Soil & Tillage Research 121 (2012) 57–62

Soil and residue carbon mineralization as affected by soil aggregate size

Pramod Jha a,*, Nikita Garg b, Brij Lal Lakaria a, A.K. Biswas a, A. Subba Rao a

a Indian Institute of Soil Science, Nabi Bagh, Berasia Road, Bhopal 462038, Madhya Pradesh, Indiab Sant Hirdaram Girls College, Bhopal, Madhya Pradesh, India

A R T I C L E I N F O

Article history:

Received 26 May 2011

Received in revised form 16 January 2012

Accepted 29 January 2012

Keywords:

Soil aggregates

Soil carbon mineralization

Residue carbon mineralization

Soil carbon pools

A B S T R A C T

The nature of the contact between fresh organic matter and soil depends mainly on the characteristics of

the plant residues and on the physical properties of the soil. In a cultivated cropping system, changes in

soil organic C cannot be entirely attributed to changes in organic matter input. Breakdown of aggregates

caused by cultivation not only affects soil organic matter but also influences the rate of mineralization of

added organic matter. Many models simulating organic matter decomposition in the field are calibrated

with laboratory data from experiments where crop residues are ground and mixed homogeneously with

soil aggregates. In the present study, soil aggregate size was used as a means of varying the contact

between crop residue and the soil. The results demonstrated that cumulative soil carbon mineralization

from different aggregates had a significant (r = 0.60, p = 0.05) and positive relationship with their

oxidizable soil carbon content. Residue carbon mineralization in different aggregate size classes was

inversely related to aggregate oxidizable soil carbon content (r = �0.95, p = 0.01), cumulative soil carbon

mineralization (r = �0.89, p = 0.01) and resistant soil carbon pool (r = �0.80, p = 0.01). Residue carbon

mineralization in different aggregate size classes was also inversely (r = �0.61, p = 0.05) related to the

active carbon content (KMnO4 oxidizable carbon) of the aggregates. There was no significant difference

in soil active carbon pool in different aggregate size classes. Determination of size and turnover of a slow

pool showed significant difference in different aggregate size classes. The slow carbon pool in different

aggregate size classes ranged from 13.7 to 25.5% with mean residence time of 1.8 to 5.4 years. Water

soluble carbon and active carbon (alkaline KMnO4 oxidizable C) were significantly higher in macro-

aggregates than in micro-aggregates.

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1. Introduction

In a cultivated cropping system, changes in soil organic Ccannot be entirely attributed to changes in organic matter input(Balesdent et al., 1998). Cultivation practices can stimulatebiodegradation of the initially physically protected C in soil,and hence it could be responsible for the decrease of soil organic C(Tisdall and Oades, 1982; Balesdent et al., 1998). Soil aggregationcan provide physical protection of organic matter against rapiddecomposition (Pulleman and Marinissen, 2004), and the aggre-gate formation seems to be closely linked with soil organic matterstorage in soils (Golchin et al., 1995). Many studies (Cambardellaand Elliott, 1993; Six et al., 1998; Ashagrie et al., 2007) havedemonstrated that breakdown of aggregates caused by cultiva-tion was responsible for the loss of soil organic matter. The soil-residue contact has 2 main physical components: the residue’spotential contact area and the actual contact area. Actual area ofcontact may be high if the bulk density of the surrounding soil is

* Corresponding author. Tel.: +91 755 2730946; fax: +91 755 2733310.

E-mail address: jha_iari@yahoo.com (P. Jha).

0167-1987/$ – see front matter � 2012 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2012.01.018

high, soil aggregate size is small and the residue evenlydistributed in the soil (Fruit et al., 1999). The contact betweensoil and plant residues has a major effect on the fate of carbon insoil. Direct contact between soil and fresh organic matter is asignificant factor regulating the decomposition of litter in forest,pasture and agricultural land. Soil-residue contact impacts on theavailability of residue carbon for degradation by microorganismsand on soil nitrogen availability. The mechanisms involved in soil-residue contact need to be properly understood (Garnier et al.,2008).

