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Estimating topsoil SOC sequestration in croplands of eastern china from 1980 to 2000.

Introduction

Global atmospheric concentrations of C[O.sub.2], C[H.sub.4], and [N.sub.2]O have increased markedly as a result of human activities since 1750, and now far exceed pre-industrial values determined from ice cores that were created over thousands of years. Annual fossil carbon dioxide emissions, for example, increased from an average of 23.5 (22.0-25.0) Pg C[O.sub.2] per year in the 1990s to 26.4 (25.3-27.5) Pg C[O.sub.2] per year in 2000-2005 (IPCC 2007). The conversion of natural ecosystems for agriculture has depleted soil organic carbon (SOC) by as much as 60% in soils of temperate regions and 75% or more in the cultivated soils of the tropics (Lal 2004). Agricultural soils are believed to have the potential to sequester atmospheric C[O.sub.2] (Lal 2004). Sustainable agricultural management has been considered a profitable option that can reduce atmospheric C[O.sub.2] over the next 20-30 years (Smith 2004).

Estimates in the IPCC Second Assessment Report suggested that 400-800 Tg C/year (equivalent to ~1400-2900 Tg C[O.sub.2]/year) could be sequestered in global agricultural soils with a finite capacity saturating after 50-100 years (Smith et al. 2007). Lal (2004) suggested that carbon sequestration by the agrosystem has the potential to offset 5-15% of global fossil-fuel carbon emissions. Several studies have indicated that the net C[O.sub.2] exchange from agricultural soils in Canada was very close to equilibrium (Monreal and Janzen 1993; Nyborg et al. 1995), while it is believed that most agricultural soils have the potential to help mitigate climate change by sequestering additional carbon, and hence decreasing C[O.sub.2] in the atmosphere (Janzen et al. 1998). Dumanski et al. (1998) estimated that implementation of appropriate management practices could result in the sequestration of ~50-75% of the total agricultural emissions of C[O.sub.2] in Canada over the next 30 years. Summarised in 4 scenarios for sustainable agricultural management, Niles et al. (2002) suggested that annual carbon sequestration rates could be 0.3-3.1 t/ha.year for arable lands and 0.1 t/ha.year for irrigated rice. Smith (2004) revealed the biological potential for carbon storage in European (EU15) cropland as 90-120 Tg C/year, with available options including reduced and zero tillage, set-aside, more efficient use of organic amendments, etc. By employing the IPCC inventory methods combined with land-use and cropping information, climate data, and tillage practices, Sperow et al. (2003) suggested that US cropland soils had the potential to increase sequestered soil C by an additional 60-70 Tg C/year over the present rate of 17 Tg C/year with widespread adoption of soil C sequestering management practices.

China has a large acreage of intensively cultivated soils, and its SOC dynamics and sequestration potential have been gaining international interest since the 1990s (e.g. Lal 2002, 2004; Li et al. 2003a). In 2004, Peng et al. (2006) measured 627 samples taken from cropland soil types typical of red earths and showed that the SOC content increased by 22.6% over that in 1979. Yu et al. (2003) reported that, from 1980 to 2000, the SOC in 69 of 80 soil-sampling sites in Jiangsu Province increased. Measurements of 207 paddy soil samples taken from 10 counties in Jiangxi Province indicated that the SOC contents increased by 3.5 g/kg from 1981 to 1997 (Luo et al. 2004). Model simulations also suggested that Chinese rice paddies covering 22.6 Mha sequestered ~0.15 [+ or -] 0.07 Pg C from 1980 to 2000 (Zhang et al. 2007). Wu et al. (2003) reported that ~57% of the cultivated soil subgroups have experienced a significant carbon loss, while the SOC has increased in paddy and irrigated soils especially in north-west China.

Based on the data collected in the Second National Soil Survey of China that was completed in the early 1980s, Song et al. (2005) estimated that the topsoil (~0.30m) SOC density was 35 [ + or -] 32 and 50 [ + or -] 47 t C/ha in cultivated and uncultivated soils, respectively, suggesting that cultivation may have induced an average decrease in the SOC density of 15 t C/ha. By running the biogeochemical model DNDC with databases of climate and agricultural managements, Li et al. (2003a) estimated that China's croplands lost 1.6% of their SOC (to a depth of 0.3 m) in 1990, although regions of rice paddy dominance located in southern China might bear lower losses or even slight gains. However, Pan et al. (2003) reported that the SOC pool of irrigation-based rice cultivation had increased by 0.28 Pg compared with dryland cultivation, by pooling the datasets from the State Extension Service of Agricultural Technology. Huang and Sun (2006) analysed data extracted from 132 articles and pointed out that, as a whole, the SOC concentration increased in the .topsoil of Chinese croplands. The SOC increased considerably in Inceptisols and Ustochrepts and decreased conspicuously in black soil. A significant C sequestration of 311.3-401.4 Tg was estimated for the top 0.20 m of soil in Chinese croplands during 1980-2000.

Although great efforts have been made towards investigating the SOC changes of the agricultural soils in China, notable uncertainties remain because attempts to estimate historical SOC loss have been complicated by high soil variability, a wide range of climate and field management practices, and a lack of adequate knowledge regarding all the factors involved (Neill and Davidson 2000). Further knowledge may therefore help to reduce the uncertainty of these estimates.

