Soil organic carbon (SOC)

8/10/2019 Soil organic carbon (SOC)

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SSSAJ: Volume 72: Number 3 MayJune 2008

Soil Sci. Soc. Am. J. 72:775785

doi:10.2136/sssaj2006.0378

Received 3 Nov. 2006.

*Corresponding author ([email protected]).

Soil Science Society of America

677 S. Segoe Rd. Madison WI 53711 USA

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Information on organic C stocks in agricultural soils is impor-tant because of the effects of SOC on climate change and oncrop production. The SOC stock at any time reflects the long-term balance between additions of organic C from differentsources and its losses through different pathways. Followingthe adoption of large-scale intensive cropping, this long-termbalance was modified since intensive cropping encourages oxi-dative losses of C due to continued soil disturbance, while italso leads to a large-scale addition of C to the soil through crop

residues. This may cause either a net buildup or a net depletionof SOC stock (Cole et al., 1993; Rasmussen et al., 1980; Konget al., 2005).

Cropping systems and management practices that ensuregreater amounts of crop residue returned to the soil areexpected to cause a net buildup of the SOC stock. Identifyingsuch systems or practices is a priority for sustaining crop pro-ductivity. To better understand the mechanisms by which Cis lost or stabilized in soil, the SOC stock is separated intoa labile or actively cycling pool, a slow pool, and a stable orpassive, recalcitrant pool with varying residence times (Partonand Rasmussen, 1994). The labile C pool is the fraction ofSOC with the most rapid turnover rates. On oxidation, thispool of SOC adds to the loading of CO2to the atmosphereto accentuate the process of global warming. At the same time,this pool is important from the point of view of crop produc-tion. It fuels the soil food web and therefore greatly influencesnutrient cycling for maintaining soil quality and its productiv-ity (Chan et al., 2001; Janzen, 1987; Majumder, 2007). Thispool is also sensitive to land management changes. The highlyrecalcitrant or passive pool is, on the other hand, altered only

Bidisha Majumder*

Jadavpur Univ.Kolkata, West Bengal 700 032

India

Biswapati MandalP. K. BandyopadhyayDirectorate of Research

Bidhan Chandra Krishi Viswavidyalaya

Kalyani, West Bengal 741 235

India

A. GangopadhyayJadavpur Univ.

Kolkata, West Bengal 700 032

India

P. K. ManiA. L. KunduDirectorate of Research


Kalyani, West Bengal 741 235

India

D. MazumdarDep. of Agril. Statistics


Mohanpur, West Bengal 741 252

India

Organic Amendments Influence Soil OrganicCarbon Pools and RiceWheat Productivity

Soil organic C (SOC) pools under long-term management practices provide information onC sequestration pathways, soil quality maintenance, and crop productivity. Farmyard manure(FYM), paddy straw (PS), and green manure (GM) along with inorganic fertilizers were usedin a 19-yr-old rice (Oryza sativaL.)wheat (Triticum aestivumL.) cropping system in sub-tropical India to evaluate their impact on SOC stock, its different poolstotal organic C(Ctot); oxidizable organic C (Coc) and its four fractions of very labile (Cfrac1), labile (Cfrac2),less labile (Cfrac3), and nonlabile C (Cfrac4); microbial biomass C (Cmic); and mineralizableC (Cmin). Cropping with only NPK fertilization just maintained SOC content, whileNPK plus organics increased SOC by 24.3% over the control, their relative efficacy beingFYM > PS > GM. A minimum of 3.56 Mg C ha1yr1was required to be added as organicamendments to compensate for SOC loss from cropping. The passive (Cfrac3+ Cfrac4) pooland Cmin constituted about 39 and 11.5%, respectively, of Ctot. Organics contributedtoward the passive pool in the order FYM > PS > GM. Most of the pools were significantly(P= 0.005) correlated with each other. Yield and sustainable yield index were strongly relatedwith Cfrac1, Coc, Cmic, and Cmin. Results suggest Cfrac1as a useful indicator for assessingsoil health, and balanced fertilization with FYM as suitable management for sustaining cropproductivity of the ricewheat system.

Abbreviations: BSR, basal soil respiration; Cfrac1, very labile carbon; Cfrac2, labile carbon; Cfrac3, less labile carbon;Cfrac4, nonlabile carbon; Cmic, microbial biomass carbon; Cmin, mineralizable carbon; Coc, oxidizable organic

carbon; Ctot, total organic carbon; FYM, farmyard manure; GM, green manure; MQ, microbial quotient; PS,

paddy straw; qCO2, respiratory quotient; SOC, soil organic carbon; SYI, sustainable yield index.


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776 SSSAJ: Volume 72: Number 3 MayJune 2008

very slowly by microbial activities and hence hardly serves as agood indicator for assessing soil quality and productivity (Weilet al., 2003; Sherrod et al., 2005; Majumder, 2007).

Most of the conventional methods used in SOC determi-nation aim to maximize oxidation of C (Walkley and Black,1934; Nelson and Sommers, 1982). This approach may notbe useful for assessing different management practices regard-ing their effect on the quality of a system for sustainable cropproduction, since in many cases it has been observed that totalSOC failed to serve as a sensitive indicator for such an assess-ment (Chan et al., 2001). The adoption of procedures that canpreferentially separate the more labile pools might be a usefulapproach for characterizing SOC and assessing the quality ofa system under different management practices. Several tech-niques based on chemical, physical, and biological principlesare used for partitioning labile pools of SOC (McLauchlan andHobbie, 2004). Some of the commonly used techniques are:oxidation under a gradient of oxidizing conditions (Walkley,1947; Blair et al., 1995; Chan et al., 2001), floatation with adense liquid (Gregorich and Janzen, 1996), sieving into differ-ent size class separates and their associated C (Six et al., 1998),mineralization under controlled temperature and moisture

(Pastor et al., 1993), and direct measurement of the pool size ofliving soil organisms and microbial biomass (Paul et al., 1999).Some of the important labile pools of SOC currently used as indica-tors of soil quality are microbial biomass C (Cmic), mineralizable C(Cmin), oxidizable organic C (Coc) fractions, and light-fraction C.

