Changes in soil organic carbon stocks as affectedby cropping systems and cropping durationin China’s paddy fields: a meta-analysis

Shan Huang & Yanni Sun & Weijian Zhang

Received: 2 August 2009 /Accepted: 31 August 2011 /Published online: 28 September 2011# Springer Science+Business Media B.V. 2011

Abstract Great uncertainties remain in the impact of cropping systems on soil organiccarbon (SOC) stocks in paddy fields that hold a large potential for carbon (C) sequestration.In this study, a meta-analysis was performed to examine trends on SOC stocks inunfertilized and fertilized fields from three of the most common rice cropping systems inChina. Results showed that rice cropping without any nutrient application (Control)significantly increased SOC stocks by 9% compared to the initial level in double ricecropping systems (DR), whereas no significant effects were observed in single ricecropping systems (SR) and rice-upland crop rotation systems (RU). Paddy soils sequesteredC in all the three cropping systems under inorganic NPK fertilization, and the magnitude ofthe increase in SOC stocks was in the order DR > RU > SR. Soil C stocks increased withthe increasing cropping duration. Continuous rice cropping for more than 20 years led toaverage SOC gains of 15% and 23% in the control and NPK treatments, respectively.Furthermore, it seems that C sequestration was still occurring in the longest fields from theincluded studies. Thus, no SOC saturation trend was found over the investigated croppingduration. However, the negative relationship between SOC changes and their initial Cstocks suggests indirectly the possibility of SOC saturation in paddy fields.

1 Introduction

The concentration of atmospheric CO2 has increased from a pre-industrial value of about280 ppm volumetrically to 379 ppm in 2005 and is expected to double by the end of the

Climatic Change (2012) 112:847–858DOI 10.1007/s10584-011-0255-x

Electronic supplementary material The online version of this article (doi:10.1007/s10584-011-0255-x)contains supplementary material, which is available to authorized users.

S. Huang : Y. SunDepartment of Agronomy, Jiangxi Agricultural University, Nanchang 330045, China

S. Huang : Y. Sun :W. ZhangInstitute of Applied Ecology, Nanjing Agricultural University, Nanjing 210095, China

W. Zhang (*)Institute of Crop Science, Chinese Academy of Agricultural Sciences, Beijing 100081, Chinae-mail: zwj@njau.edu.cn

twenty-first century (IPCC 2007). Great attention has been paid to carbon (C) sequestrationto reduce the atmospheric CO2 concentration for mitigating global climate change(Solomon et al. 2009; Meinshausen et al. 2009). The global soil organic C (SOC) pool isabout two times the atmospheric pool and three times the biotic pool, and thus Csequestration in soil has been widely considered as a promising measure for mitigating theincreasing atmospheric CO2 concentration (Lal 2004; Smith 2008). Cropland soils arecharacterized by relatively low SOC content due to intensive cultivation, and are frequentlyaffected by great human disturbance, thus suggesting the large potential for C sequestrationunder appropriate management practices (Lal 2004).

China is the most important rice-producing country in the world. The sown area of riceaccounted for 18.8% of the total crop area in China (National Bureau of Statistics 2008).Previous studies have shown that paddy soils in China held large C stocks and exhibited ahigh potential for C sequestration (Pan et al. 2003; Liu et al. 2006; Wang et al. 2010).Recently, Pan et al. (2010) reported that rice paddies not only held higher SOC stockscompared to dry croplands in China, but showed greater sequestration rates from 1985 to2006. Furthermore, ample evidence indicated that SOC stocks increased in China’ ricefields from 1980s to 2000s (Xie et al. 2007; Yu et al. 2009; Xu et al. 2009; Wang et al.2009; Wang et al. 2010). In addition, Huang and Sun (2006) estimated that topsoil Csequestration occurred in paddy fields contributed 74% to the increase of the SOC pool inChina’s croplands over the last two decades. Much research has gone into the evaluation onchanges in SOC stocks and the effect of agricultural management practices on SOCsequestration in rice fields (Guo and Lin 2001; Yu et al. 2009; Lu et al. 2009; Wang et al.2009; Wang et al. 2010). However, little attention has been paid to the quantitativeassessment concerning SOC changes as affected by rice cropping systems and croppingduration (Pan et al. 2003).