Microorganisms are a fundamental component of soils, playinga key role in essential processes such as organic matter dynamics,nutrient cycling, degradation of residues and development of soilstructure and aggregation. Microorganisms create their ownhabitats in soil by enhancing aggregation, as explained in themodel of Tisdall and Oades (1982), but each stage of aggregationresults in the production of microsites with a different habitat orenvironment indicating the effects of aggregate size on microbialactivity. The crop residue inputs in soil influence the aggregatestructure by changing carbon content of soil (Haynes and Swift,1990; Hulugalle and Cooper, 1994; Martens et al., 2003).Furthermore, aggregation affects carbon storage by occluding

P. Jha et al. / Soil & Tillage Research 121 (2012) 57–6258

organic materials, making them inaccessible to degrading organ-isms and their enzymes (Sollins et al., 1996).

Many studies have been conducted on the effect of residuequality and size of residue particles on carbon mineralization(Bremer et al., 1991; Sorensen et al., 1996; Angers and Recous,1997; Tarafdar et al., 2001). In the present study, we attempted toquantify the effect of soil aggregate size on externally addedresidue carbon mineralization. While several studies haveinvestigated the influence of management practices on soil organicmatter dynamics and C sequestration, the exact fate of newlyadded C and the level at which C is affected by soil aggregate size isstill unclear. Furthermore, it is important to know-how and towhat extent size of soil aggregates affects decomposition ofrecently added residue C in view of conservation agriculture.Therefore, this work was undertaken with the hypothesis thataggregates having different carbon contents will affect the addedresidue carbon mineralization in soil. The aim of the present studywas to gain a quantitative knowledge about the mineralization ofadded plant residues as affected by the size of the soil aggregates.

2. Materials and methods

2.1. Collection of soil samples

Soil samples were collected from the Research Farm (conven-tionally tilled) of Indian Institute of Soil Science, Bhopal (Latitude238200N; Longitude 778300E). Samples were collected using a coreaugar from 0 to 15 cm soil depth from the field of soybean–wheatcrop rotation. The bulk sample was mixed thoroughly, homoge-nized and stored for further analysis. The study area is in the sub-tropical humid region that receives an average annual precipita-tion of 1080 mm and mean annual temperature of 25 8C. The soil ofthe study area (Vertisol) was clayey in texture with neutral toslightly alkaline in soil reaction. These soils belong to the finemontmorillonitic hyperthermic family of Typic Haplusterts.

The samples were air-dried for 2 weeks at room temperature(25 8C), ground and sieved through different mesh size. Samples ofsoil were fractionated using a standard dry sieving technique todetermine different aggregate size fractions. Five 20-cm soil sieves(4 mm, 2 mm, 1 mm, 0.5 mm, 0. 25 mm) were used to create 5 soilfractions for analysis and incubation. Air-dried soil was gentlypoured onto a sieve with a screen size matching the largest sizedaggregates in a size class (passed through 4 mm and collected onthe 2 mm sieve). The sieve was held in one hand above a piece ofbrown paper and tapped with the other hand. The number of tapswas same for all the aggregate size classes. Accordingly, aggregatesizes of 2–4 mm, 1–2 mm, 1–0.5 mm, 0.5–0.25 mm and <0.25 mmwere collected and subsequently used for chemical characteriza-tion and incubation. The descriptions of each aggregate size classare provided in Table 1. The moisture content of sieved materialwas determined by oven drying at 105 8C overnight. Allconcentrations were expressed on an oven-dried weight basis.Replicated samples were pooled to obtain large composite samplesfor C mineralization experiments.

Table 1Aggregate size distribution, C within aggregates and C distribution within aggregates o

Descriptions Aggregate size (mm) Aggregate content (

Large macro-aggregates 2–4 17.6 a

Small macro-aggregates 1–2 29.1 b

Small macro-aggregates 0.5–1 22.1 b

Small macro-aggregates 0.25–0.5 18.9 a

Micro-aggregates <0.25 12.3 c

Note: Values within a column followed by the same letter are not significantly differen