Eastern China, including the provinces of Jiangsu, Anhui, Zhejiang, Jiangxi, and Fujian, and Shanghai City, is located between latitudes 24[degrees]29'N and 35[degrees]02'N and longitudes 113[degrees]34'E and 123[degrees]10'E. It covers 6.7% of the national total territorial area and contains 13.8% of the cropland of mainland China. The average annual precipitation is 90-1560 mm and the mean temperature is 16.4-20.5[degrees]C. Plentiful precipitation and higher temperatures occur in the southern portion of this region. The dominant soils are mainly Inceptisols, Ustochrepts, Aridisols, Alfisols, Udert, Udalf, and Ultisols in terms of the Soil Taxonomy of the USDA (Shi et al. 2006; Zhao et al. 2006). Double and triple cropping systems with rice and/or upland crops have been dominant. Crop yields in this region account for ~20% of the national total. According to the State Soil Survey Service of China (SSSSC 1993), the acreage of paddy soils is 9.25 Mha, accounting for 51.7% for croplands in eastern China.

Over the last 2 decades, Chinese researchers have made several field surveys in eastern China, which offer a great opportunity to evaluate the changes in SOC as a function of time. In this paper, we compiled these SOC data extracted from field surveys. The main objective was to estimate the carbon sequestration of cropland soils in this area. A further objective will focus on identifying the role of cropland soils in mitigating greenhouse gases on regional and/or national scales.

Data sources

Data were collected from 50 published papers that reported SOC dynamics in the cultivated layers (Ap horizon) over time, by field surveys and long-term oriented observations in eastern China. These datasets accord with the criteria of: (1) the reference period is the Second National Soil Survey (SNSS) that was completed in the early 1980s; (2) the methods to measure SOC content and bulk density are the same as that used in the SNSS; and (3) with the same sampling sites or coverage as the SNSS. The datasets from the model plots for high-yield cultivation were not taken into account in this study.

The potassium dichromate volumetric method (ISSAS 1978; Lu 2000; Schulz 2002) was used to measure SOC content. Soil bulk density was determined as a ratio of oven-dried mass to field volume of soil sample (ISSAS 1978; Lu 2000). The soil sample was taken by driving a cylindrical metal sampler into the soil to a desired depth and carefully removing it to preserve the volume of the sample as it existed in situ, oven-drying at 110[degrees]C, and weighing to obtain soil bulk density.

For sites where the SOC concentrations were recorded as soil organic matter (SOM), a conversion factor of 0.58 (Post et al. 1998; Evrendilek et al. 2004) was used to convert into SOC. The constructed database also included the number of samples or the number of long-term oriented observation sites, the area of the field surveys or the long-term oriented observations covered, and soil orders (Appendix A).

Datasets in the 50 published papers (Appendix A) covered 76% of the total croplands in eastern China, with >8500 soil sample measurements, representing paddy soil (Inceptisols) and upland soils (Ustochrepts, Aridisols, Alfisols, Udert, Udalf, and Ultisols). Figure 1 shows the spatial distribution of national croplands and soil sampling sites in eastern China, where 1 dot represents 100 or 200 soil sample measurements.

Intervals between the initial and final field measurements were 9-22 years in these field surveys (Appendix A). The intervals of 9-10 years, 11-15 years, 16-20 years, and 21-22 years accounted for 3.6%, 37.4%, 55.4%, and 3.6%, respectively, of the total measurements reported.

Methods

Batjes (1996) reported that globally the 0-0.30m topsoil contributed to ~46% of the total SOC pool. Although some changes in the SOC may occur below 0.30 m, the vast majority of the changes occur here (Smith et al. 2000). Calculations with the data in the China Soil Series (SSSSC 1993) indicated that the average thickness of the cultivated layer (Ap horizon) of the croplands was 14.8 [ + or -] 2.5 cm (n-209) and the sub-cultivated layer (P horizon) extended a further 10.3 [ + or -] 4.9 cm (n = 209) in eastern China. To capture the majority of the SOC changes, it was hypothesised that the changes in the SOC pool occurred mainly in the topsoil (Ap and P horizons). The Ap and P horizons for paddy soils refer to the plough layer and ploughpan (Pan et al. 2003), respectively.

[FIGURE 1 OMITTED]

Because the datasets from these published papers (Appendix A) were specified for the Ap horizon, the changes in SOC density in the topsoil were determined from a relationship between the changes that occurred in the topsoil and in the Ap horizon.

Calculations of changes in SOC density ([DELTA][D.sub.oc])

The SOC density ([D.sub.oc], t C/ha) was calculated as (Pan et al. 2003):

[D.sub.oc] = SOC x [gamma] x H x (1 - [[delta].sub.2mm]/100) X [10.sup.-1] (1)

where SOC and [gamma] are the content of organic C (g/kg) and soil bulk density (g/[cm.sup.3]), respectively. A negative exponential relationship of [gamma] = 1.377 x [e.sup.-0.0048 x SOC] (Song et al. 2005) was used to estimate the values of [gamma] when they were not reported in the literature (Appendix A). The [[delta].sub.2mm] is the fractional percentage (%) of >2 mm sands and has an average value of 6 in eastern China according to SSSSC (1993). H is the thickness (cm).

