An increase in topsoil SOC stock of China’s croplands between 1985 and 2006revealed by soil monitoring

Genxing Pan a,1,*, Xinwang Xu a,b,1, Pete Smith c, Weinan Pan a, Rattan Lal d

a Institute for Resource, Ecosystem and Environment of Agriculture, Nanjing Agricultural University, 1 Weigang, Nanjing 210095, Jiangsu, Chinab Department of Geography and Environment Sciences, Chizhou College, Anhui 247000, Chinac Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB24 3UU, UKd Carbon Management and Sequestration Center, Ohio State University, 422B Kottman Hall, 2021 Coffey Rd, Columbus, OH 43210, United States

Agriculture, Ecosystems and Environment 136 (2010) 133–138

A R T I C L E I N F O

Article history:

Received 26 August 2009

Received in revised form 9 December 2009

Accepted 9 December 2009

Available online 8 January 2010

Keywords:

C stock

Climate change mitigation

Croplands

Soil organic carbon

Topsoil

A B S T R A C T

Soil C sequestration in cropland could play an important role in mitigating the rapidly increasing CO2

emissions in China. Many efforts had been dedicated to estimating the potential for C sequestration in

croplands. Potential increases in SOC in China’s croplands had been recently evaluated using inventory-

up-scaling simulation and crop-soil C process-based modeling. In this study, data of SOC change at

monitoring sites from croplands across mainland China were collected from publications available from

1985 to 2006 to perform a statistical analysis. The data set comprises 1081 observations (404 from rice

paddies, RPs and 677 from dry croplands, DCs). Frequency analysis indicates that over 70% of

observations show an increase in SOC, which is higher among RPs than DCs. To quantify SOC dynamics, a

Relative Annual Change Index in g kg�1 year�1 (RAC, g kg�1 year�1) is defined and calculated using the

initial and final SOC values for the duration of monitored observations. RAC values ranged from�0.806 to

0.963 g kg�1 year�1 for DCs and from�0.597 to 0.959 g kg�1 year�1 for RPs, respectively. From this data,

the average is estimated to be 0.056 � 0.200 g kg�1 year�1 for DCs, and 0.110 � 0.244 g kg�1 year�1 for RPs,

with an overall estimate for China’s croplands, with RPs and DCs combined, of 0.076 � 0.219 g kg�1 year�1. A

mean increase in topsoil C (0–20 cm) stock of China’s croplands was estimated to be 25.5 Tg C year�1

(8 Tg C year�1 in RPs and 17.5 Tg C year�1 in DCs) between 1985 and 2006, with a total topsoil C stock

increase of 0.64 Pg C over the whole period. The annual stock increase may offset �20%, on average, of the

total CO2 emissions of China for 1994. This study suggests an important role of China’s croplands, especially

rice paddies, for C sequestration to mitigate climate change.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journal homepage: www.e lsev ier .com/ locate /agee

1. Introduction

Change in soil organic carbon (SOC) storage at global andregional scales is of critical importance for understanding theprocesses and mechanisms of terrestrial ecosystem C balance. This,in turn, is important for increasing the certainty with projections offuture atmospheric CO2 concentration, and the future trend ofglobal climate change (Houghton, 2007). Recently, SOC dynamicsat regional scale, and the possible impacts of climate change, havebeen paid much attention (Sleutel et al., 2003; Bellamy et al., 2005;Saby et al., 2008). At the same time, increasing emphasis has beenput on soil C sequestration as an effective method to mitigate theincrease of atmospheric CO2 levels (McCarl et al., 2007), and is

Abbreviations: RP, rice paddy; DC, dry cropland; GHGs, greenhouse gases; RAC,

relative annual change.

* Corresponding author. Tel.: +86 25 8439 6027; fax: +86 25 8439 6027.

E-mail address: pangenxing@yahoo.com.cn (G. Pan).1 These authors contributed equally.