2.2. Soil analysis

The particle size distribution of the soil was determined byInternational pipette method (Piper, 1966). The texture class wasdetermined by textural triangle chart developed by the Interna-tional Society of Soil Science. Soil pH and EC was determined insoil:water suspension (1:2) by the method outlined in Jackson(1967). Total carbon content was determined by dry combustionmethod using Shimadzu total organic carbon (TOC) analyzer witha NDIR detector (SSM5000A). Organic carbon in the soil samplewas also determined by the wet digestion method of Walkley andBlack as outlined in Jackson (1967), as this method determinesonly oxidizable form of the organic C in soil. Water soluble carbonwas determined by mixing 10 g of soil with 20 ml of distilledwater and shake for 1 h. This was followed by centrifugation for5 min at more than 6000 rpm, filtration and titrimetric determi-nation. Dehydrogenase activity was determined by following themethodology outlined by Cassida et al. (1964). In brief, 1 g of airdried soil was taken in air tight screw capped test tube. This wasfollowed by addition of 0.2 ml of 3% triphenyl tetrazoliumchloride (TTC) solution and addition of 0.5 ml of 1% glucosesolution in each tube. Then it was incubated at 28 � 0.5 8C for 24 h.The reaction was terminated by addition of 10 ml of methanol, andthe formation of triphenyl formazan (TPF) was measured with aspectrophotometer at a wavelength of 485 nm. Active carbon wasdetermined by the modified method of Blair as outlined by Weil et al.(2003).

2.3. Carbon mineralization experiment

Potential C mineralization was studied in a laboratoryincubation experiment for 183 days. The soil moisture wasadjusted at 80% of FC (field capacity) to simulate average fieldmoisture conditions and incubation was carried out at roomtemperature (25 8C). Each soil aggregate size class (50 g) wasamended with and without 1% of wheat straw (weight basis), andeach size class was replicated 3 times. The carbon and Kjeldahl Ncontent of ground material were 423 and content of 5.2 g kg�1,respectively.

A vial containing 10 ml of 2 M NaOH was placed inside the flasksuspended on a thread, and flasks were sealed (air-tight) with wax.Each treatment was replicated 3 times with and without wheatstraw at each sampling date. The vials were taken out on 2, 5, 9, 14,23, 33, 44, 55, 69, 89, 103, 117, 134, 157 and 183-day interval fromthe day of incubation. The amount of C-CO2 evolved was calculatedby using the formula:

C-CO2 evolved ðmg 100 g�1Þ ¼ ðA � BÞ � N � 6 (1)

where A and B is the volume of HCl consumed for titrating 10 ml of2 M NaOH in control and amended soil, and N is the normality ofHCl. Decomposition is reported as a percentage of the original Cremaining:

Net C evolved from wheat straw = C amended � C control.

f a vertisol.

%) Soil organic C (g kg�1) C distribution among aggregates (%)

17.9 ab 17.9

19.0 b 31.4

16.7 a 21.0

16.4 a 17.6

17.4 ab 12.2

t at p = 0.05.

P. Jha et al. / Soil & Tillage Research 121 (2012) 57–62 59

2.4. Soil carbon pools

Soil carbon content was divided into 3 pools (active, slow andresistant pool). Resistant organic C (Cr) in soil samples wasdetermined using the method suggested by Rovira and Vallejo(2002). In brief, one gram of oven-dry sieved (<0.25 mm) soil samplewas hydrolyzed with 25 ml of 6 M HCl at 110 8C for 18 h withoccasional shaking. After cooling, the unhydrolyzed residue wasrecovered by centrifuging. The process of centrifuging (at 20 8C) anddecantation was repeated several times with de-ionized water untilsamples were free from chloride. Residues were then transferred topre-weighed vials, dried at 60 8C to constant weight, and total C wasmeasured by dry combustion technique using Shimadzu TOCanalyzer (SSM5000A). The remaining pools, Ca (labile/active) andCs (slow) were estimated by using the double decomposition model.SOC (soil organic carbon) pools are divided into Ca and Cs poolsaccording to their turnover time with the assumption that negligibleamount of CO2 was evolved from the resistant pool (Cr) during theshort incubation period (Paul et al., 1999). The use of extendedlaboratory incubations of soil with measurements of CO2 has beenwidely used to differentiate the Ca and Cs functional C pools(Motavalli et al., 1994; Paul et al., 1999; Collins et al., 1992). Thismethod can be considered as a biological fractionation of organicmatter whereby Ca of soil organic matter (SOM) are rapidlymineralized by soil microorganisms and subsequent slow fractionCs fractions are more slowly mineralized. The CO2 evolved during Cmineralization was used to determine the size and kinetics of thefunctional C pools of soil for each site (Paul et al., 1999).