Cropping systems may play a critical role in the change of SOC stocks by affecting thebalance between C inputs through litter additions and losses through decomposition. Fieldexperiments focusing on the effect of rice cropping on SOC stocks have reached mixingconclusions. For example, Hao et al. (2008) reported that compared to initial levels, SOCstocks in the unfertilized treatment increased by 66% in a long-term double rice-croppingsystem, whereas significant declines were observed in rice-corn rotation systems. Pan et al.(2009) showed that long-term rice-rape rotation did not significantly affect SOC stocks inthe unfertilized plots, while resulting in considerable increases in the inorganic fertilizedtreatments. In contrast, Shen et al. (2007) reported that soils in the unfertilized treatmentcontained 27.4% C more than the initial soil in a long-term rice-wheat rotation system. Riceis mainly planted in three cropping systems in China: single rice cropping per year, rice-upland crop rotations, and double rice cropping annually. Various cropping systems arecharacterized by differential whole-year crop yields, and rice yields vary largely even in thesame cropping system, thus resulting in differences in C inputs through crop residues (Yu etal. 2009). On the other hand, climatic and soil conditions differ significantly among thethree rice cropping systems. For example, wet-dry cycles in rice-upland crop rotationsystems may increase decomposition of SOC due to intense disturbance, while permanentwater flooding in the double rice-cropping system is likely to prevent microbialdecomposition and benefit SOC stocks (Cai et al. 2003). In contrast, SOC decompositionrates may be lower in the single rice cropping system due to low temperature. Therefore, wehypothesized that the direction and magnitude of changes in paddy SOC stocks likelyvaried with rice cropping systems.

Growing evidence indicates that the capacity of SOC stocks is finite, particularly in thesurface soil, implying an upper limit or saturation level for SOC stocks (Hassink 1997; Six

848 Climatic Change (2012) 112:847–858

et al. 2002; Stewart et al. 2007). No further C can be sequestered with time when anequilibrium C content is reached under steady C inputs (West and Six 2007). Thus,changes in SOC stocks likely depend on the duration over which rice is cropped. Inaddition, due to the increase in rice yields and thus the growing C inputs resultingfrom inorganic fertilization and the improvement of crop varieties and managementpractices over the recent three decades in China, the saturation level of SOC in paddyfields may have increased as well (Stewart et al. 2007; Yu et al. 2009). However, littleinformation exists regarding the effect of rice cropping duration on SOC stocks in paddyfields (Cheng et al. 2009).

Meta-analysis provides a quantitative method used to integrate results from manyindependent studies with an attempt to estimate the direction and magnitude of a treatmenteffect (Gurevitch and Hedges 1999; Guo and Gifford 2002). Therefore, in the present study,we assessed the direction and magnitude of the effect of rice cropping systems and croppingduration on SOC stocks in China’s paddy fields using the meta-analytical method.

2 Materials and methods

2.1 Data collection

We extracted results for SOC changes in rice fields from all peer-reviewed literaturepublished in Chinese and English journals before May 2009. Data published in Chinesewere collected from the Chinese Journal Net (CJN) full-text database and those in Englishfrom the Science Citation Index (SCI) of the Institute for Scientific Information. Wefocused on field studies in which changes in SOC stocks were measured. To include asmany studies as possible, studies that did not report the measure of variance were alsoconsidered in our analysis. Consequently, 44 published papers were used for the presentmeta-analysis (Appendix 1). Detailed information of rice experimental sites is shown inAppendix 2. In addition, crop yields in the corresponding experimental site were collectedto qualitatively represent the difference in C inputs returned through crop residues amongrice cropping systems.