0167-8809/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.12.011

recognized as a potential major contributor to mitigation inagriculture (Smith et al., 2007a,b, 2008). For the last 5 years, muchattention has been paid to the capacity and duration of Csequestration by croplands in the study of climate change (Smith,2004; Bruce et al., 2007). Many studies have demonstrated Csequestration potential, the number of years over which seques-tration is effective, and the major controls on soil C sequestrationby meta-analysis of data from various sources, under varyingmanagement and bio-climatic conditions (Smith et al., 1997, 1998,2000; West and Post, 2002; Ogle et al., 2005; West and Six, 2007).

China had a total area of croplands of 138.7 Mha including32.3 Mha for RPs and 106.4 Mha for DCs in 1982 (SSSC, 1996). By2006, China had an arable land area of 142.6 Mha (FAOSTAT, 2006),which is about 7% to the global total. SOC dynamics in thesecroplands has been widely studied (Lal, 2002; Pan et al., 2003; Panand Zhao, 2005; Huang and Sun, 2006; Tang et al., 2006; Liao et al.,2009; Wang et al., 2000; Xu et al., 2004; Yu et al., 2003, 2006). Greatefforts have been given to understanding the accumulation of SOCin the croplands of China by qualitative or semi-quantitativeanalysis of survey and inventory data in comparison to the baseline

Table 1Distribution of observations of SOC dynamics of China among regions and monitoring periods.

Geographical region Total observations Initiated before 1985 Finalized in

Before 1990 1991–1995 1996–2000 2001–2006

NE China 120(6) 87 12 4 42 62

N China 241(12) 183 26 16 100 99

E China 356(220) 305 11 53 143 149

S China 119(101) 86 1 2 71 45

NW China 148(5) 106 1 15 70 62

SW China 97(60) 76 3 4 41 49

Total 1081(404) 840 54 94 467 466

The numbers in brackets are observations from rice paddies.

G. Pan et al. / Agriculture, Ecosystems and Environment 136 (2010) 133–138134

in 1980 (Pan et al., 2003; Zhang et al., 2004; Li and Wu, 2006; Xuet al., 2004; Liao et al., 2009). By using the DNDC model, Li (2000)and Tang et al. (2006) argued that an annual loss of�70 Tg C couldbe possible in China’s croplands in comparison to an annual gain of70 Tg C in USA’s croplands over the 1990s. Contrary to theestimates of Li (2000), a meta-analysis by Huang and Sun (2006)demonstrated a general trend of increasing topsoil SOC(300–400 Tg C) estimated roughly for all of China’s croplandsbetween 1980 and 2000, using observation data from publicationsavailable on current contents of Chinese journals. The increasingtrend is also supported from other studies of C dynamic usingmodeling and up-scaling (Yu et al., 2006; Zhang et al., 2007). Xieet al. (2007) estimated a topsoil SOC stock increase of23 Tg C year�1 in China’s croplands during the last two decadesusing data from a small number of observations collected from84 papers available in Chinese journals. Recently, an increasein a range of 26 � 11 Tg C from the estimates by Huang and Sun(2006) and Lal (2004) was used for evaluating the terrestrial C balanceof China during the 1980s and 1990s (Piao et al., 2009). A comparativestudy of data from the 2nd national soil survey conducted in1979–1982 and a geological survey conducted in 2004 yielded aquantitative estimate of topsoil SOC stock increase of 24.0 � 3.7 Tg inJiangsu, China over 1982 and 2004 (Liao et al., 2009). However, areliable estimate of topsoil SOC stock increase in China’s croplands isstill unavailable.

By conducting a holistic statistical analysis of soil monitoringdata from various sources reported since the 2nd National SoilSurvey in early 1980s, the purpose of this study is to characterizethe relative SOC increment and its regional distribution, and toestablish a more quantitative estimate of total topsoil Csequestration of China’s croplands over the last 2 decades. Byquantifying SOC dynamics in China’s croplands the paper aims toprovide a scientific basis for potential climate mitigation measuresin China’s agriculture.

