SOILS, SEC 1 • SOIL ORGANIC MATTER DYNAMICS AND NUTRIENT CYCLING • RESEARCH ARTICLE

Stability and saturation of soil organic carbon in ricefields: evidence from a long-term fertilizationexperiment in subtropical China

Yanni Sun & Shan Huang & Xichu Yu & Weijian Zhang

Received: 1 December 2012 /Accepted: 9 June 2013 /Published online: 19 June 2013# Springer-Verlag Berlin Heidelberg 2013

AbstractPurpose Soil organic carbon (SOC) sequestration in crop-lands plays a critical role in climate change mitigation andfood security, whereas the stability and saturation of thesequestered SOC have not been well understood yet, partic-ularly in rice (Oryza sativa L.) fields. The objective of thisstudy was to determine the long-term effect of inorganicfertilization alone or combined with organic amendmentson SOC stability in a double rice cropping system, and tocharacterize the saturation behavior of the total SOC and itsfractions in the paddy soil.Materials and methods Soils were collected from a long-term field experiment in subtropical China where differentfertilization regimes have been carried out for 31 years. Thetotal SOC pool was separated into four fractions, character-istic of different turnover rates through chemical fraction-ation. Annual organic carbon (C) inputs were also estimatedby determining the C content in crop residues and organicamendments.

Results and discussion Relative to the initial level, long-termdouble rice cropping without any fertilizer application signif-icantly increased SOC concentration, suggesting that doublerice cropping facilitates the storage and accumulation of SOC.The partial substitution of inorganic fertilizers with organicamendments significantly increased total SOC concentrationcompared to the unfertilized control. Total SOC increasedsignificantly with greater C inputs and did not show anysaturation behavior. Increased SOC was primarily stored inthe labile fraction with input from organic amendments.However, other less labile SOC fractions showed no furtherincrease with greater C inputs exhibiting C saturation.Conclusions While the paddy soil holds a high potential forSOC sequestration, stable C fractions saturate with increas-ing C inputs, and thus, additional C inputs mainly accumu-late in labile soil C pools.

Keywords Carbon stability . Double-cropped rice . Organicamendments . Saturation . Soil organic carbon fractions

1 Introduction

Carbon (C) sequestration means the process of transfer andsecure storage of atmospheric CO2 into long-lived C pools(Lal 2010). Soils are the world’s largest terrestrial C pool,storing about three times the amount of C in vegetation andtwo times the amount in the atmosphere (Lal 2004). Due tointensive cultivation, cropland soils contain 25 to 75 % lesssoil organic C (SOC) than their counterparts in undisturbed ornatural ecosystems (Lal 2010). Thus, C sequestration in crop-lands has a large potential to mitigate the increasing atmo-spheric CO2 through adopting appropriate management prac-tices (Smith 2008; van Wesemael et al. 2010; Minasny et al.2012). It remains to be determined, however, whether thesequestered C can be stored over the long term (i.e., stability)and if there is a limit to how much C can be stabilized (i.e.,

Responsible editor: Thomas H. DeLuca

Y. Sun :W. Zhang (*)Institute of Applied Ecology, Nanjing Agricultural University,Nanjing 210095, People’s Republic of Chinae-mail: zwj@njau.edu.cn

S. HuangDepartment of Agronomy, Jiangxi Agricultural University,Nanchang 330045, People’s Republic of China

X. YuJiangxi Institute of Red Soil, Jinxian 331700, People’sRepublic of China

W. ZhangInstitute of Crop Science, Chinese Academy of AgriculturalSciences/Key Laboratory of Crop Physiology and Ecology,Ministry of Agriculture, Beijing 100081,People’s Republic of China

J Soils Sediments (2013) 13:1327–1334DOI 10.1007/s11368-013-0741-z

saturation) in farmlands (Stewart et al. 2007; Schmidt et al.2011; Dungait et al. 2012).