The ricewheat cropping system occupies about 13.5 mil-lion ha in the Indo-Gangetic Plains of South Asia and providesfood for 400 million people (Ladha et al., 2003). The cropsare grown with adequate amounts of fertilizer and differentorganics, as available. Several reports, however, have indicateda widespread declining or stagnating trend of yields in thelong-term ricewheat cropping system in Asia. A decrease inSOC has been identified as the major cause for this (Swarup

et al., 2000; Yadav et al., 2000; Ladha et al., 2003). To offsetsuch a decrease (in SOC), different organic amendments suchas manure (farmyard manure or green manure), compost, andcrop residues (particularly rice straw) are commonly recom-mended. When applied, a part of their C is stabilized into SOCand distributed among different pools. This process is governedby an interplay of factors including climate, substrate biochem-istry, C loading, soil, associated precinct, and so on. As such,the different organic amendments are likely to differentiallyaffect the amount of C stabilized and the size and dynamics ofSOC pools and ultimately crop productivity. Accordingly, it ishypothesized that the long-term ricewheat cropping system

with balanced fertilization in combination with or without dif-

ferent organic amendments may influence SOC content, thesize of its active or labile pools, and hence the soil quality andsustainable productivity of the system.

A number of studies have been done on the changes inlabile pools of SOC under different soil management prac-tices, but most of them are incomplete in the sense that theyincluded only one or two or at best three labile pools of SOC.Furthermore, they have been confined to the cooler, temperateregions of the world (Wu et al., 2003; Sherrod et al., 2005).Only a very few such studies have been done so far in tropicaland subtropical regions of the world (Rudrappa et al., 2006),

although these studies are of special importance in the regionsince the organic C stock of soils here is inherently low. Thislow C stock poses a serious threat to soil health and thus long-term sustainable crop production in the region (Mandal, 2005;Sharma et al., 2005). In India, a series of long-term fertilityexperiments was started, using both organic and inorganicsources of nutrients, during the late 1960s and early 1970s

when fertilizer-responsive high-yielding cultivars of crops wereintroduced. Productivity analysis, nutrient balance, and soilquality studies were made for some of these experiments (Singhet al., 2004; Chaudhury et al., 2005; Mandal, 2005; Sharmaet al., 2005), but no attempt was made to study the impact oflong-term continuous cropping using balanced fertilizer withor without different organic amendments on the SOC stockand dynamics of its various pools in relation to crop productiv-ity. In the present study, an attempt has been made toward thisend using a 19-yr-old experiment with a ricewheat croppingsystem on an Inceptisol located in the hot, humid, subtropicalregion of India with an ultimate aim of identifying a manage-ment practice for the system that would improve pools of SOCstock and, at the same time, crop productivity.

MATERIALS AND METHODSSite DescriptionA long-term field experiment with a ricewheat cropping system

on an Inceptisol located in the hot, humid subtropics was established at

the University Teaching Farm, Bidhan Chandra Krishi Viswavidyalaya,

West Bengal, India (23 N, 89 E, 9.5 m above sea level), in 1986

along the new alluvial soil zone of the state. The experimental site

receives an average annual rainfall of approximately 1480 mm and

experiences mean annual minimum and maximum temperatures of

12.5 and 36.2C, respectively. The soil is sandy loam (hyperthermic

Aeric Haplaquept according to U.S. Soil Taxonomy), with pH 7.2,

and contains 500, 295, and 205 g kg1of sand, silt, and clay, respec-

tively. The initial surface soil (00.2 m) of the experimental site had

total (estimated from an archaic sample) and oxidizable (Walkley andBlack, 1934) organic C of 14.2 and 8.8 g kg1, respectively, CaCO3

equivalent of 1.0 g kg1, bulk density of 1.2 Mg m3, and cation

exchange capacity of 22.0 cmolckg1. Because of high rainfall, a rice-

based cropping system predominates in this region.

Treatments and Crop ManagementTwo crops, rice and wheat, were grown annually with necessary

plowing, on average, to a depth of 0.15 to 0.20 m using a power t iller.

The experiment was laid out in a randomized block design with the

following treatments: (i) control (no NPK fertilizers or organics),

(ii) inorganic NPK fertilizer, (iii) NPK + FYM, (iv) NPK +

PS, (v) NPK + GM, and (vi) a fallow where no crop was grown

since the initiation of the experiment. Each treatment was replicated

four times. Farmyard manure, PS, and GM were used as organics. For

green manuring, Sesbania sesban(L.) Merr. was grown in an adjacent

field, and the aboveground biomass of the 40- to 45-d-old crop was

collected and chopped into small sizes. This GM and well-decom-

posed FYM were manually spread uniformly on the surface of the

specified plots (size 8 by 8 m) at 8.0 and 7.5 Mg ha1, respectively, on

a wet-weight basis and mixed thoroughly with the soil using a power

tiller 2 to 5 d before puddling (i.e., breaking down of soil aggregates

through tillage under submergence to create a soft layer at the top for

easy transplanting of rice seedlings and an impervious layer below


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for curbing leaching). For using PS as an organic, a good amount of it

was collected after threshing, chopped, and kept in a pit with adequate

moisture (1:0.8 PS/water) for 4 to 5 mo to partially decompose it, and

thereafter it was similarly applied at 10.0 Mg ha1on a wet-weight basis

and mixed well with the soil using a power tiller 3 wk before puddling.

Native vegetation (shrubs, herbs, grasses, etc.) of the site was

allowed to grow freely in the fallow plots without any inputs (fertil-

izer, water, or plant protection chemicals). During land preparation

for transplanting rice, the native vegetation was cut and incorporated

into the soil through a shallow plowing (0.10-m depth) so that it was

not blown away by the wind. As such, the fallow plots experienced

little stresses from cultivation and maintained a good amount of labile

SOC and microbial activities. These plots were a part of the original

experimental design maintained throughout the experimental period

(19 yr) for assessing the relative ability of different treatments for their

aggradingdegrading effects on soil quality and also for accommodat-

ing any additional treatments of relevance, if required, in the future

into this long-term experiment.

The remaining plots were kept flooded with water for a day, then

puddled (0.20-m depth) with the power tiller, followed by a ladder-

ing for leveling the land. One week thereafter, on average, 30-d-old

rice seedlings were transplanted in July with a spacing of 0.20 by

0.20 m. After the rice was harvested in the last week of October, landpreparation (plowing and laddering) was done and wheat was sown

in the second week of November with a spacing of 0.2 m between

rows, and harvested in April. The NPK fertilizers, at 1206060

(NP2O5K2O) for rice and 1006040 (NP2O5K2O) for wheat

as recommended by the State Agricultural Department for those crops

on the basis of average soil fertility indices of the region, were applied

in the form of urea, single superphosphate, and muriate of potash

following standard schedules uniformly throughout the experiment.