2. Materials and methods

2.1. Data source

Data used in this study originate mainly from the croplandfertility monitoring work managed by the administrations ofChina’s agricultural agencies, mostly initiated shortly after thefinalization of the 2nd National Soil Survey in 1982–1985 all overChina (Zhong, 2003), and scientific observations related to soilfertility dynamics in China. The data were collected frompublications available at www.cqvip.com and www.cnki.net bythe end of 2007. In total, 547 publications since 1985 were foundwith observations of SOC dynamics in monitoring sites. Weselected data with monitoring duration �5 years. All of theobservation data were from topsoil sampled at 0–20 cm depth, andthe sampling and SOC measurement with wet digestion potassiumpyrochromate in sulfuric acid as the protocol described in Lu

(1999) was consistent throughout the monitoring duration. Thereported SOC values were within the range of mean plus/minustriple standard deviations for the all observations. By screeningusing these criteria, a total of 242 publications, including 1081observations, with 404 from RPs and 677 from DCs across China,were collected (Table 1). The spatial distribution of the monitoringsites of the observations is shown in Fig. 1. The number ofmonitored observations for DCs is slightly smaller than that for RPswhen compared by their area proportion in 1982 (cf., Pan et al.,2004). The majority (77.7%) of the monitoring observations wereinitiated before 1985, and most (87.5%) of the observations werefinalized after 1996 (also see Table 1). The dataset is thereforeconsidered suitable for estimating cropland SOC dynamics in thelast two decades following completion of the 2nd National SoilSurvey of China.

2.2. Data processing and analysis

All the data were normalized to express SOC content in g kg�1

using a conversion factor of 0.58 when the values were reported ascontents of soil organic matter (SOM). We defined the SOCdynamics as the difference between the initial value and the finalvalue in the reported monitoring period. It has recently beenshown that SOC changes often depend on the baseline SOC values,and the relative change to the baseline may reflect better the SOCchanges under climate change and land use management (Larket al., 2006; Saby et al., 2008). Similar to Bellamy et al. (2005), weuse the relative annual change in SOC (RAC) for tracing SOCdynamics, which is calculated using the following equation:

RAC ðg kg�1 year�1Þ ¼ SOCt � SOC0

t(1)

where SOC0 is the initial SOC content at the beginning ofmonitoring and SOCt is the final SOC content after t years ofmonitoring.

2.3. Statistical analysis

Data processing was conducted with MS-EXCEL 2003 in MS-WINDOWS 2003. Significance of the differences in SOC levels/dynamics between cropland types and soil regions were testedwith an analysis of variance procedure (ANOVA) using theSPSS11.0 statistical package. Statistical significance was deter-mined over the 95% confidence level at p < 0.05.

3. Results

3.1. Overall dynamics of SOC

The data show an overall increase of SOC in the topsoil ofChina’s croplands. The frequency distribution of the initial and thefinal SOC contents of the overall monitored croplands is shown in

Fig. 1. A sketch map of distribution of topsoil samples from soil monitoring sites used in this study.

Fig. 2. Frequency distribution of the initial (blank) and final (shaded) SOC contents

of the monitored croplands.Fig. 3. Frequency distribution of RAC observation values for samples of dry

croplands (blank) and of rice paddies (shaded).

G. Pan et al. / Agriculture, Ecosystems and Environment 136 (2010) 133–138 135

Fig. 2. The final SOC contents are significantly (p < 0.01) higherthan the initial ones, indicating an overall increase in SOC contentfor croplands across China. As seen also in Table 2, 71% of the totalobservations showed an increase in SOC content, but 29% showed adecrease an increase in SOC was observed in 70.2% and 72.6% of theobservations from DCs and RPs, respectively.

The frequency distribution of the calculated RAC values of thesingle observations is given in Fig. 3. Following a normaldistribution, the RAC values ranged from �8.0% to 7.8% for allsamples, and fall in a range of �0.60 to 0.96 g kg�1 year�1 for RPsand �0.81 g kg�1 year�1 to 0.96 g kg�1 year�1 for DCs, respective-ly.

3.2. Variation of SOC dynamics with land use types

The croplands are divided into two major types of RPs and DCsin this study. There was a great difference in both the initial and

Table 2Distribution of monitored observations with different cropland types.