Increasing evidence indicates that the capacity of SOCstocks is finite, particularly in the surface soil, implying anupper limit or saturation level for SOC storage (Six et al. 2002;Stewart et al. 2007; 2012; Castellano et al. 2012). Soil Csaturation means C can no longer increase in soil when anequilibrium C content is reached under specific conditions(i.e., climate, productivity, and management practices) (Westand Six 2007; Chung et al. 2010). Furthermore, C saturationlimits the rate and efficiency of C stabilization in soils, therebyinfluencing the quantity and duration of soil C sequestration(West and Six 2007; Virto et al. 2012). Meanwhile, SOC iscomposed of a complexmixture of organic matter and consistsof various fractions that are stabilized by specific mechanismsand have unique stability and saturation behaviors (Jastrowet al. 2007; Stewart et al. 2008; Dungait et al. 2012). For soilsto act as a C sink to mitigate climate change, organic C needsto be stored in the soil’s stable C fractions (i.e., increasing thestability of SOC) (Jagadamma and Lal 2010; Carrington et al.2012). Therefore, characterizing the saturation and stability oftotal SOC and its fractions will help in selecting appropriatemanagement practices that benefit both climate change miti-gation and food security (Lal 2004; Powlson et al. 2011;Stewart et al. 2012).

China is one of the most important rice-producing coun-tries in the world, accounting for 28.0 % of the world’s totalproduction in 2011 (FAOSTAT 2013). Previous studies haveshown that paddy fields in China contained large amounts ofSOC and exhibited a high potential for C sequestration (Yuet al. 2009; Sun et al. 2010; Huang et al. 2012). Recent resultsfrom direct measurements of SOC also confirmed that SOCstocks in paddy soils increased significantly from the early1980s to the late 2000s in China, though their original valueswere already high (Yan et al. 2011). Although a large body ofresearch has been conducted regarding C sequestration inpaddy soils, most studies addressed the dynamic of the totalSOC pool (Huang et al. 2012). Relatively less attention wasgiven to the saturation behavior and stability of SOC and itsfractions in rice paddy soils (Pan et al. 2003; Schulz et al.2011; Song et al. 2012).

Fertilizer management, particularly organic amendments,plays a critical role in increasing SOC stocks in croplands(Lal 2010; Zhang et al. 2012). Meanwhile, long-term fertil-ization experiments provide good opportunities to examinethe effect of fertilizer management on the stability and satu-ration of SOC sequestered in croplands, as both of them mayneed to be observed on a relatively long-time scale (Mandalet al. 2008; Huang et al. 2010; Chung et al. 2010). Therefore,based on a 31-year experiment in subtropical China, thepresent study objectives were (1) to determine the long-term effect of inorganic fertilization alone or combined withorganic amendments on SOC content in the double rice

cropping system, (2) to characterize the stability of SOC bymeans of chemical fractionation, and (3) to investigate thesaturation behavior of total SOC and its fractions in thepaddy soil.

2 Materials and methods

2.1 Site description

The long-term experimental site is located at the Institute ofRed Soil, Jinxian county, Jiangxi province, China (28°21′N,116°10′E). This site is under a typical subtropical climatewith a distinct arid (July to September) and humid (March toJune) season. Mean annual temperature and rainfall are18.1 °C and 1,727 mm, respectively. A double rice (Oryzasativa L.) cropping system with winter fallow has beeninitiated in a long-term field experiment since 1981. Theearly rice was transplanted in late April and harvested inmid-July, while the late rice was transplanted in late July andharvested in late October.

The soil was developed from Quaternary red clay and wasclassified as a typic Stagnic Anthrosol. The soil of the plowlayer (0–15 cm), sampled in 1981, contained 16.2 g kg−1

organic C, 1.6 g kg−1 total nitrogen (N), 0.5 g kg−1 totalphosphorus (P), 143.7 mg kg−1 available N (alkali hydrolyz-able N), 10.3 mg kg−1 available P (Olsen-P), 38.2 mg kg−1

available potassium (K) (ammonia acetate extractable K),and 260.0 g kg−1 clay (<0.001 mm), with an initial pH of 5.7.

2.2 Experimental design

The experimental design was a randomized complete blockwith three replicates (plot size 60 m2). The experiment hadnine treatments, including seven organic amendments com-bined with reduced rates of inorganic N, P, and K fertilizers(T3–T9), one inorganic fertilization alone treatment with fullrates of inorganic N, P, and K fertilizers (NPK), and onecontrol treatment without any fertilizer application (control)(Table 1). The application rates of inorganic N, P, and Kfertilizers were 160, 16.4, and 100 kg ha−1 in the NPK treat-ment, and 70, 6.5, and 28 kg ha−1 in the T3–T9 treatments foreach rice growing season, respectively (Table 1). The fertili-zation rates were recommended by the Jiangxi Institute of RedSoil, which were based on soil tests when initiating the exper-iment. All inorganic fertilizers were applied as basal fertilizersin all treatments except for the NPK treatment, in which twothirds of N was applied as basal fertilization and one third astopdressing for both early and late rice. Inorganic N, P, and Kfertilizers were applied in the form of urea, calcium–magne-sium phosphate, and potassium chloride, respectively. Theexperiment included tree organic sources: legume green ma-nure (GM, Astragalus sinicus L.), farmyard manure (FYM),