In meta-analyses, one assumption is the independence of studies included (Gurevitchand Hedges 1999). Therefore, if multiple observations were contained in the assembleddataset for the same field plots, we restricted our analysis to the latest sampling date (exceptfor the test of the effect of cropping duration).

Paddy field experiments in China were divided into three rice cropping system categories:single rice (Oryza sativa L.) cropping per year (SR), rice-upland crop (including winter wheat(Triticum aestivum L.), spring corn (Zea mays L.), and rape (Brassica campestris L.))rotations (RU), and double rice cropping annually (DR) (Fig. 1). We examined the effect ofrice cropping without any nutrient application (Control) on SOC stocks. In addition, thecombined nitrogen, phosphorus, and potassium fertilization treatment (NPK) was alsoincluded in the present analysis, because the balanced fertilization regime is widely employedby farmers in rice production in China. The cropping duration was divided into fourcategories: 1–5, 6–10, 11–20, and >20 years. Temperature was not considered in our analysisbecause of the colinearity with cropping systems.

In some studies where only SOC concentrations were reported, SOC stocks werecalculated as follows:

Cs ¼ Cc � BD� H � 10�1 ð1Þ

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where Cs and Cc are SOC stocks (t ha−1) and concentrations (g kg−1), respectively. BD is thesoil bulk density (g cm−3), and H is the soil sampling depth (cm). If soil organic matter wasreported, SOC equals to soil organic matter multiplied by a conversion coefficient of 0.58.

In cases where soil bulk densities were missing, the densities were estimated as follows,using the equation of Pan et al. (2003) that was derived from the empirical relationshipbetween soil bulk density and SOC concentration:

BD g cm�3� � ¼ �0:220� ln Cc g kg�1

� �þ 1:780 ð2Þ

In this study, we focused only on changes in SOC stocks in the surface layer (0 to 15–20 cm) of paddy fields due to limited data on the subsurface layer.

2.2 Meta-analyses

The effect size was calculated as the natural log of the response ratio (r), which is themean SOC stock of the treatment (i.e. Control and NPK) divided by the mean of theinitial SOC stock. The percentage changes in SOC stocks expressed in the text wereestimated by (r−1)×100%. Positive percentage changes indicate an increase in SOCstocks, while negative values indicate a decrease.

An un-weighted meta-analysis was used because a limited number of studies reporteddata that would allow calculation of sample variance (standard deviations or standard errorswith replicate size). In the present analysis, a mixed effect model was used, with theassumption that differences among studies within a category are due to both sampling error

SR

RU

DR

Fig. 1 Locations of paddy field experiments in different rice cropping systems in China. SR: single ricecropping systems; RU: rice-upland crop rotation systems; DR: double rice cropping systems

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and random variation. Bias-corrected 95% confidence intervals (CIs) were calculated foreach mean effect size by a bootstrapping procedure (5,000 iterations) using MetaWinsoftware (Rosenberg et al. 2000). Means were considered to be significantly different fromzero if their 95% CIs did not overlap zero. Statistical analyses for crop yields wereperformed using SPSS software 11.0 (SPSS Inc., Chicago, IL, USA) and P<0.05 wasconsidered significant.

3 Results

3.1 Crop yields in different rice cropping systems

Since rice-wheat rotations dominated the RU system, wheat yields were used to comparethe difference in year-round crop productivity among rice cropping systems. Application ofNPK significantly increased the year-round crop yield by 84.0%, 78.4%, and 57.2%compared with the control in the SR, RU, and DR cropping systems, respectively (Fig. 2).Rice yields were significantly higher in the control in the SR (4,044 kg ha−1) and RU(4,062 kg ha−1) systems than those of early rice in the DR (3,035 kg ha−1). Both early andlate rice yields in the NPK treatment in the DR (5,202 and 5,166 kg ha−1, respectively)were significantly lower than those in the SR (7,441 kg ha−1) and RU (6,744 kg ha−1). Theyear-round crop productivity was greater in the RU and DR than in the SR. However, nomarked difference in the year-round crop productivity was observed between the RU andDR systems with 6,166 and 6,594 kg ha−1 in the control (P=0.395), and 11,002 and10,368 kg ha−1 in the NPK treatment, respectively (P=0.629).