Their amounts in the NPK plus organic treatments were adjusted,

however, making allowance for the nutrients contained in the added

organics, which were estimated annually following standard meth-

ods described by Page et al. (1982). Other recommended practices

for raising the crops such as weeding, irrigation, and plant protec-tion measures were followed. All the aboveground biomass for the

crops was harvested (leaving 57-cm short stalks on the ground) on a

whole-plot basis manually by a sickle and removed from the field for

mechanical threshing by a paddle thresher. Grain and straw yield for

both crops on a whole-plot (8- by 8-m) basis were measured and then

expressed in kilograms per hectare.

Crop Residue and Organic-Derived CarbonInputs into Soil

After harvesting the crops, representative samples of the leftover

stubble (the short stalks left after harvest) and roots were collected

from each of the four replicated plots of the different treatments for

the last 5 yr (since 2000), processed, and analyzed for their C content.

Sampling for the roots was done following the method described by

Thangaraj and OToole (1986). The total biomass yield of stubble,

roots, and rhizodeposition were computed as 190, 25, and 150 g kg1,

respectively, for rice and 220, 30, and 126 g kg1, respectively, for

wheat of the total aboveground biomass harvested at maturity for the

38 ricewheat crops during the 19 yr of the experiment (Bronson et

al., 1998). The cumulative crop residue (stubble, roots, and rhizode-

position) C inputs into the soils were then calculated by multiplying

the above total biomass amount of stubble, roots, and rhizodeposition

by their respective mean C concentrations measured annually during

the last 5 yr. The extra C input into the soil through photosynthetic

aquatic organisms of the rice field was also estimated following Saito

and Watanabe (1978).

The cumulative C inputs into the soil through organics (FYM,

GM, and PS) were similarly computed by multiplying the total dry

weight of the organics added during the 19-yr period of the experiment

with their respective mean C concentrations measured during the last

5 yr. The biochemical composition of the organics was also determined

for the last 5 yr following the methods described by Rahn et al. (1999).

Soil Sampling and AnalysisThree representative field-moist soil samples were collected from

each of the plots in each replication from 0- to 0.2-, 0.2- to 0.4-, and

0.4- to 0.6-m depths with a bucket auger on the seventh day after

rice harvest. They were pooled together to make a composite sample

for each depth and replication, then hand crushed, passed through a

2.0-mm sieve, stored at 4C, and used fresh within 24 h for estimating

soil microbial biomass C and mineralizable C. A portion of the field-

moist soil samples was oven dried, passed through the same sieve, and

used for analysis of different pools of C. Additional triplicate samples

were taken from all three depths using a core sampler (0.05 m in

diameter, 0.08 m in length) for measuring the bulk density of the soil

following the method described by Blake and Hartge (1986).

Total Organic CarbonThe soil samples were oven dried, powdered, and passed through

a 2.0-mm sieve, while the organics (FYM, PS, GM, stubble, and

roots) were oven dried and finely ground in a mechanical grinder fol-

lowing the methods described by Nelson and Sommers (1982). They

were all analyzed for C by a LECO CHN analyzer (Foss Heraeus

Elemental Analyzer CHN-O-RAPID, Hanau, Germany). Soil sam-

ples were also analyzed for inorganic C titrimetrically by digesting

them with dilute HCl following the method of Bundy and Bremner

(1972). The Ctot(obtained by Ctot= Leco C HCl-C), expressed as

megagrams per hectare for each of the three (00.2-, 0.20.4-, and

0.40.6-m) depths, was computed by multiplying the Ctotcontent(g kg1) by the bulk density (Mg m3) and depth (m). The amount of

C left over, stabilized, and sequestered in the entire 0- to 0.6-m depth

was estimated as

( )NPK+or gC0.6

1

NPK

0.0

C left over in soil from

applied organics (Mg ha ) Cd

=

= [1]

( )0.6

0.0

19

NPK+org NPK

org1

C CC stabilized

C

d

y

=

=

=

[2]

where CNPK+orgrepresents C in NPK fertilizer plus organic amend-

ments (FYM, GM, or PS), CNPKis the C in the NPK treatment at

0- to 0.6-m depth, and Corgrepresents C applied through FYM, GM,

or PS during 19 yr.

1

current initC sequestered (Mg ha soil) SOC SOC = [3]

where SOCcurrentand SOCinitindicate the SOC stocks in 2004 (cur-

rent) and at the initiation of the experiment (in 1986). Positive and

negative values indicate SOC gains and losses, respectively, for the

cropping system. A relationship (linear regression equation) between

such changes in SOC (Ctot) (Y) and the total cumulative C inputs

(crop residue + organics) to the soils (X) over the years was computed.


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The critical amounts of C inputs causing a zero change in SOC in

soils were calculated (X) from the equation having Y= 0.

Oxidizable Organic Carbon and Its FractionsThe content of Coc and its different fractions in the soil was

estimated through the Walkley and Black (1934) method as modi-

fied by Chan et al. (2001) using 5, 10, and 20 mL of concentrated

(18.0 mol L1) H2SO4and K2Cr2O7solution. This resulted in three

acid-aqueous solution ratios of 0.5:1, 1:1, and 2:1 that corresponded

to 6.0, 9.0, and 12.0 mol L1H2SO

4, respectively, and caused the

production of different amounts of heat of reaction to bring about

oxidation of SOC of different oxidizability. The amounts of Cocthus

determined allowed separation of Ctot into the following four frac-

tions of decreasing oxidizability as defined by Chan et al. (2001):

Fraction I (Cfrac1, very labile): organic C oxidizable under 6.0

mol L1H2SO4

Fraction II (Cfrac2, labile): the difference in Cocoxidizable under 9.0

mol L1and that under 6.0 mol L1H2SO4

Fraction III (Cfrac3, less labile): the difference in Cocoxidizable

under 12.0 mol L1and that under 9.0 mol L1H2SO4

(the 12.0 mol L1 H2SO4 is equivalent to the standard

Walkley and Black method)

Fraction IV (Cfrac4, nonlabile): residual organic C after oxidation

with 12.0 mol L1H2SO4when compared with Ctot

Mineralizable CarbonA 25-g portion of field-moist soil samples was wetted to 50%

water-filled pore space. It was placed in 1.0-L canning jars along with

vials containing 10 mL of 1.0 mol L1NaOH to trap the evolved

CO2, and incubated for 24 d at 25 2C. Alkali traps were replaced

at 3 and 10 d and finally removed at 24 d. Evolved CO2was esti-

mated by titrating the excess alkali in the traps with 1.0 mol L1HCl

to a phenolphthalein endpoint (Anderson, 1982). Basal soil respira-

tion (BSR), an estimate of potential microbial activity, was calculatedas the linear rate of respiration during 10 to 24 d of incubation. The

total amount of CO2C evolved during the 24-d incubation was

taken as a measure of the potential Cminof the soil (Franzluebbers

and Arshad, 1996).