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and rice straw (RS). Green manure was cut from other fieldsand incorporated into soil during tillage before early ricetransplanting. Farmyard manure in the form of fresh pigmanure was applied before rice transplanting. Rice strawobtained directly from experimental plots was either incorpo-rated into soil before the transplanting of late rice or used asmulching during winter fallow. Otherwise, following the har-vest of late rice, the fields were left unmanaged during thefallow period. The weight of organic amendments was basedon fresh mass with the water content of 90, 50, and 10 % forGM, FYM, and RS, respectively (Table 1) (Liu et al. 2009).Based on dry weights at 60 °C, the C concentration of organicamendments was 467, 340, and 421 g kg−1 for GM, FYM, andRS with C: N ratios of 17, 12, and 58, respectively (Liu et al.2009). Other field management practices such as ricetransplanting, irrigation, and plant protection were identicalto local farmers.

2.3 Determination of carbon input

Rice grain and straw yields were determined following cropharvest in each rice growing season. Annual organic C inputsinto soils included extra C inputs through organic amend-ments and the return of rice roots and stubbles. The biomassof roots and stubbles returned to fields was estimated by 50and 63 % of the total aboveground biomass (rice grain andstraw) in the fertilized treatments and the control, respective-ly (Liu et al. 2009). Carbon inputs were then calculated bythe amount of returned crop residues and organic amend-ments after calibrating for the water and organic C content.

2.4 Soil sampling and analyses

Soils were sampled by the core method following harvest oflate rice in November 2011. In each plot, six cores (4 cm in

diameter) were taken to a depth of 15 cm and were pooled asa composite sample. Samples were air-dried and then passedthrough a 2-mm sieve for analyses.

Total soil organic C (Ctot) was measured by dry combus-tion with an elemental analyzer (Elementar, Vario Max,Germany). Carbonates were not present in the samples. AsSOC was determined using a wet oxidation method withK2Cr2O7 and concentrated H2SO4 in 1981, a comparativemeasurement was conducted and showed that the content ofSOC was significantly higher by 8.1 % with the wet oxida-tion method than with the elemental analyzer. Thus, SOCcontent in 1981 was corrected for comparison.

2.5 Oxidizable organic carbon and its fractions

Oxidizable SOC content was determined by the methodproposed by Walkley and Black (Walkley and Black 1934).Its different fractions were estimated through a modifiedWalkley and Black method as described by Chan et al.(2001) using 5, 10, and 20 mL of concentrated (18.0 molL−1) H2SO4 that resulted in three acid-aqueous solutionratios of 0.5:1, 1:1, and 2:1 (corresponding to 6.0, 9.0, and12.0 mol L−1 H2SO4, respectively). Thus, the determinedamount of C allowed separation of Ctot into the followingfour fractions according to their decreasing order ofoxidizability (Mandal et al. 2008):

Fraction 1 (most labile): organic C oxidizable by 6.0 molL−1 H2SO4

Fraction 2 (labile): the difference in C oxidizable by 9.0and that by 6.0 mol L−1 H2SO4

Fraction 3 (less labile): the difference in C oxidizable by12.0 and that by 9.0 mol L−1 H2SO4

Fraction 4 (non-labile): the difference between Ctot and Coxidizable by 12.0 mol L−1 H2SO4

Table 1 Organic amendments and inorganic fertilizer application under different treatments

Treatment Early rice Late rice Winter fallowkg ha−1 kg ha−1 kg ha−1

Control Unfertilized

NPK 160 N, 16.4 P, 100 K 160 N, 16.4 P, 100 K

T3 22,500 GM+70 N, 6.5 P, 28 K 70 N, 6.5 P, 28 K

T4 45,000 GM+70 N, 6.5 P, 28 K 70 N, 6.5 P, 28 K

T5 22,500 GM+22,500 FYM+70 N, 6.5 P, 28 K 70 N, 6.5 P, 28 K

T6 22,500 GM+70 N, 6.5 P, 28 K 22,500 FYM+70 N, 6.5 P, 28 K

T7 22,500 GM+70 N, 6.5 P, 28 K 22,500 FYM+70 N, 6.5 P, 28 K 4,500 RS

T8 22,500 GM+70 N, 6.5 P, 28 K 70 N, 6.5 P, 28 K 4,500 RS

T9 22,500 GM+70 N, 6.5 P, 28 K 4,500 RS+70 N, 6.5 P, 28 K

The weight of the organic amendments was based on their fresh mass. The inorganic N, P, and K fertilizers applied were urea, calcium–magnesiumphosphate, and potassium chloride, respectively

GM green manure, FYM farmyard manure, RS rice straw

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According to Chan et al. (2001), fractions 1 and 2 togetherrepresent the active C pool while fractions 3 and 4 togetherconstitute the passive pool of organic C in soils.