The size and turnover rates of each pool were estimated bycurve fitting the CO2 evolved per unit of time (Ct) using a two-component first-order model:

Ct ¼ Cae�kat þ Cse�kst (2)

where Ct is total organic carbon at time t; Ca and Cs are the sizes ofthe active, and slow carbon pools; ka and ks are respective pool’sdecay constants.

2.5. Model fitting and statistical analysis

The double decomposition equation was fitted with the non-linear regression (SPSS window version) that was used in theMarquardt algorithm and an iterative process to find the parametervalues that could minimize the residual sum of squares. The resultantpool sizes and their mineralization rate constants are generallysensitive to the initially assigned parameter values, and the iterativesteps size. It was found that the automatically estimated initialparameters resulted in acceptable parameter values. The onlycaution was taken that decay constant of carbon pools should not benegative, and the sum of the slow and active pool should not exceedthe acid hydrolyzable (6 M HCl) soil carbon pool. The model whichgave the lowest value of RMS (root mean square) and high F-valuewas chosen as the best fit. The mean rates of CO2-C evolutions atdifferent day intervals were used to compare the treatment effects onsoil organic C mineralization. Analysis of variance (ANOVA) wasconducted to assess the impact of aggregate size on the mean rates ofCO2-C evolution over the different incubation periods. Responses ofsoil CO2-C evolutions to the aggregate size classes were comparedusing the least significant difference (LSD) at p � 0.05.

3. Results and discussion

3.1. Aggregate size distribution

Soil organic matter accumulation and storage are stronglyrelated to soil aggregation and facilitated by the formation and

stabilization of micro-aggregates and macro-aggregates. Aggre-gate size distribution, aggregate associated C and C distribution indifferent aggregates are shown in Table 1. About, 70% of aggregatecorrespond to small macro-aggregates (0.25–2 mm). This isfollowed by large macro-aggregates (>2 mm) which constitutesabout 17.6%. The proportion of large macro-aggregates is the mostimportant fraction to evaluate the effect of management practiceson soil aggregation, because it exerts a strong influence on themean weight diameter, a comprehensive index for evaluating soilaggregation (Jiao et al., 2006; Razafimbelo et al., 2008). Majumdaret al. (2010) also reported that long-term fertilization mainlycontributes to the formation of small macro-aggregates.

Aggregate associated C, and its distribution are shown in Table1. It was found to be the highest in 1–2 mm aggregate size anddecreased as the size of aggregate decreased up to 0.25–1 mm.However, carbon content in less than 0.25 mm was found to behigher than the 0.25–1 mm size class. The C concentrations weresimilar within macro-aggregate size classes, but the greatestproportion of C occurred in the >1 mm size class. Similar resultswere reported by Sotomayor-Ramirez et al. (2006). Approximately,50% of the total SOC associated in aggregates was present in these 2aggregate size classes. In vertisol, a large part of the total aggregateC could be protected by microbial attack by its physical isolationwithin macro-aggregates, and by the high SOC initially present.The higher content of SOC in smaller aggregate size class(<0.25 mm) could be the result of greater stabilization of soilorganic matter in silt and clay associated fraction. In the presentstudy, the samples collected were from the conventionally tilledplot, the results are in accordance to the findings of Bossuyt et al.(2002), who also reported more recently added C in the 2–4 mmaggregate size class. Majumder and Kuzyakov (2010) also reportedincreased residue 14C retention in micro-aggregate size fractionsunder long-term fertilization. Benbi and Senapati (2010) alsoreported that continuous application of rice straw and farmyardmanure (FYM) either alone or in conjunction with inorganicfertilizers improved macro-aggregate associated C in rice-wheatcropping on a sandy loam soil after 7 years of cropping. Theyfurther reported that macro-aggregates had higher C and Nconcentration as compared to micro-aggregates. In another study,Wright and Hons (2005) reported that majority of SOC storage at0–5 cm depth was observed in 0.25–2-mm aggregates, and at 5–15 cm depth, in the >2-mm and 0.25–2-mm fractions. Similarly,Benbi and Senapati (2010) also reported that macro-aggregateshad higher C and N concentration as compared to micro-aggregates.