Microbial Biomass CarbonField-moist soil samples (25 g) were fumigated with CHCl3

vapor for 24 h and then extracted with 0.5 mol L1K2SO4(Vance

et al., 1987). A second, unfumigated set of samples was also similarly

extracted. The difference between the C obtained from the fumigated

and unfumigated samples was taken to represent the microbial C

flush. The Cmic

was calculated using the relationship Cmic

= 1/0.41

C flush (Voroney and Paul, 1984). A microbial quotient (MQ) was

calculated as the ratio of Cmic to Ctot. The respiratory quotient or

metabolic quotient (qCO2) was calculated as the ratio of Cminto Cmic

and expressed as grams CO2evolved per day per gram of Cmic.

Sustainable Yield Index and Yield EquivalentTotal crop productivity of the ricewheat system was calculated

through a sustainable yield index and equivalent rice yield using yield

data for the 38 ricewheat crops harvested during the entire 19-yr

period of the experiment. This was done to offset annual variations in

yield and highlight performance of the treatments during the entire

experimental period. The sustainable yield index is defined as

max

SYI Y

Y

= [4]

where Y is the estimated average yield of a practice across the years,

is its estimated standard deviation, and Ymaxis the observed maxi-

mum yield in the experiment during the years of cultivation (Singh et

al., 1990). Average equivalent rice yield for wheat was calculated as

( )( )Equivalent grain yield of wheat unit price of wheatrice yield

unit price of rice=

where unit priceis the procurement price fixed by the government for

a unit quantity of the respective grain during the harvesting season.

Statistical AnalysisStatistical analysis was performed by the Windows-based SPSS

program (Version 10.0, SPSS, Chicago, IL). The SPSS procedure was

used for analysis of variance to determine the statistical significance

of treatment effects. Duncans multiple-range test was used to com-

pare treatment means. Simple correlation coefficients and regression

equations were also developed to evaluate the relationships among the

response variables (equivalent rice yield, sustainable yield index [SYI],

Cmin, Cmic, Cfrac1, Coc, etc.) using the same statistical package. The

0.05 probability level is regarded as statistically significant.

RESULTSBiomass Yield and Quantity and Quality of

Organic ResidueOn average, grain yield was higher for both crops with

NPK plus organic amendments than only NPK or con-trol treatments (Fig. 1). This effect of organics was more pro-nounced with FYM for rice and with PS for wheat (separatedata not presented). Only with the control treatment wasthere a declining trend in system productivity during the 19-yrperiod of the experiment. This was true particularly for rice. Ofthe five tested treatments, the equivalent rice yield and annualC input return to soil were highest with NPK + FYM, fol-lowed by NPK + PS > NPK + GM > NPK > control

Fig. 1. Combined (rice and wheat) grain yield of the ricewheat crop-ping system under different treatments (19862004): NPK, in-organic NPK fertilizer; GM, green manure; PS, paddy straw;FYM, farmyard manure.


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treatments (Table 1). Sustainable yield index values were alsohigher with NPK plus organics compared with only NPKor control treatments.

The quantities of stubble, root biomass, and rhizodepo-sition (rice and wheat together) were higher in NPK plusorganics than in NPK only or control treatments (Fig. 2).On average, C concentrations (on an oven-dry basis) in stubbleand roots were 318 21 and 412 33 g kg1, respectively, forrice and 352 26 and 391 37 g kg1, respectively, for wheat.The C concentrations for the three organics were 333 51, 420

23, and 415 18 g kg1

for FYM, PS, and GM, respectively(Table 2). The FYM also contained the highest amount of ash(253 36 g kg1), lignin (175 15 g kg1), and polyphenol(10.8 3.2 g kg1); PS had the highest C/N value (97.7) andcellulose content (350 29 g kg1); GM had the lowest valuesfor all these parameters.

Total Soil Organic Carbon and Carbon StabilizationContinuous cropping without the use of NPK fertil-

izer or organics (i.e., control) for 19 yr caused a net decrease(13.3%) in Ctot content compared with that under fallow,whereas cropping with the use of NPK fertilizers and organ-ics increased the Ctotcontent by 26.7, 24.1, and 22.0% with

FYM, PS, and GM, respectively, over that of the control (Table3). Out of 9.49, 7.98, and 6.31 Mg of C added through FYM,PS, and GM (on a dry-weight basis), respectively, only 6.44,4.57, and 3.09 Mg, respectively, could be accounted for in thesoil in these treatments after 19 yr of cropping (Table 4). A sig-nificant positive linear relationship (R2= 0.98, P= 0.001) wasobserved between changes in SOC (Ctot) and total cumulativeC inputs (crop residue plus organics) (Fig. 3), which is describedby the following equation: SOC sequestered (Mg ha1) = 0.14 cumulative C input (Mg ha1) 9.19. Figure 3 shows thatC inputs of 3.56 Mg ha1yr1to soils can keep the SOC levelunchanged. This is taken as the critical amount of C inputs formaintenance (no depletion) of SOC level.

Oxidizable Organic Carbon and Its FractionsCropping both with and without (control) NPK fer-

tilizer caused a decrease (4.5 and 11.3%, respectively) in Coccompared with that in the no-cultivation fallow treatment(Table 3). The NPK fertilizer combined with FYM, PS, orGM caused an increase in Cocof as much as 12.4 and 20.9,10.4 and 18.8, and 8.2 and 16.4% over that in the NPK andcontrol treatments, respectively.

The amounts of SOC fractions extracted under a gradi-ent (6, 9, and 12 mol L1 H2SO4) of oxidizing conditions

Table 1. Annual return of crop residue C to soil, equivalent rice yield, and sustainable yield index (SYI) under different treatments of thericewheat system (mean of 19 yr of experimentation).