Cropland Monitoring observations Initial SOC (g kg�1) Final SOC (g kg�1)

DCs 677 10.06�6.72A 10.83�5.86A

RPs 404 15.74�6.06B 17.37�6.31B

Overall 1081 12.51�7.99 13.65�7.57

Note: RPs, rice paddies; DCs, dry croplands; �, plus and minus the standard deviation. The

dry croplands and rice paddies, respectively.

final measurements of topsoil SOC contents between RPs and DCs.Nevertheless, there was a significant difference in RAC valuesbetween the observations from RPs and from DCs (see Table 2).While the SOC contents of RPs was higher than of DCs by 5.7 g kg�1

at initial and 6.3 g kg�1 at final on average, the mean RAC values ofRPs was higher than DCs by 0.05 g kg�1 year�1 though a similarfrequency (�70%) of increase in SOC of was observed for both typesof croplands.

3.3. Geographical variation of SOC dynamics

Table 3 shows the pattern of SOC change across thegeographical regions of China. The final SOC contents weresignificantly higher than the initial ones in croplands from allregions except for Northeast and Southwest China, where thevariability of SOC contents is higher. However, the numbers of

Frequency of increase (%) Mean RAC (g kg�1 year�1) Mean duration (year)

70.2 0.056�0.200B 15.5�5.3A

72.6 0.110�0.244A 14.9�5.1A

70.9 0.076�0.219 15.2�5.3

capital letters in a same column indicate very significant difference at p< 0.01 between

Table 3Variation of SOC dynamics with geographical regions of China.

Geographical region Monitored observations Initial SOC (g kg�1) Final SOC (g kg�1) Frequency of increase (%) Mean RAC (g C kg�1 year�1)

NE China 120 16.02�8.77b 16.20�7.85b 50.8 0.019�0.024d

N China 241 8.32�6.06d 9.29�4.58d 80.1 0.067�0.178a

E China 356 12.34�6.00 c 14.01�6.10c 78.4 0.112�0.117a

NW China 148 8.53�3.40d 9.45�3.99d 72.5 0.066�0.171abc

S China 119 18.21�6.25a 19.33�6.45a 65.0 0.082�0.313bc

SW China 97 15.83�8.36ab 16.41�9.00ab 55.1 0.049�0.267c

Total/mean 1081 12.51�7.79 13.65�7.57 70.9 0.076�0.219

Note: �, plus and minus the standard deviation. The lower case characters in a same column mean significant difference between regions at p<0.05, respectively.

G. Pan et al. / Agriculture, Ecosystems and Environment 136 (2010) 133–138136

observations with increasing SOC were higher than those showingSOC decline in all regions. Increase in SOC was found in over70% of total observations in East, North and Northwest China, andin 50–65% of total observations in Northeast, South and SouthwestChina. Thus, there did occur an overall tendency for an increase intopsoil SOC across China over the last 2 decades, although half ofthe observations from Northeast China show a decline. SOCincrease was most frequently (over 70% of the total observations)observed in East China with high mean RAC values around0.11 g kg�1 year�1 and in North and Northwest China, withmoderate mean RAC values of 0.067 g kg�1 year�1, but lessfrequently in Northeast and Southwest China which showed lowRAC values (0.02 and 0.05 g kg�1 year�1, respectively).

4. Discussion

4.1. Changes in the SOC dynamics with cropland type and

geographical regions

C dynamics of China’s croplands had been much concerned forthe last decade. Huang and Sun (2006) reported an SOC increasefrequency of was 60% for all the China’s croplands and significantlyhigher increase frequency for RPs than for DCs, using a smallerobservations size in a semi-quantitative assessment of SOCchanges. Thus, the soil monitoring data evidences a definiteincreasing trend of C dynamics in China’s croplands in contrast tothe overall decreasing trend shown by DNDC modeling approach(Tang et al., 2006). The present study further indicated a higherfrequency of SOC increase for overall China’s croplands with usinga larger sized and longer duration (those monitored for shorterthan 5 years excluded) of observations in a similar time span. Thepresent study revealed, by using a parameter of RAC calculated asannual SOC increase rate, significantly higher SOC increase rate ofRPs than of DCs though the frequency of SOC increase of theobservations was similar. This supports the previous findings ofhigher SOC accumulation rate for RPs than for DCs (Zhang et al.,2004; Pan et al., 2005a,b; Li and Wu, 2006; Hou et al., 2007). As RPsholds topsoil C 10 t ha�1 more than DCs on average of the China’scroplands (Pan et al., 2004; Song et al., 2005), the significantlyhigher RAC up to 0.05 g kg�1 year�1 by RPs than DCs found in thisstudy may suggest further the crucial role of rice paddy protectionand C management in China’s cereals production and C sequestra-tion in offsetting the increasing greenhouse gases emission of thestate (Pan and Zhao, 2005; Pan, 2009).