3.2. Labile carbon indicators and dehydrogenase activity in different

aggregate size classes

Oxidizable carbon, water soluble carbon, active carbon anddehydrogenase activity in different aggregate size classes are givenin Table 2. Soil oxidizable carbon content (Walkley and Blackcarbon) content in less than 0.25 mm was significantly higher incomparison to other aggregate size classes. Oxidizable C contentvaried from 7.6 to 10.0 g kg�1. Oxidizable organic carbon wasfound in the following order: less than 0.25 mm > 0.25–0.50 mm > 0.5–1.0 mm > 1–2 mm > 2–4 mm. In general, as theaggregate size decreased, oxidizable C content of soil increased.The decrease in size of soil aggregate might have increased thespecific surface area of soil, which leads to the higher content ofoxidizable carbon in smaller size aggregates.

Water soluble carbon (WSC) or dissolve organic carbon (DOC)is one of the most actively cycling SOC pools. WSC, which is theindicator of the labile carbon pool, was ranged from 20 to40.8 mg kg�1 in different soil aggregate class (Table 2). It wasfound to be the highest (40.8 mg kg�1) in 0.5–1 mm aggregate

Table 2Oxidizable carbon, water soluble carbon, active carbon, dehydrogenase activity, cumulative soil and residue carbon mineralization in different aggregate size classes.

Aggregate size

classes

Oxidizable

C (WBC) (g kg�1)

WSC

(mg kg�1)

AC (active carbon)

(mg kg�1)

DHA (mg TPF g�1) Cum soil C min

(mg CO2-C 50 g�1 soil)

Cum res C min

(mg CO2-C 50 g�1 soil)

2–4 mm 7.7 a 20.0 a 182.9 a 21.7 a 68.2 a 133.6 a

1–2 mm 8.0 a 23.8 b 237.7 b 23.1 a 66.5 a 129.8 ab

0.5–1 mm 8.7 b 40.8 c 272.7 b 23.4 a 72.6 ab 125.0 b

0.25–0.5 mm 8.9 b 30.7 d 259.4 b 24.7 a 79.0 b 119.2 c

<0.25 mm 10.0 c 32.1 d 261.6 b 19.2 a 85.8 c 114.0 d

Note: WBC, oxidizable carbon (Walkley and Black carbon); WSC, water soluble carbon; AC, active carbon (alkaline KMNO4 oxidizable); DHA, dehydrogenase activity; Cum C

min, cumulative soil carbon mineralization in 183 days; Cum res C min, cumulative residue carbon mineralization in 183 days. Values within a column followed by the same

letter are not significantly different at p = 0.05.

Fig. 1. Cumulative C mineralized (CO2-C mg 50�1 g soil) from wheat straw in

different size of soil aggregates.

P. Jha et al. / Soil & Tillage Research 121 (2012) 57–6260

size class, which was significantly higher than the otheraggregate size class. This was followed by less than 0.25 mm(32.1 mg kg�1) and 0.25–0.50 mm (30.73 mg kg�1) size class,respectively. In general, waters soluble carbon and active carbon(alkaline KMnO4 oxidizable C) were significantly higher in macro-aggregates than micro-aggregates. Sotomayor-Ramirez et al.(2006) also noticed greater concentration of labile and microbialbiomass carbon in macro-aggregates in comparison to micro-aggregates. Relationships between aggregate size and indicatorsof labile C have been reported by several authors (Cambardellaand Elliott, 1993; McLauchlan and Hobbie, 2004; Mikha and Rice,2004). The greater amount of total C within macro-aggregatesresults in a proportionally greater amount of mineralizable C andsuggests that macro-aggregates contribute more to short termnutrient cycling than micro-aggregates (Mikha and Rice, 2004).Active carbon content (KMnO4 oxidizable carbon) in differentaggregate size class ranged from 182.8 to 272.6 mg g�1 It wasfound highest in aggregate size class 0.5–1.0 mm followed by lessthan 0.25 mm (261.5 mg g�1) and 0.25–0.50 mm (259.4 mg g�1)aggregate size class. Higher value of labile carbon in aggregatesize class of 0.5–1.0 mm is the indication of higher activity of

Table 3Correlation matrix.