TreatmentC returned to soil through

Equivalent rice yield SYIStubble Roots Rhizodeposition Aquatic growth (in rice) Total

Mg ha1

Control 0.08 0.02 0.56 0.12 0.62 0.13 0.70 1.96 0.32 2.35 0.24 0.62

NPK 0.16 0.03 1.29 0.22 1.56 0.18 0.70 3.70 0.42 6.67 0.35 0.87

NPK + FYM 0.18 0.06 1.42 0.19 1.80 0.21 0.70 4.10 0.39 7.47 0.26 0.92

NPK + PS 0.18 0.04 1.39 0.23 1.75 0.26 0.70 4.02 0.28 7.31 0.31 0.93

NPK + GM 0.17 0.05 1.36 0.21 1.71 0.29 0.70 3.93 0.33 7.03 0.29 0.91

Control, no NPK fertilizers or organics used; NPK, recommended rate of inorganic NPK fertilizer; FYM, farmyard manure; PS, paddy straw;GM, green manure.

Fig. 2. Combined (rice and wheat) amount of crop residue (stubble,roots, and rhizodeposition) returned to the soil under differenttreatments in the ricewheat cropping system (19862004):NPK, inorganic NPK fertilizer; GM, green manure; PS, paddystraw; FYM, farmyard manure.


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varied significantly (P = 0.001) among the treatments anddepths compared (data not given, pooled for the layers, Table3). The means of all treatments were in the order C

frac2 >

Cfrac4Cfrac1> Cfrac3, constituting about 32.1, 29.7, 28.9,and 9.3%, respectively, of the Ctot. The NPK + FYM andNPK + PS treatments had higher values, whereas the controltreatment had lower values for almost all the fractions. Such

values for the fallow treatment were also higher, for most of thefractions, than that of the control. Again, there was a greateraccumulation of all the fractions in the surface compared withthe subsurface layers (data pooled for the layers).

Mineralizable Carbon, Soil Respiration, andMicrobial Biomass Carbon

Mineralizable C content of the soil varied from 1.15 to

1.92, with a mean value of 1.65 g CO2C kg1

soil, consti-tuting about 11.4% of the Ctot (Table 5). This was higher(P= 0.001) in NPK + FYM (12.4%) and fallow (12.3%)than in the other treatments, with similar trends for BSR. TheCmiccontent varied from 0.28 to 0.52, with a mean value of0.43 g C kg1soil. The Cmiccontent of the different treatments

were in the following order: NPK + FYM (0.52 g C kg1)NPK + PS (0.48 g C kg1) fallow (0.47 g C kg1) NPK + GM (0.45 g C kg1) NPK (0.41 g C kg1) >control (0.28 g C kg1). The MQ also varied widely between0.023 and 0.033 kg Cmic kg

1 Ctot, with a mean value of0.030 kg Cmickg

1Ctot(Table 5). Its values were higher in allthe fertilized treatments than in the control. On the contrary,

the qCO2was higher under the control and NPK than inthe NPK plus organic treatments, with no variation, how-ever, among the three different organic amendments used.

Relationship among Different Organic Carbon Poolsand with Crop Productivity

Correlation analyses (data not shown) showed that mostof the C pools were significantly correlated (P= 0.005) with

each other. Such correlations forCfrac1 and Coc were comparativelystronger. When the relationship wasdrawn between SOC pools and cropproductivity, data fit well with the lin-ear predictive model for Cmic, Cfrac1,Coc, and Cmin(Table 6).

DISCUSSIONSoil Organic Carbon Depletion and AppliedCarbon Stabilization

Cropping during the years caused a net decrease (13.3%of fallow) in Ctot. Crop cultivation needs plowing, which dis-turbs the distribution and stability of soil aggregates (Six et al.,2002), thereby exposing the organic C stock in soils to rapidoxidation. This results in depletion of the stock. Many research-ers (Jenny and Raychaudhuri, 1960; Swarup et al., 2000), how-ever, have reported a much greater magnitude (3060%) of thisdepletion due to cropping in this subtropical part of India. Thecomparatively lower magnitude of depletion we observed wasfirst due to the soils remaining under a unique aquic (flooded)moisture regime for 3 to 4 mo for the rice crop and second due

to the soils being inherently low in organic C content (34.4 Mgha1for the 00.6-m depth). These conditions possibly slowedthe rate of C oxidation in the soil. Cropping with NPK +FYM, PS, or GM brought about a significant increase in Ctotdue to an additive effect of NPK and the organics and interac-tions between them. Moreover, a large amount of plant residue(stubble, root biomass, and rhizodeposition) C was left over inthe soil owing to increased yield under these treatments (Table2, Fig. 2). A similar buildup of SOC due to cropping withthe application of balanced fertilizer combined with manure(Rudrappa et al., 2006), paddy straw (Verma and Bhagat, 1992),and green manure (Yadav et al., 2000) was also reported fromlong-term experiments.

It was observed that 67.8, 57.3, and 49.0% of the Capplied through FYM, PS, and GM, respectively, was stabi-lized in the form of SOC (Table 4). This shows that the Capplied through FYM was the most resistant while that appliedthrough GM was the least resistant to decomposition. This isbecause FYM and PS have a higher C/N ratio and lignin andpolyphenol contents than GM (Table 2). The higher content oflignin and polyphenol in FYM and PS led to the formation of

Table 2. Composition of the organic amendments used.

Organic amendment Ash Total C Total N C/N Cellulose Lignin Polyphenol

g kg1 g kg1

Farmyard manure 253 36 333 21 5.0 1.5 66.6 231 36 175 15 10.8 3.2

Paddy straw 98 4 420 23 4.3 1.3 97.7 350 29 110 11 6.0 2.9

Green manure 123 8 414 18 17.0 2.2 24.3 100 15 89 13 3.2 1.9

Table 3. Soil organic C and its fractions at 0- to 0.6-m depth under different treatments after 19 yr of ricewheat cropping.