There had been great regional variation of cropland C dynamicsacross China. In this study, high frequency (72–80%) of increaseand high RAC between 0.066 and 0.112 g kg�1 year�1 on averagewere observed in East, North and Northwest China (Table 3). Manyother studies have described SOC increase in croplands, particu-larly in RPs dominant in East China (Li and Wu, 2006; Zhang et al.,2004; Liao et al., 2009; Hou et al., 2007) and North China GreatPlain soils (Yang, 1991; Sun and Liu, 2009). The increase incroplands from Northwest China could probably be attributed tothe enhanced crop production favored, in part, by the increasing

rainfall due to climate change over the last decades (Ding et al.,2006). Non-significant increase at a frequency of 50–55% wasobserved for Northeast and Southwest China in this study. Somestudies have argued that Northeast China’s croplands have beenlosing SOC (Han et al., 2004), with large SOC losses recorded wherenatural soils, indigenously rich in SOC, were converted tocroplands (Liu and Zhang, 2005; Song et al., 2005; Xu et al.,2004; Wang et al., 2007). Huang and Sun (2006) evaluated a smallreduction in SOC in the soils of Northeast China. Under long-termagricultural use, however, an increase in SOC due to enhanced cropbiomass production could be observed in case studies in this area(Yin et al., 2006; Wang et al., 2000). Nevertheless, the overall SOCdecline in croplands from Northeast China may be impacted by thesignificant warming and drying trend under climate change (Dinget al., 2006). The lower frequency and insignificant increase oftopsoil SOC in croplands from Southwest China, mainly RPsobserved, may be partly attributed to a general trend of increasingrice-wheat double cropping instead of winter fallow with water-logging since1980s in the hilly region (Liu, 2005). The increase inSOC in the croplands from South China is significantly lower thanfrom East China though RPs are extensive in both regions. Whereas,RPs from South China mostly monitered here are higher in initialSOC contents (Table 3) as derived mainly from wetlands (Li, 1992).In addition, there has been a trend of soil drying except for summerin this area for last decades, which has been attributed by someauthors to climate change (Huang et al., 2004). The changes in SOCdynamics with geographical regions, therefore, reflect impactsboth of crop production and/or climate change.

4.2. Topsoil C stock increase in China’s croplands between 1985 and

2006

Using the data on soil bulk density and soil area reported by Panet al. (2004) and Song et al. (2005) from the 2nd National SoilSurvey, an estimate of topsoil SOC stock increase can be madeusing the mean of RAC values for all RPs and all DCs, or for differentgeographical regions (Table 4). An overall increase in topsoil of 0–20 cm SOC was estimated as 24.9 � 2.7 Tg C year�1, with increasesof 7.8 � 0.8 Tg C in RPs and 17.1 � 1.9 Tg C in DCs over the period of1985–2006. Of course, the real topsoil SOC increase with any possiblechange of ploughing layer (Sleutel et al., 2006) is still unknown. Here,the RPs contribute almost 30% of the total stock increase but occupyonly 23% of the area. However, the estimated yearly gain in SOC stockby rice paddies is lower than the modeled gain of 13 Tg C year�1 fromrice biomass input over the period of 1980–1994 (Zhang et al., 2007).The sequestration of SOC from crop input may be subject to influenceby a number of management factors.