2.6 Statistical analyses

Analyses of variance were performed to determine the effectof long-term fertilization on mean annual rice yield, C inputs,and the concentration and proportion of SOC. Means weretested using the least significant difference with a significancelevel of p <0.05. Linear regression analyses were employed toevaluate relationships between SOC concentrations of thewhole soil and various fractions and mean annual C inputsat p <0.05. All statistical analyses were performed using SPSSsoftware 11.0 (SPSS Inc., Chicago, IL, USA).

3 Results

3.1 Rice yield and carbon input

Compared to the unfertilized control, both inorganic fertilizerapplication alone (NPK) and partial substitution of inorganicNPK with organic amendments (T3–T9) significantly in-creased the mean annual crop yield in the double rice croppingsystem (Table 2). The highest mean annual rice yield wasobtained in the FYM amendment treatments (T5, T6, and T7).

Like the mean annual rice yield, mean annual C inputswere significantly greater in both NPK and organic amend-ment treatments as compared to the control (Table 2). Organicamendments resulted in higher C inputs relative to the NPKfertilization alone, with the highest annual C input rates oc-curring in the FYM amendment treatments as well. Comparedto the control, on average, 51.7 % of the increased C input in

organic fertilization treatments was attributable to organicamendments while 48.3 % from increased crop production.

3.2 Total soil organic carbon

In comparison to the control, organic amendments signifi-cantly increased the concentration of total SOC, while NPKfertilization alone had no significant effect (Table 2). Thehighest concentration of total SOC was found in the T4treatment, where a doubled rate of GM was applied. Inaddition, relative to the initial level, long-term double ricecropping significantly increased the concentration of totalSOC by 31.4 %, even in the unfertilized control.

3.3 Oxidizable organic carbon and its fractions

The following comparisons were all done across treatmentswithin each fraction. Relative to the unfertilized control, allfertilization treatments increased the C concentration in frac-tion 1; except for the T9 treatment which had a lower, but notsignificant, C concentration than that in the control (Fig. 1).The C concentration of fractions 2 and 4 did not differ signif-icantly among treatments. No significant difference in the Cconcentration of fraction 3 was found among most of thetreatments except between the T9 treatment and the control.

However, long-term different fertilization did not signifi-cantly alter the proportion of various SOC fractions (Fig. 2).It is likely due to the minor difference in C concentrations ofvarious fractions among treatments and the disproportionatedistribution of total SOC among fractions. In addition, nomarked trends were observed among different fertilizationtreatments except the non-labile fraction 4, where its propor-tion was consistently lower in all organic amendment plots(T3–T9) than that in the control and NPK treatments.

3.4 Relationship between SOC concentration and carbon input

The concentration of total SOC increased significantly withhigher mean annual C inputs (Fig. 3). Likewise, the C con-centration of fraction 1 exhibited a significant positive rela-tionship with C inputs. However, fractions 2, 3, and 4 showedno marked increase in SOC with greater C inputs. In addition,the increase in SOC concentration per unit of C input (i.e., theslope of the linear regression equation) was in the order: totalSOC > fraction 1 > fraction 2 > fraction 3 > fraction 4.

4 Discussion

4.1 Soil organic carbon sequestration in rice paddies

Long-term continuous double rice cropping (31 years) signifi-cantly increased SOC concentration by 31.4 % in the surface

Table 2 Annual rice yield and organic carbon input averaged over theentire experimental duration and the concentration of total soil organiccarbon (SOC) in 2011 under different treatments

Treatment Rice yield Carbon input Total SOCMg ha−1 year−1 Mg ha−1 year−1 g kg−1

Control 5.25d 2.91g 19.7e

NPK 8.24b 5.86f 22.2de

T3 7.76c 6.30e 24.1bcd

T4 8.12b 7.76d 28.4a

T5 8.78a 11.16b 27.0abc

T6 8.80a 11.07b 26.9abc

T7 8.98a 12.91a 27.8ab

T8 8.08bc 8.32c 23.6cd

T9 8.19b 8.47c 25.2abcd

Different lowercase letters within the same column indicate significantdifferences among treatments (p <0.05)