3.2 Soil C stocks in different rice cropping systems

Rice cropping resulted in a significant increase of 9% in SOC stocks in the DR system in thecontrol treatment compared to the initial level, whereas no significant changes in SOC stockswere observed in the SR and RU cropping systems (95% CIs of means overlap zero) (Fig. 3a).

0

2000

4000

6000

8000

10000

Rice Rice Wheat Early rice Late rice

SR RU DR

Cropping system

Gra

in y

ield

(kg

ha-1

)

Control NPK

Fig. 2 Crop yields in the controland NPK treatments under dif-ferent rice cropping systems. SR:single rice cropping systems; RU:rice-upland crop rotation systems;DR: double rice cropping sys-tems. Bars represent mean ±standard error

Climatic Change (2012) 112:847–858 851

Soil organic C stocks in paddy fields increased significantly in all the three cropping systemsin the NPK treatment compared to the initial level (Fig. 3b). On average, rice cropping resultedin SOC gains of 2%, 13%, and 19% in the SR, RU, and DR cropping systems, respectively.

3.3 Effects of cropping duration on SOC stocks in paddy fields

Soil organic C stocks in paddy fields increased with the increasing cropping duration. Ricecropping for more than 20 years significantly increased SOC stocks by 15% in theunfertilized plots compared to the initial level, whereas no marked changes in SOC stockswere observed in rice fields cropped for less than 20 years (Fig. 4a).

Overall, SOC stocks showed the same increasing trend with time in the NPK treatmentas in the control (Fig. 4b). However, the increase in SOC stocks was much greater in theNPK treatment than in the control. Rice cropping for 1–5 years led to a significant increaseof 8% in SOC stocks under NPK fertilization. The magnitude of the increase in SOC stocksin paddy fields cropped for 6–10 years was similar as that for 11–20 years in the NPKtreatment, with average C increases of 17% and 15%, respectively. Rice cropping for over20 years increased SOC stocks by 23% under NPK fertilization. Thus, no soil C saturationtrend was observed in both the control and NPK treatments over the investigated cropping

(a)

(6)(13)

(21)

-10

0

10

20

30

SR RU DRCropping system

SR RU DRCropping system

SOC

cha

nge

(%)

(b)(20)

(14)

(7)

-10

0

10

20

30

40

SOC

cha

nge

(%)

Fig. 3 Effects of rice croppingsystems on changes in soil or-ganic carbon (SOC) stocks in thecontrol (a) and NPK (b) treat-ments compared to their initiallevels. SR: single rice croppingsystems; RU: rice-upland croprotation systems; DR: double ricecropping systems. The bars show95% confidence intervals. Thenumber of observations is indi-cated in parentheses

852 Climatic Change (2012) 112:847–858

duration, indicating that C sequestration may still occur even in the longest fields from theincluded studies.

3.4 Relationship between SOC changes and their initial C stocks

Although no soil C saturation trend was observed in the paddy fields included in the meta-analysis (Fig. 4a-b), changes in SOC stocks may be affected by their initial C levelsaccording to the hypothesis of soil C saturation (Stewart et al. 2007). Therefore, alogarithmic function was used to describe the relationship between SOC changes and theirinitial C stocks in the present study (Fig. 5). Changes in SOC stocks were significantlynegatively correlated with the initial SOC stocks (P<0.001).