Accordingly, 523.3 � 52.9 Tg C is likely to have been sequesteredin China’s cropland soils between 1985 and 2006. Huang and Sun(2006) reported, by a statistical analysis of qualitative and quantita-tive SOC dynamics observations, an estimated enhancement of�300–400 Tg C for China’s croplands over the last 2 decades. This numberfalls in the range of 25–37 Tg C year�1 estimated by Lal (2004). Xieet al. (2007) projected a yearly increase of 23.6 Tg C in China’s

Table 4Estimated increases in topsoil SOC of croplands from different geographical regions

of China.

Geographical

region

Cropland

area (Mha)

Average

RAC (g C kg�1 year�1)

Total SOC stock

increase (Tg C year�1)

NE China 21.17 0.019 4.07

N. China 18.06 0.067 3.48

E China 37.82 0.112 7.28

NW China 25.21 0.066 4.85

S China 7.73 0.082 1.49

SW China 19.50 0.049 3.75

Total/mean 129.49 0.076 24.92

Data of soil bulk density and soil area from Song et al. (2005).

G. Pan et al. / Agriculture, Ecosystems and Environment 136 (2010) 133–138 137

croplands using data from 84 papers with SOC dynamics availablefrom 1996 to 2006. Liu et al. (2004) figured out an China’s croplandSOC increase at 79.0 � 7.7 Tg C in topsoil of 0–30 cm during 1990–2000 using up-scaling of GIS-based vegetation inventory. Our recentwork (Liao et al., 2009) reported a topsoil SOC increase in the whole ofJiangsu Province of China of 24.0 � 3.7 Tg C for a soil area of 6 Mhaover the period 1982–2004. The significant net SOC gain estimatedhere supports the findings of overall C sequestration in China’scroplands over the last decades, in contrast to the findings by Li(2000) and Tang et al. (2006). While a overall decline in cropland SOCstock may be observed in other countries of the world in temperatezone with dry croplands (Bellamy et al., 2005; Saby et al., 2008),China’s croplands may have had a positive effect on the terrestrial Csink over the last two decades. This may be attributed to an increasein NPP, increased crop residue return, and the extension of goodfertilization practice schemes discussed by Huang and Sun (2006). Asthe total CO2 emission of China amounted to 0.73 Pg C in 1994 (PRC,2004) and 0.87 Pg C in 2000 (Anon., 2007), the total C sequestered inChina’s croplands could be offsetting about 1/5 of China’s 20%emission reduction target for total carbon emissions. Thus, China’scroplands may play a significant role in climate change mitigation.

5. Conclusions

The statistical analysis of monitoring data on topsoil SOCdynamics presented here shows an overall increase in SOC inChina’s croplands since the 1980s. The mean net relative annualincrement was estimated at 0.067 g C kg�1 year�1since the 2ndnational soil survey was finalized. The increase was much higherfor RPs than for DCs and in East and North China than in South andNortheast China. An annual topsoil SOC stock increase (0–20 cm) of24.9 � 2.7 Tg C is estimated, giving a total SOC stock increase of0.52 � 0.05 Pg C between 1985 and 2006. These findings demonstratethe potentially important role in the mitigation of climate change ofagricultural soils in China. RPs show a higher C sequestration rate dueto higher C storage and, therefore, have particular importance formitigation of GHGs, as well as for food security in China. Theimplementation and extension of best agricultural managementpractices and incentives within the framework of a sound climatepolicy will help to enhance the capacity of Chinese soils to mitigateChina’s increasing CO2 emissions.

Acknowledgments

This work is partly supported by the China Natural ScienceFoundation under a grant number 401700190002 and by theMinistry of Education for a priority project on China’s cropland Cstock. This collaboration is also partly supported by travel fundsfrom a UK BBSRC China Partnership Award. PS is a Royal Society-Wolfson Research Merit Award holder. The authors are grateful toprofessor Huang Y from the Institute of Atmospheric Physics,Chinese Academy of Sciences, for his helpful discussion on topsoil Cstock dynamics over China’s croplands. This work was revisedfollowing comments from two anonymous referees and followingdiscussion with Steve Ogle and Keith Paustian during the firstauthor’s visit to Colorado State University in 2009. The authors arealso grateful to them for their help.

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