Treatment Total organic C Oxidizable organic C Very labile C Labile C Less labile C Nonlabile C

Mg ha1

Fallow 81.7 c 58.5 d 24.9 abB 27.0 bcA 6.6 bC 23.1 bB

Control 70.8 d 51.9 f 21.7 eB 23.6 dA 6.7 bD 18.9 cC

NPK 83.3 c 55.8 e 23.4 dB 27.7 bA 4.8 cC 27.4 aA

NPK + FYM 89.7 a 62.8 a 25.0 aB 26.6 cA 11.2 aC 26.9 aA

NPK + PS 87.8 b 61.7 b 24.8 bC 27.0 bcA 9.9 aD 26.2 aB

NPK + GM 86.4 b 60.4 c 24.5 cC 28.6 aA 7.4 bD 25.9 aB

Overall mean 83.3 58.5 24.0 B 26.7 A 7.8 C 24.7 B

Fallow, no cultivation since inception; control, no NPK fertilizers or organics used; NPK, recommended rate of inorganic NPK fertilizer;FYM, farmyard manure; PS, paddy straw; GM, green manure.

Within a column of a C pool, numbers followed by different lowercase letters are significantly different between treatments atP0.05 by Dun-cans multiple-range test.

Within a row of a treatment, numbers followed by different uppercase letters are significantly different between C pools at P0.05 by Duncansmultiple-range test.


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stable complexes with proteins of plant origin and thus madethe FYM and PS C more resistant to decomposition than thatof the GM (Tian et al., 1992). Villegas-Pangga et al. (2000)also obtained an inverse relationship between cumulative Crelease and the C/N ratio in rice straw. The differences in themagnitude of loss among the organic sources (GM 51.0%, PS42.7%, and FYM 32.2%) were narrowed (6371%), however,when crop residue C inputs were also included (Table 4). Highash, lignin, and polyphenol contents in the stubble and rootsof the rice and wheat crops gave them resistance to decomposi-tion. Results thus suggest that both the quality and quantity ofC inputs into the soil have an important role in building upSOC. The total amount of C applied to the soil through allthree organics (FYM, PS, and GM) together during the entireperiod (19 yr) of the experiment was 23.8 Mg C ha1(Table4), of which 14.1 Mg C ha1, constituting, on average, 58.1%of the added amount, could be accounted for in the form ofCtotin the soil. Furthermore, the combination of organics withNPK fertilizer caused a significant increase in crop yield (Fig.1, Table 1), with an attendant increase in the amount of cropresidue C returned to the soil (Fig. 2, Table 1). If this is alsotaken into consideration, the value of leftover C declines to

only 33% of the applied amount (organic plus crop residue C)(Table 4). A strong oxidative force of high temperature (dur-ing peak summer months, the temperature can reach 4045C)compared with cooler, temperate regions coupled with the dis-rupting effect of intensive cultivation led to a rapid oxidationof soil organic C in this region.

The existence of a linear relationship (R2= 0.98, P= 0.001;Fig. 3) between the changes in SOC (Ctot) and the total cumu-lative C inputs (organics plus crop residue) to the soil indicatesthat, even after 19 yr of C additions at a reasonably high ratethrough FYM, PS, and GM (7.510.0 Mg ha1yr1on a wet-

weight basis) and crop residues (3.934.10 Mg ha1yr1), thesoils of the present experiment still have a capacity for storing

C and therefore have great potential for further C sequestration.The slope of the curve (Fig. 3) represents the rate of conver-sion of input C to SOC. This is about 14% of each additionalmegagram of C input per hectare in this cropping system. We

wanted to compare our values with those of others, if any, insubtropical parts of the world but failed to do so since suchinformation in the literature is rare; however, our values weresimilar to those reported by Rasmussen and Collins (1991)(14.021.0%) and Rasmussen and Smiley (1997) (14.8%)

from cooler, temperate climatic regions (the United States andCanada) but higher than those obtained by Kong et al. (2005)

(7.6%) under Mediterranean-like climatic conditions.This study also reveals that, to maintain SOC levels

(zero change), the critical amount of C input to the soil is3.56 Mg C ha1yr1(Fig. 3). This is similar to that reportedby Kong et al. (2005) (3.1 Mg ha1yr1) in Davis, CA, undera Mediterranean-like climate. The quantity of crop residue Cinputs (3.7 Mg ha1yr1) under the NPK treatment almostfailed to bring about any significant change (~3.6%) in SOCeven after 19 yr of cultivation. In view of this, the observedcritical amount of 3.56 Mg C ha1yr1seems to be a realisticone; however, more such studies need be undertaken under hot,subhumid, tropical climate to substantiate the observed valueand look for suitable alternate profitable cropping systems that

might provide the amount necessary for upkeep of soil health.

Oxidizable Organic Carbon and Its FractionsReports of loss of Coc content in soils due to cropping

are many from this subtropical region because of the hightemperature (Bhattacharyya et al., 2004, p. 44; Swarup et al.,2000). Carbon supplementation through FYM, PS, and GMincreased Coc; however, the percentage increase in Cocover thecontrol was relatively lower than that of Ctot. Accumulation

Fig. 3. Relationship between cumulative C inputs to soil and change in soilorganic C (SOC) stock (critical C input value derived for zero change)under different treatments in the ricewheat cropping system (errorbars represent the standard error of means of change in SOC).

Table 4. Cumulative amount of C inputs and leftover C in soils at 0- to 0.6-m depth after 19 yr of ricewheat cropping.

Organicamendment

C input throughorganic amendments

C input throughcrop residues

Leftover C in soil from

organic amendments

Leftover C as percentage applied from

Passivepool

TotalOrganic

amendmentsOrganic amendments +

crop residues

Mg ha1 %

Farmyard manure 9.49 7.60 4.24 6.44 67.8 37.7

Paddy straw 7.98 6.08 2.38 4.57 57.3 32.5

Green manure 6.31 4.37 1.38 3.09 49.0 29.0

Mean 7.93 6.02 2.67 4.70 58.0 33.0

Total 23.78 18.05 8.00 14.10

Crop residue C = (crop residue C in fertilizer + organic treatment) crop residue C in fertilizer treatment.

Leftover C in soil = (soil C in fertilizer + organic treatment) soil C in fertilizer treatment.

Passive pool = (less labile C + nonlabile C).


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of Cocunder organic inputs is likely to be influenced by theircomposition, C/N ratios, and decomposability or degradability.In spite of having significant differences in these parameters andalso in the amounts of C added through the three organic sources(FYM, PS, and GM), there were little differences in the Coccon-

tent (62.8, 61.7, and 60.4 Mg C ha1

soil, respectively) in the soilprofile (up to 0.6 m) even after 19 yr of their application.