WBC WSC AC

WBC 1.00

WSC 0.57* 1.00

AC 0.61* 0.68* 1.00

DHA �0.06 0.16 0.18

Cr �0.77** �0.63* �0.76**

Cum soil C min 0.60* 0.13 0.31

Cum res C min �0.95** �0.48 �0.61*

N 15.00 15.00 15.00

Note: WBC, oxidizable carbon (Walkley and Black carbon); WSC, water soluble carbon;

resistant pool soil carbon; C min, soil carbon mineralization in 183 days; Res C min, re* Correlation is significant at the 0.05 level.** Correlation is significant at the 0.01 level.

microbes probably due to easily availability of substrate (wheatstraw) for microbial decomposition. No significant difference indehydrogenase activity was observed among the differentaggregate size class. Its activity in different aggregate size rangedfrom 19.2 to 24.7 mg TPF g�1 soil. The activity was higher inmacro-aggregates in comparison in micro-aggregates. Highdehydrogenase activity in 0.25–0.50 aggregate size class is theindication of strong catabolic activity of soil microorganisms.There are reports regarding linkages of soil dehydrogenaseactivity and microorganisms population (Deng and Tabatabai,1997; Klose et al., 1999).

3.3. Cumulative soil and residue carbon mineralization in different

aggregate size class

Table 2 and Fig. 1 depict the cumulative soil carbon minerali-zation in different aggregate size class. Cumulative soil carbonmineralization in different aggregate size ranged from 65.8 to85.8 mg CO2-C 50 g�1 soil. Soil carbon mineralization was faster inthe beginning of the incubation study thereafter decreasedprogressively with the advancement of time (Fig. 1). The decreasein decomposition rate in the latter phase is probably due to risingconcentrations of structural carbohydrates (such as lignin andhemicellulose) as a result of the loss of other constituents (sugarsand starches) in the detritus. The structural carbohydrates oftenconstitute the major portion of the detrital biomass and are usuallyvery resistant to decomposition (Mfilinge et al., 2002). Aggregatesize class significantly affected the efflux of CO2 from soil. Thelowest value was recorded in aggregate size of 1–2 mm, which wasat par with 2–4 mm size class. The finer aggregate size class (0.25–0.50 mm) mineralized significantly more of native SOC incomparison to other aggregate size classes. During 183 days ofincubation, CO2-C emission ranged from 66.5 to 85.8 mg CO2-C50 g�1 of soil (Fig. 1). Correlation matrix was developed among thevarious soil carbon pools (Table 3) and cumulative soil carbonmineralization showed significant (r = 0.60, p = 0.05) positiverelationship with soil oxidizable carbon content. Finer aggregatesize class having a higher amount of oxidizable SOC, mineralized

DHA Cr Cum soil C min Cum res C min

1.00

�0.14 1.00

0.07 �0.22 1

�0.06 �0.80** �0.89** 1

15.00 15.00 15.00 15.00

AC, active carbon (alkaline KMNO4 oxidizable); DHA, dehydrogenase activity; Cr:

sidue carbon mineralization in 183 days.

Table 4Soil carbon pools and their MRT in soil aggregate classes.

Soil aggregate classes Active pool C (%) MRT (days) Slow pool C (%) MRT (years) Resistant pool C (%) Soil organic C (%)

2–4 mm 4.1 (36.7) a 45.5 13.7 (122.8) a 1.8 81.6 a 1.79 ab

1–2 mm 4.5 (42.8) a 52.6 25.5 (242.5) b 5.4 69.5 b 1.90b

0.5–1 mm 6.1 (52.13) a 55.6 17.0 (143.0) c 4.1 76.8 b 1.67 a

0.25–0.5 mm 5.7 (47.0) a 41.7 18.4 (150.2) c 2.5 75.5 b 1.64 a

<0.25 mm 4.7 (40.8) a 34.1 24.4 (212.3) d 2.5 69.1 b 1.74 ab

Note: Value in parenthesis is absolute C content (mg 50 g�1 soil). Values within a column followed by the same letter are not significantly different at p = 0.05.