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layer (0–15 cm) under no fertilizer application when comparedto the initial value (Table 2), suggesting rice cropping facilitatesthe storage and accumulation of SOC (Yu et al. 2009; Huanget al. 2012; Minasny et al. 2012). This observation is consistentwith previous studies conducted in double rice cropping sys-tems in subtropical China (Tong et al. 2009; Zhang et al. 2012).A recent analysis summarized the results of independent ex-periments showing that rice cropping without any nutrientapplication can significantly increase SOC stock by 9 % com-pared to the initial level in double rice cropping systems but nosignificant effects in single rice cropping and rice-upland croprotation systems (Huang et al. 2012). The large SOC accumu-lation in double rice cropping systems may be due to their highcrop biomass production and long-time duration of submer-gence resulting in higher C inputs (e.g., stubble, root, andrhizodeposition) and lower decomposition and mineralizationrates of both amended organic matter and native SOC (Moore

et al. 2008; Kögel-Knabner et al. 2010; Zhang et al. 2012). Incontrast, Mandal et al. (2008) reported that continuous doublerice cropping without any fertilization caused a net decrease inthe content of surface SOC (0–20 cm) relative to the initial valuein a 36-year-old fertility experiment in subtropical India. Inaddition to differences in climate, soil, andmanagement practicesbetween the experimental sites, the duration and timing of“fallowing” may influence the storage of SOC (Halvorsonet al. 2002). In the present study, a “winter fallow” period lastsfor 5–6 months during the annual rice cropping cycle, when thedecomposition of SOC is likely to be low due to low-air tem-perature (Liu et al. 2009). However, the land was kept “summerfallow” in Indian for about 2.5 months during the peak summer,thereby leading to a substantial amount of the oxidative loss of C(Mandal et al. 2008).

4.2 Stability of soil organic carbon in the paddy soil

Long-term fertilization, particularly organic amendments,increased the concentration of SOC (Table 2) primarily dueto C accumulation in the most labile fraction (fraction 1)(Fig. 1). Chen et al. (2011) fractionated SOC pools by acidhydrolysis showing that increases in SOC storage underlong-term organic amendments (fresh legume residues andpig manure) were mainly accumulated in labile fractions,thus reducing the proportion of recalcitrant C in a double-cropped paddy soil. In contrast, using the same fractionationmethod as in this study, Mandal et al. (2008) reported thatlong-term application of organic compost promoted the shiftin SOC towards passive pools (fractions 3 and 4), and thusbenefited the long-term C storage.

Fig. 1 Carbon concentration of various fractions under different treat-ments. Bars represent mean + standard error. Different lowercase lettersindicate significant differences among treatments within each fraction(p <0.05)

Fig. 2 Carbon proportion of various fractions under different treat-ments. Bars represent mean + standard error. Different lowercase lettersindicate significant differences among treatments within each fraction(p <0.05)

Fig. 3 Relationship between mean annual carbon input and soil organiccarbon (SOC) concentration across treatments within the whole soil andvarious fractions. Fitted linear regression equations are as follows: TotalSOC: y=0.7736x+18.5620 (R2 = 0.70, p <0.01); fraction 1:y=0.4846x+9.7954 (R2=0.52, p <0.05); fraction 2: y=0.1369x+4.4490(R2=0.24, p=0.18); fraction 3: y=0.1055x+1.3746 (R2=0.12, p=0.37);fraction 4: y=0.0466x+2.9430 (R2=0.03, p=0.68)