The amounts of the four fractions (Cfrac1, Cfrac2, Cfrac3,and Cfrac4) of SOC having different lability varied significantlyamong the treatments and depths (Table 3). These variations

were more prominent with Cfrac1and Cfrac2than with Cfrac3and Cfrac4. The current methodology for estimating C frac-tions would thus be helpful in monitoring even small differ-ences in the effects of different treatments. On average, thehigher values of different C fractions under FYM, PS, and GMtreatments were ascribed to the increased yield and C returnedto the soil. These higher values of Cfrac1under the NPK +FYM and NPK + PS treatments particularly were associated

with the high content of polysaccharides (cellulose and hemi-cellulose) in FYM and PS that could lead to the productionof higher amounts of labile fraction (Cfrac1) SOC than withGM (Seneviratne, 2000). Many researchers (Halvorson et al.,2002; Dormaar and Pittman, 1980; Rasmussen and Collins,1991) have shown that there was, in general, a decrease in thevalues of different pools of C in soils under fallow treatmentand the magnitude varied directly with the length of the fallowperiod. They had fallow periods of varying length between twocrops where land was subjected to normal plowing and otherforms of perturbation, except during the fallow period. Duringthe fallow period, the land remained bare and was exposed to

high temperatures that accelerated the oxidation of SOC. Inour experiment, however, the fallow treatment was maintaineddifferently, allowing native vegetation to grow with its subse-quent in situ incorporation with minimum tillage and no per-turbation for cropping since the initiation of the experiment in

1986. This explained the observed variations in results underthe fallow treatments between this study and the others men-tioned above.

The first two most easily oxidizable fractions, Cfrac1andCfrac2, together constituted about 87.6 and 61.0% of the Cocand Ctot, respectively. Chan et al. (2001), while comparing theeffectiveness of different pasture species in maintaining labilepools of SOC, observed a similar proportion (~65%) of Ctotinthose two labile fractions in semiarid areas of Australia. Theyfurther observed that the other two fractions (Cfrac3and Cfrac4)accounted for a smaller proportion (~35%) of the Ctotthat falls

within the range of 30 to 40% assigned to the passive poolof SOC used in the Century Model (Parton and Rasmussen,

1994), which is very close to our observations (39% of Ctot).Further, while fractionating C into these four pools in soilsfrom five long-term experiments with different managementpractices and cropping systems, Majumder (2007) observedstrong relationships of Cfract 1and Cfract 2with Cmicand Cminand crop productivity of the systems, although such relation-ships for Cfract 3and Cfract 4were weak. Accordingly, Cfract 1and Cfract 2might constitute the active pool while Cfract 3andCfract 4might represent the passive pool of SOC (Chan et al.,2001; Majumder, 2007). Comparing our results with those ofChan et al. (2001), it appears that the type of crops cultivated(cereals or legumes and grasses) does not have much effect onthe relative proportion of C

tot

in labile pools. Climatic condi-

tions (semiarid or subtropical) were also found to have no dis-cernible effect in this regard under the management practicesand cropping system tested.

An attempt was made to find out how the application ofdifferent organic amendments helped to build up the passivepool (Cfrac3+ Cfrac4) of SOC (Table 4). It was observed thatout of 6.44, 4.57, and 3.09 Mg of C inputs in the form ofFYM, PS, and GM, respectively, 5.97, 3.86, and 1.08 Mg con-stituting 92.8, 84.5, and 35.1%, respectively, found their wayto the passive pool, leaving the rest to participate actively in soilC cycling. Of the sources, PS and FYM thus contributed more

Table 5. Microbial biomass C (Cmic), mineralizable C (Cmin), basal soil respiration (BSR), microbial quotient (MQ), and respiratory quo-tient (qCO2) in soils at the 0- to 0.2-m depth under different treatments after 19 yr of ricewheat cropping.

Treatment Cmic Cmin(024 d) BSR (1024 d) MQ qCO2

g C kg1 g CO2C kg1C d1 kg Cmickg

1Ctot kg CO2C kg1Cmicd

1

Fallow 0.47 ab 1.72 ab (12.3) 0.33 a 0.033 a 0.15 b

Control 0.28 d 1.15 c (9.5) 0.21 c 0.023 c 0.17 a

NPK 0.41 c 1.65 b (11.5) 0.31 b 0.029 b 0.17 a

NPK + FYM 0.52 a 1.92 a (12.4) 0.33 a 0.033 a 0.15 b

NPK + PS 0.48 ab 1.76 ab (11.5) 0.30 b 0.032 a 0.15 b

NPK + GM 0.45 b 1.68 b (11.3) 0.31 b 0.031 ab 0.15 bOverall mean 0.43 1.65 (11.4) 0.30 0.030 0.16

Fallow, no cultivation since inception; control, no NPK fertilizers or organics used; NPK, recommended rate of inorganic NPK fertilizer;FYM, farmyard manure; PS, paddy straw; GM, green manure.

Within a column of a C pool, numbers followed by different lowercase letters are significantly different between treatments at P0.05 by Dun-cans multiple-range test.

Values in parentheses indicate percentage of total organic C (Ctot).

Table 6. Relationship of different soil organic C (SOC) pools withequivalent rice yield and sustainable yield index (SYI) under dif-ferent treatments after 19 yr of ricewheat cropping.

Yield SYI

SOC pool Equation R2 Equation R2

Cmic Y= 3.60X+ 3.06 0.95 Y= 0.21X+ 0.67 0.68

Cmin Y= 1.05X+ 2.66 0.68 Y= 0.05X+ 0.71 0.30

Cfrac1 Y= 1.33X+ 10.56 0.92 Y= 0.07X+ 0.09 0.63

Coc Y= 0.61X+ 8.36 0.92 Y= 0.03X+ 0.03 0.63

Cmic, microbial biomass C; Cmin, mineralizable C; Cfrac1, very labile C;Coc, oxidizable organic C.


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than GM in enriching the passive pool and thus subsequentlyhelped in building up C stocks in the soil. If one considers thehighly stable passive pool of SOC with turnover times in thecenturies used in the Century Model (Parton and Rasmussen,1994), however, the stability of the currently built-up passivepool of Cfract 3and Cfract 4through added organics during thelast 19 yr of the present experiment may not be alike.