4 Discussion

4.1 Soil C sequestration depending on rice cropping systems

Our study showed that SOC stocks in rice fields significantly increased, particularly in thelong term. Pan et al. (2003) reported that C stocks in China’s paddy soils are comparable to

(a) (8)

(31)

(28)

(44)

-10

0

10

20

30

1-5 6-10 11-20 >20Duration (years)

SOC

cha

nge

(%)

(b) (8)

(36)(37)

(42)

-10

0

10

20

30

40

1-5 6-10 11-20 >20

Duration (years)

SOC

cha

nge

(%)

Fig. 4 Effects of cropping dura-tion on changes in soil organiccarbon (SOC) stocks in the con-trol (a) and NPK (b) treatmentscompared to their initial levels.The bars show 95% confidenceintervals. The number of obser-vations is indicated in parentheses

Climatic Change (2012) 112:847–858 853

those in the US grasslands and are higher than those in the cultivated upland soils in Chinaand the US. Results from soil monitoring at the national scale also indicated that SOCstocks in rice paddies were greater than those in upland fields, and the C sequestration ratein paddy soils was two times larger than that in upland soils over the recent two decades(Pan et al. 2010). In contrast, an overall decline in cropland SOC stocks was observed inother countries of the world in temperate zone with upland cropping (Bellamy et al. 2005;Saby et al. 2008). Increasing rice yields and anaerobic conditions were attributed to thesequestration of SOC in China’s paddy fields (Yu et al. 2009).

The present meta-analysis indicated that changes in SOC stocks in rice fields varied withcropping systems. Previous evidence has shown that increases in temperature wouldpromote the decomposition of SOC (Mo et al. 2005; Zheng et al. 2009). However, themagnitude of the increase in SOC stocks was highest in the DR system, though the meanannual temperature increased in the order DR > RU > SR, thus suggesting that temperaturemay be not the main factor controlling SOC stocks in rice fields. Indeed, C dynamics insoils are a multifactor-controlled process, particularly in cropland soils that are influencedby intensive human management (Lohila et al. 2003; Gesch et al. 2007).

Mandal et al. (2007) suggested that SOC stocks could be enhanced in cropping systemsthat provided more C than the critical value of C losses. The year-round crop productivitywas high in the DR cropping system, implying great above and below ground C inputs(Zhang et al. 2007). In addition, rainfall is abundant in southern China, and thus rice fieldsare generally flooded or waterlogged in the DR cropping system even in the non-ricegrowing season (Cai et al. 2003). Previous studies have shown that, under continuousanaerobic conditions, both the decomposition of amended organic matter and mineraliza-tion of native SOC were lower than those under aerobic conditions (Witt et al. 2000; Guoand Lin 2001). Therefore, multiple rice cropping characterized by high productivity andrelatively low C losses may benefit SOC sequestration in China. In contrast, despite highcrop productivity, SOC sequestration was lower in the RU cropping system than in the DR.It is likely that periodical wet-dry cycles alter soil conditions (e.g. temperature, moisture,and aeration), thereby increasing the decomposition rate of SOC in the RU cropping system(Wassmann et al. 2004; Zou et al. 2005). Furthermore, aggregates are susceptible todisruption when soil is exposed to wet-dry cycles that may reduce the physical protection ofsoil aggregates for SOC (Lundquist et al. 1999; Jastrow et al. 2007). The present results

y = -21.048Ln(x ) + 79.941

R 2 = 0.152, P <0.001

-40

0

40

80

120

160

0 20 40 60Initial SOC stocks (t ha-1)

SOC

cha

nge

(%)

Fig. 5 Relationship betweenchanges in soil organic carbon(SOC) stocks and their initialSOC stocks

854 Climatic Change (2012) 112:847–858

showed that SOC sequestration was lower in the SR system than in the RU and DR, whichcan be partially explained by the lower whole-year crop productivity. In addition, SOCsequestration may be constrained by the high initial SOC stocks in the SR cropping systemthat is located mainly in northeastern China where black soils (phaeozems, Food andAgriculture Organization (FAO) of the United Nations) dominate and natural soil fertility ishigh due to relatively short reclamation history (Huang and Sun 2006; Yu et al. 2009).