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The mixed results may be primarily due to the difference inthe quality of C added through organic manure amendments.The organic manure applied in Mandal et al. (2008) was well-decomposed compost that contained large amounts of ligninand polyphenols. These biopolymers exhibit a high degree ofresistance to microbial degradation under prevailing anaerobicconditions in double rice cropping systems (Olk et al. 2006).Furthermore, the high content of phenolics and lignin in thecompost is likely to suppressmicrobial activity involved in thedecomposition of returned crop residues (Mandal et al. 2008).Recently, Yu et al. (2012) demonstrated that application offermented compost significantly decreased C mineralizationrates in an intensively cultivated sandy loam soil. In contrast,in the present study, large amounts of fresh legume greenmanure were applied in all organic amendment treatments.Legumes generally have high litter quality (low C: N) andhigh litter decomposition rates (Fornara and Tilman 2008).Thus, legume manure application increased the content oftotal SOC, but mainly affected the most labile fraction (frac-tion 1) (Fig. 1). In contrast to active pools (fractions 1 and 2) of51.2 % of total SOC (Mandal et al. 2008), the dominance ofactive pools in total SOC (77.9 %) in this study reflected thelong-term effect of legumemanure amendments on the qualityof SOC (Fig. 2). Finally, inputs of large amounts of labile Cthrough legume green manure may stimulate the activity ofsoil microbes, and thus increase C loss through mineralizationof recent as well as old SOC (Liu et al. 2009). Therefore,legume manure amendments could facilitate SOC storage, butmay have a limited capability for increasing the stability ofSOC (Jastrow et al. 2007). However, it should be noted thatemploying different SOC fractionation methods may reachmixed conclusions. For example, using physical fractionationtechniques, Huang et al. (2010) reported that SOC originatingfrommanure amendments was mainly stored in fractions withslow turnover rates in a double-cropped rice field. In addition,the present fractionation method should be applied to otherexperiments under similar cropping systems and managementregimes before solid conclusions can be drawn.

4.3 Saturation behavior of soil organic carbon pools

Total SOC increased significantly with higher C inputs andexhibited no saturation behavior (Fig. 3), suggesting the highpotential for soil C storage in the double rice field (Huanget al. 2012). Even after 36 years of continuous C additions ata reasonably high rate through compost and crop residues,Mandal et al. (2008) also found a significant positive linearrelationship between SOC sequestration and C inputs due totheir high capacity for storing C. On the other hand, it ispossible that SOC requires a broader range of C inputs toelicit C saturation behavior, and thus C accumulation appearsto be linear across shorter segments of a C saturation curve(Stewart et al. 2007; Virto et al. 2012).

Like total SOC, themost easily oxidizable fraction (fraction 1)showed no C saturation, while other less labile fractionsexhibited no further increase in SOC concentration in responseto greater C inputs (Fig. 3). Furthermore, the lower theoxidizability of the fractions, the less is the increase in SOCconcentration per unit of C input (i.e., the slope of the linearregression equation). Chung et al. (2008) reported that, comparedto those in macroaggregates and coarse particulate organic mat-ter, stable C fractions (C occluded in microaggregates or associ-ated with soil silt and clay) exhibited saturation behaviors due toa smaller C sequestration potential and slower C turnover. In aCanadian agroecosystem exposed to a broad range of C inputs,Gulde et al. (2008) also showed that different SOC pools satu-rated at different rates with stable fractions saturating at lower Cinputs than labile ones, thus concluding that as C inputs in-creases, the stable C fraction of soil becomes saturated, andconsequently, additional C inputs will only accumulate in labilesoil C pools. In other words, greater C inputs to soils wouldpreferentially increase C storage in labile SOC fractions ratherthan in stable components (Stewart et al. 2008; 2012; Chunget al. 2010; Carrington et al. 2012).

Furthermore, it is also possible that the passive soil C poolmay be already saturated in the rice field, and thus the addedC was mainly stored in labile C pools that have relativelyfaster turnover rates (Gulde et al. 2008; Carrington et al.2012; Stewart et al. 2012). Thus, agricultural practices thataim to enhance SOC sequestration in croplands should placemore emphasis on the dynamics of stable C fractions forsynchronizing climate change mitigation with food security(Lal 2004; Powlson et al. 2011).

5 Summary

Compared to initial levels, long-term double rice croppingled to a marked increase in SOC content, even without anyfertilizer application. Organic amendments significantly in-creased the level of total SOC relative to the control, whereasincreased C was mostly stored in the labile fraction. Ingeneral, total SOC and the labile C fraction increased signif-icantly with greater C inputs, showing no C saturation be-havior. However, other less labile fractions did not increasewith C inputs exhibiting C saturation behavior. Stable Cfractions in the soil may have already been saturated, andthus, additional C inputs were mainly stored in the labile Cpools that have faster turnover rates. Therefore, given thelarge potential for SOC sequestration in rice fields, moreattention should been focused on increasing the content ofstable C fractions other than just the total C pool to benefitboth climate change mitigation and food security.

Acknowledgments This work was supported by the National KeyTechnology Support Program of China (2011BAD16B14,

1332 J Soils Sediments (2013) 13:1327–1334

2012BAD14B14), the Foundation of Jiangxi Province (GJJ12245,QN201102), the innovation program of Chinese Academy of Agricul-tural Sciences and New Century Excellent Talents Program, China(NCET-05-0492).

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