Mineralizable Carbon, Soil Respiration, and

Microbial Biomass CarbonThe amount of C mineralized (11.4% of C tot) that weobserved (Table 5) within a short span of 24 d is typical in soilsof tropical and subtropical regions (Rudrappa et al., 2006). Theamount varied under different treatments, being lowest in thecontrol. This is due to the variable amounts of labile organicC fractions in soils under the treatments (Table 3). The higher

value of Cmin content in the NPK fertilizer with organicamendments and fallow treatments may be attributed to thegood supply of labile C substrate in those treatments. The basalsoil respiration (Cminat 1024 d and BSR) per unit of SOCrepresents the performance of soil organic C decomposers andalso the quality of SOC under decomposition. Like Cmin, the

BSR also was found to be higher in the NPK with organicsupplementation and fallow treatments. As mentioned above,the availability of easily decomposable organic matter and alsoreadily available nutrients in these treatments provided a con-ducive environment for microbial activity, resulting in a higherrate of respiration (Sayre et al., 2005). The mineralizable C,

which provides an early indication of a possible degrading oraggrading effect of different management practices on soilquality (Powlson, 1994), was also lowest in the control treat-ment. This seems to be related to the unfavorable environmentin the control arising out of the depletion of nutrients due tocontinuous cropping without any fertilization. Balanced fertil-ization with C supplementation, however, provided a congenial

environment for microbial growth and thus caused an increasein soil Cmicover that in the control.

The MQ signifies microbial activity. It varied widely(0.0230.033) among the treatments (Table 5). This is, how-ever, within the range of 0.01 to 0.05 kg Cmickg

1Ctotadvo-cated by Anderson and Domsch (1980). Its higher values inNPK plus organics and fallow treatments compared withthe control may be attributed to a better nutritional environ-ment in the former treatments. The results also suggest thatthe organic C under the former treatments is more stable thanin the control (Sparling et al., 1992). The lowest value of MQin the control soils indicates that the capacity of the soil for Ccycling has been impaired, signifying a degradation of its qual-

ity (Dalal, 1998). A relationship between MQ and Ctotshowedthat MQ was directly influenced by Ctot(r= 0.80, P= 0.001).Haynes and Tregurtha (1991) also reported a decline in MQfrom 0.023 to 0.011 kg Cmickg

1Ctotdue to a decrease inSOC from 65 to 15 g C kg1soil.

The respiratory or metabolic quotient (the ratio of Cminand Cmic, qCO2) is an indicator of the efficiency of soil micro-organisms in processing residue or available soil C. Its value

was significantly higher under the control and NPK fertilizerthan in the fallow and organic-amended soils (Table 5). Thehigher value of qCO2in the former treatments suggests a less

efficient use of available C by the microbes there, whereas itslower value in the latter (fallow and organic-amended treat-ments) indicates their higher efficiency in preserving C in soils.There was, however, no difference in qCO2among the threeorganic amendments. Higher and lower values of qCO2asso-ciated with fertilizer and organic-amended treatments, respec-tively, have also been reported by many researchers (Fauci andDick, 1994; Lupwayi et al., 1998; Rudrappa et al., 2006).

Although qCO2 undoubtedly indicates microbial efficiency,several other factors such as soil moisture and qualitativechanges within the microbial population (e.g., an increase inthe proportion of fungi) may explain the differences in qCO2

within the treatments. For all these active C fractions, the fal-low treatment rather than the control treatment compared well

with the fertilized and organic-amended treatments, mainlybecause the fallow treatment had experienced little stress fromcultivation and also contained high amounts of labile C pools.

Relationship among Different Organic CarbonPools and with Crop Productivity

The observed significant correlations among the pools ofC in the soil indicate the existence of a dynamic equilibrium

among them. This means that depletion or enrichment in onewould shift the equilibrium and affect the size of the others.Such relationships, particularly for Cfrac1 and Coc with oth-ers, were stronger. This emphasizes the importance of monitor-ing these two pools for a better understanding of C cycling inthe soil. Relationships were also drawn between these pools ofC in the soil and the crop productivity of the system. Theserelationships of both yield and SYI were strong, particularly

with Cfrac1, Coc, Cmin,and Cmic(Table 6). They accountedfor as much as 63, 63, 30, and 68% of the variability in SYIand 92, 92, 68, and 95% of the variability in rice equivalentyield, respectively. This suggests the importance of these poolsof SOC in influencing crop yield, possibly through maintain-

ing better soil quality. Considering the relatively low cost andease of estimation and also its influence on yield, SYI, and Ccycling, Cfrac1appears to have an edge over the other pools ofC in the soil for its inclusion as a good indicator for routinemonitoring of soil health in the ricewheat agroecosystem.

CONCLUSIONSWhile continuous ricewheat cropping without balanced

fertilization (NPK) caused a significant depletion of SOC,the same with balanced fertilization maintained the level and, incombination with organics, significantly improved it. A majorfraction (~67%) of the C supplemented in the system throughorganic amendments (FYM, PS, and GM) was lost, however,

and a small fraction was left to be stabilized into SOC. The rel-ative efficacy of the amendments for SOC building was FYM >PS > GM. To offset the loss and to maintain SOC, a minimumof 3.56 Mg C ha1yr1needed to be incorporated into thesoil in the form of organics. Of the several C pools analyzed,the passive pool constituted about 39% of the Ctot. A higherproportion (70.8%) of C that got stabilized into SOC from Cinputs, however, found its way to the passive pool, leaving asmaller part in the active pool. Mineralizable C (24 d) consti-tuted a large (~11.5%) proportion of Ctotin this subtropicalregion. Of the pools, Cfrac1, Coc, Cmin, and Cmic explained


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the variability in crop yield to a greater extent. Because of thelow cost and ease of estimation and also its influence on yieldand SYI, the Cfrac1 fraction of SOC can reasonably be usedas a good indicator for assessing soil health and productivity.

An overall consideration of the results indicates that, of all thetreatments tested, balanced NPK fertilization along with anadequate amount of FYM is most suitable for the continuousricewheat cropping system in the subtropical Indo-GangeticPlains of India for improving the SOC stock and sustainingcrop productivity.

ACKNOWLEDGMENTSWe are extremely grateful to Dr. L.N. Mandal, former professor ofsoil science, Bidhan Chandra Krishi Viswavidyalaya, West Bengal,for kindly going through a draft manuscript and offering valuablesuggestions for strengthening it. We also express our immense gratitudeto Dr. Bill Hardy, Science Editor and Publisher, Communicationand Publications Services, IRRI, for editing the manuscript. We arealso thankful to the Indian Council of Agricultural Research, NewDelhi, for funding the work through the World Bank assisted multi-institutional collaborative National Agricultural Technology Project.

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