% NPK + 25% N by FYM 202 153 212 181 207 167

100% NPK + Green manure 177 157 206 168 192 163

100% NPK + Crop residues 169 129 172 175 171 152

100% organic source 178 152 181 166 179 159

Blank plot 168 133 164 162 166 148

LSD (P < 0.05) Fertilizer – 11 Fertilizer – 15

Puddling – 15 Puddling – NS

Figure 3. Effect of puddling and tillage practices on soil organic carbon (SOC) and soil microbial

biomass carbon (MBC).


TABLE V

Status of soil microbial biomass carbon at wheat harvest with tillage and organic amendments

Microbial biomass carbon (mg kg−1)

Wheat 2001 Wheat 2002 Mean

Treatment Tilled No tilled Tilled No tilled Tilled No tilled

After puddled rice

Control (no N) 151 162 151 151 151 156

100% NPK 180 171 161 168 170 169

75% NPK + 25% N by FYM 178 185 178 174 178 180

100% NPK + Green manure 183 179 169 170 176 174


100% organic source 170 172 175 176 173 174

Blank plot 155 167 145 155 150 161

After non-puddled rice

Control (no N) 158 162 148 152 153 157

100% NPK 173 174 158 169 165 171

75% NPK + 25% N by FYM 174 186 174 170 174 178

100% NPK + Green manure 180 179 174 165 177 172


100% organic source 174 175 171 171 172 173

Blank plot 163 166 144 151 153 158

LSD (P < 0.05) Fertilizer – 5 Fertilizer – 6

Tillage – NS Tillage – NS

Puddling – NS Puddling – 3

in soil. A favourable temperature in the puddled soil compared to the aerobic soils(Gajri and Majumdar, 2002) also contribute towards enhanced microbial activityand higher MBC in soil.

3.5. EFFECT OF ORGANIC AMENDMENTS AND TILLAGE ON MBC IN WHEAT

Fertilizer and organic amendments had significant effect on MBC in wheat (Ta-ble V). In the year 2001 the maximum value of MBC was recorded in the FYMtreatment (185 mg kg−1) in no tilled plot followed by the green manure treatment(183 mg kg−1) in tilled plot. Plots receiving crop residues showed significant in-crease in soil MBC compared to the control.

In 2002 also fertilizer application significantly increased soil MBC. MaximumMBC (178 mg kg−1) was in the FYM treatment in tilled soil while the value was176 mg kg−1 in 100% organic source treated no-tilled soil (Table V). Similar to the2001 wheat crop MBC content in soil was highest (164 mg kg−1) in no-tilled crop


residue treatment (Table V). In this wheat crop also tillage treatments had no effecton MBC.

3.6. TRENDS OF MBC IN RICE-WHEAT CROPPING SYSTEM

Two conditions of rice establishment, i.e., puddled and non-pudddled direct seeded,differed in terms of trends in MBC content in soil in the rice-wheat systems. In caseof puddled, transplanted rice followed by either tilled or no tilled wheat, the MBCremained unchanged during the two years of the cropping period (Figures 4a and4b). But in case of non-puddled, direct seeded rice followed by either tilled orno tilled wheat, there was increase in MBC (Figures 5a and 5b). In direct seededrice-wheat system, though initially MBC was much lower than that of transplantedrice-wheat system, after two years of cropping MBC became on par. This suggestedthat puddling had initial advantage in terms of higher MBC and the non-puddledrice system had a lag phase up to 2 years to build up the microbial biomass.

Figure 4. Microbial biomass carbon (MBC) content of soil in (a) puddled-transplanted rice and tilled

wheat and (b) puddle-transplanted rice and no-tilled wheat.


Figure 5. Microbial biomass carbon (MBC) content of soil in (a) non-puddled direct seeded rice and

tilled wheat and (b) nonpuddled direct seeded rice and no-tilled wheat.

3.7. RELATIONSHIP BETWEEN SOC AND MBC

Microbial biomass carbon is a good indicator of soil quality. However, it is dif-ficult and expensive to measure routinely. Therefore, it will be useful to developa relationship between MBC and SOC, which is easy to measure and routinelydetermined in the soil-testing laboratories all over the world. In the present studythe following relationships between MBC and SOC were established.

Relationship between MBC (mg kg−1) and SOC (%) in rice (Figure 6a):

MBC = 317.1 SOC − 27.7 (2)

Relationship in wheat (Figure 6b):

MBC = 201.3 SOC + 44.3 (3)


Figure 6. Relationship between soil organic carbon (SOC) and soil microbial biomass carbon (MBC)

in (a) rice, (b) wheat and (c) rice-wheat systems.

Relationship using the pooled data for rice and wheat (Figure 6c):

MBC = 253.3 SOC + 11.3 (4)

Several researchers (Jenkinson and Ladd, 1981; Leita et al., 1999) earlier showedthat there is a significant linear relationship between MBC and SOC in temperateenvironments. Our results showed that similar relationships exist in tropical envi-ronments as well where the SOC levels are very low. The 95% confidence interval


TABLE VI

Confidence interval (95%) of the regression equations showing the relationship between soil organic

C and microbial biomass C in rice-wheat cropping systems

Intercept Slope

Coefficient Lower 95% Upper 95% Coefficient Lower 95% Upper 95%

Rice −28 −50 −6 317 282 352

Wheat 44 25 63 201 172 231

Rice-Wheat 11 −3 26 253 231 276

of the intercepts ranged from −50 to −6 in Equation (2), 25 to 63 in Equation (3)and −3 to 26 in Equation (4) (Table VI). This showed that there was no overlap-ping between the intercept values in the above equations. Similarly in case of slopealso the ranges were 282 to 352; 172 to 231 and 231 to 276 in Equations (2)–(4),respectively (Table VI). Thus the relationships between SOC and MBC were dif-ferent for rice and wheat crops and also the rice-wheat cropping systems. Thisproved that change in MBC with SOC depended on crop type and cropping se-quence. Such relationships are, however, likely to be affected by several otherfactors such as soil type, temperature and pH of the soil, which needs furtherinvestigation.

4. Conclusion

Adoption of intensive agricultural practices has led to the depletion of soil nutrientsin many productive land use systems such as the rice-wheat cropping systems ofsouth Asia. Therefore, there is a need to manage SOC for sustainability of theseland use systems. The study showed that application of organic matter in rice-wheatsystems in the northwestern IGP resulted in increased SOC. Soil MBC was alsohigher in organic amended plots compared to unamended plots. A linear relationshipexisted between SOC and MBC, therefore, SOC can be used as a proxy indicatorfor MBC in the rice-wheat systems of the IGP. Further studies should be carried outin different locations of the IGP to find out the effect of agronomic managementpractices on SOC and MBC dynamics.

Acknowledgments

The first author is grateful to the Post Graduate School, Indian Agricultural ResearchInstitute, New Delhi and University Grants Commission for providing fellowshipduring the course of the study.


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