P. Jha et al. / Soil & Tillage Research 121 (2012) 57–62 61

maximum (85.8 mg CO2-C 50 g�1 soil). SOC mineralization isinfluenced by soil structure such as pore size distribution andaggregation, both of which result in heterogeneity in thedistribution of soil water and microbial activity.

The contact between soil and plant residues has a major effecton the fate of carbon in soil. In order to the isolate the effect of soilaggregate size on wheat straw carbon mineralization an incuba-tion study was carried out for 183 days. Wheat carbon minerali-zation was found to be in following order 2–4 mm > 1–2 mm > 0.5–1 mm > 0.25–0.5 mm > less than 0.25 mm (Table 2and Fig. 2). Wheat straw carbon mineralization ranged from 114.0to 133.6 mg C-CO2 50 g�1 in 183 days of incubation study (Fig. 2)from 1% wheat straw amended soil. Wheat straw carbonmineralization was significantly affected by the aggregate size.In general, significantly higher wheat straw carbon mineralizationwas occurred in soil macro-aggregates than the soil micro-aggregates. The degree of contact between crop residues andthe soil matrix, as determined by the method of residueincorporation, affects decomposition dynamics both under naturaland experimental conditions (Henriksen and Breland, 1999).Garnier et al. (2008) reported that larger residues decomposedmore slowly because it provides less surface area in contact withthe soil. Residue C stabilization is primarily based on the silt + clayprotective capacity SOC accumulation in excess of the silt + clayprotective capacity would be subject to higher rates of decompo-sition. Here, it was probably due to more associations of straw withmacro-aggregates as compared to micro-aggregates. Correlationmatrix (Table 3) clearly depicted that residue carbon mineraliza-tion was inversely related to soil oxidizable carbon content(r = �0.95, p = 0.01), cumulative soil carbon mineralization(r = �0.89, p = 0.01) and resistant soil carbon content (r = �0.80,p = 0.01). Residue carbon mineralization is also inversely(r = �0.61, p = 0.05) related to active carbon content (KMnO4

oxidizable carbon) of soil. These correlations clearly demonstratedthat residue C mineralization in soil could be attributed to theavailability of substrates (incorporated wheat straw) for soilheterotrophs.

Fig. 2. Cumulative C mineralized (CO2-C mg 50�1 g soil) from different size of soil

aggregates.

3.4. Soil carbon pools in different aggregate size class

Data on soil carbon pools in different aggregate size classes arepresented in Table 4. Active carbon pool in different aggregate sizeclasses ranged from 4.1 to 6.1% of the total C, and MRT (meanresidence time) varied from 34.1 to 55.6 days with an averagevalue of 45 days (Table 4). There was no significant difference insoil active carbon content in different aggregate size class.Determination of the size and turnover of the slow pool (Cs)showed significant differences in different aggregate size classes(Table 4). Slow carbon pool in different aggregate size class rangedfrom 13.7 to 25.5%, and MRT was 1.8–5.4 years. Slow carbon poolwas found to be the highest (25.5%) and lowest (13.7%) in 1–2 mmand 2–4 mm, respectively. MRT of 1–2 and 0.5–1.0 mm sizeaggregate was found 5.4 and 4.1 years, respectively. It could be dueto the formation of micro-aggregate protected carbon inside themacro-aggregates.

No significant difference in resistant soil C, determined by acidhydrolysis, to total soil C was found in different aggregate sizeclasses except for the 2–4 mm aggregate size class whichcontained greater percentage (81.6%) of non-acid hydrolysablecarbon (Table 4) and which was significantly higher than theresistant carbon content of other aggregate classes.

4. Conclusions

The study clearly demonstrated that residue carbon minerali-zation is significantly affected by the size of soil aggregate. Residuecarbon mineralization was inversely related to soil oxidizablecarbon content and soil carbon mineralization. Higher the soilcarbon mineralization lesser will be the residue carbon minerali-zation. In other words, greater will be the chances of residuecarbon stabilization.

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