In the present study, greater SOC sequestration occurred in all cropping systems in theNPK fertilization treatment than in the control, which is mainly associated with morereturns of crop residues and root-related C resulting from higher crop productivity in theNPK treatment compared with the control (Mandal et al. 2007; Pan et al. 2009).

4.2 Carbon sequestration duration in paddy soils

Our results showed that SOC stocks increased with the increasing cropping duration, beinghighest in rice fields cropped for over 20 years in both the control and NPK treatments.Thus, no C saturation trend was observed over the investigated cropping duration, as noleveling-off or declining trends in SOC stocks were found. We expect that the relativelyshort experimental duration may result in the lack of SOC saturation, as no study durationwas more than 30 years in the dataset. Thus, SOC sequestration in paddy fields may occurover a longer period, at least more than 20 years. Indeed, evidence from chronosequentialchanges in SOC stocks indicated that 30–50 years might be needed for rice fields to reachan equilibrium soil C content (Li et al. 2005; Cheng et al. 2009).

On the other hand, the negative relationship between changes in SOC and their initial Cstocks suggests indirectly that soil C saturation may exist in paddy fields (Fig. 5). Reicoskyet al. (2002) also found no increase in SOC stocks after long-term straw retention,suggesting that SOC sequestration may be affected by the amount of C already present inthe soil (i.e. a non-linear increase in SOC stocks as C inputs increase) (Stewart et al. 2008).A decreased SOC sequestration potential was observed in soils with high C content relativeto those with low C content, which is consistent with the concept of soil C saturation(Campbell et al. 1991). The lower the initial C content, the further a soil is from saturationlevel, the greater the SOC sequestration potential (Stewart et al. 2007). In addition, Stewartet al. (2008) reported that initial soil C contents influenced the stabilization of added C, andsuggested that soils with low C stocks might have the greatest potential and efficiency tosequester C because they were further from their saturation level.

The 2006 IPCC guidelines provide a default factor of 10% for the C stock increasein paddy soils that are cropped for rice more than 20 years (IPCC 2006). In the IPCCmethodology, however, country-specific and management-specific stock change factorsare also encouraged to be employed to reduce the uncertainty in assessment associatedwith SOC stock changes (IPCC 2006). In the present study, rice cropping for more than20 years resulted in SOC sequestration of 15% and 23% in the control and NPKtreatments, respectively. Therefore, estimating SOC stock changes should take intoaccount the effects of cropping systems, cropping duration, and management practices. Inour study, no C saturation trend was observed over the investigated cropping duration. Itseems that C sequestration was still occurring in the longest fields from the includedstudies. In addition, given the growing fertilization rates and rice yields since the 1980s(Zhu and Chen 2002), China’s paddy soils may progressively sequester C over the recentthree decades (Yu et al. 2009). However, while improving crop yields and consequentlyincreasing SOC stocks, fertilization will enhance other greenhouse gas (e.g. methane andnitrous oxide) emissions in rice fields (Zou et al. 2005). Therefore, an overall evaluation

Climatic Change (2012) 112:847–858 855

of net greenhouse effects is needed to better understand the role of rice cropping in globalwarming (Qiu et al. 2009).

5 Conclusions

Our meta-analysis indicated that SOC sequestration in paddy fields varied with croppingsystems. Rice fields showed higher rates for C sequestration in the DR cropping systemthan in the SR and RU. Soil C stocks increased with the increasing cropping duration. Ricecropping for more than 20 years resulted in average SOC gains of 15% and 23% in thecontrol and NPK treatments, respectively. In addition, no C saturation trend was found overthe investigated cropping duration. However, the negative relationship between SOCchanges and their initial C stocks suggests that an upper limit or saturation level for soil Cmay exist in paddy fields.

Acknowledgements This work was supported by the National Key Technology Research and DevelopmentProgram (2006BAD02A15, 2006BAD15B02), the Program for New Century Excellent Talents in University(NCET-05-0492), and the National Natural Science Foundation of China (30571094). We are grateful to theeditors and the reviewers for their helpful comments on the manuscript.

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