Changes in SOC density ([DELTA][D.sub.oc]) over a 20-year period were determined by:

[DELTA][D.sub.oc] = 20 x ([D.sub.oc_F] - [D.sub.oc_R]/t) (2)

where [D.sub.oc_F] and [D.sub.oc_R] are the SOC density in the final and the reference year, respectively; t is the time interval between the final and the reference year.

Datasets from papers (Appendix B) that reported SOC values in the upland soil profiles were used to calculate changes of SOC density in the Ap and P horizons by using Eqns 1 and 2. These papers provided temporal data with long-term field experiments under various treatments including synthetic fertiliser application, manure and crop residue incorporation, and tillage (Appendix B).

Because very few papers could be found that recorded the changes in SOC in both the Ap and P horizons, particularly in paddy soils, we calculated the changes in SOC density with the 'space-for-time substitution' method in order to determine the relationship between the changes that occurred in the topsoil and in the Ap horizon. In the use of 'space-for-time substitution' (e.g. Pickett 1989; Sparling et al. 2003; Zaitsev et al. 2006), soils of different types and stages of development at separate locations ('space') are identified to obtain a chronosequence of ages ('time'). Thus, changes in the SOC density ([DELTA][D.sub.oc]) were calculated by:

[DELTA][D.sub.oc] = [D.sub.oc] - [[bar.D].sub.oc]] (3)

where [D.sub.oc] refers to the SOC density in either the Ap or the P horizon, and [[bar.D].sub.oc]] is the average value of [D.sub.oc] for total recorded spatial patches in eastern China. The [[bar.D].sub.oc]] was calculated for paddy soils and upland soils, respectively. By using Eqn 1, [D.sub.oc]. was computed from the spatial data in the China Soil Series (SSSSC 1993), which recorded detailed information on the SOC content, bulk density, and thickness of the Ap and P horizons taken at different sites in eastern China.

Quantification of relationship between [DELTA][D.sub.oc] in topsoil and in the Ap horizon

Assuming that the changes in topsoil SOC density ([DELTA][D.sub.oc_T]) are proportional to that in the Ap horizon ([DELTA][D.sub.oc_A]), linear regression was used to estimate the relationship between [DELTA][D.sub.oc_T] and [DELTA][D.sub.oc_A] (Fig. 2).

The open circles (n = 42) and the solid circles (n = 128) in Fig. 2a were calculated from the spatial serial data (SSSSC 1993), respectively using Eqns 1 and 3 and from the temporal serial data (Appendix B) using Eqns 1 and 2. The slope of the regression in Fig. 2a suggests that the change in the SOC density in the topsoil was ~1.14 times that of the Ap horizon. Figure 2a also implies that the [DELTA][D.sub.oc] computed by 'space-for-time substitution' method [Eqn 3] is acceptable.

Figure 2b presents the relationship between the changes in the SOC density in the topsoil and in the Ap horizon for paddy soils when the [DELTA][D.sub.oc] was determined from the spatial data (SSSSC 1993) by Eqns 1 and 3. Note that the value of the slope in Fig. 2b is greater than in Fig. 2a, suggesting that the P horizon of the rice paddy contributed more than that of the upland soils to carbon sequestration in topsoil.

Estimation of the changes in SOC stock

By employing Eqns 1 and 2, the changes in the SOC density the Ap horizon ([DELTA][D.sub.oc_A]) were calculated for each administrative region in Appendix A. Regional means of [DELTA][D.sub.oc_A] on a provincial scale were estimated by weighting the area covered by the sampling in the given province (Appendix A). Calculations for the data in the China Soil Series (SSSSC 1993) suggested that the average thickness of the Ap horizon was 15.8 [+ or -] 2.6 (mean [+ or -] s.d.) for upland soils (n = 44) and 14.6 [ + or -] 2.3 cm (n = 165) for paddy soils in eastern China. Because wide variations in the thickness existed, the lower and upper estimates of [DELTA][D.sub.oc_A] were computed under the scenarios of H - s.d. and H + s.d. [Eqn 1], respectively.

Analogous to the estimates of the changes in the SOC stocks that were observed on the basis of the administrative region, the means of [DELTA][D.sub.oc_A] were estimated on the basis of soil order when they were specified (Appendix A). Due to a lack of sufficient data for Ustochrepts, Ultisols, Aridisols, and Udalf (Appendix A), we simply merged these soils into the upland soil in order to calculate the area-weighted mean [DELTA][D.sub.oc_A].

[FIGURE 2 OMITTED]

The changes in the SOC stock ([DELTA][P.sub.oc]) were estimated by:

[DELTA][P.sub.oc] = [summation over (i)] [S.sub.i] x [[bar.[DELTA]D].sub.oci]] (4)

where [bar.[DELTA][D.sub.oci] is the area-weighted mean [DELTA][D.sub.oci] in a given sector i; [S.sub.i] is the area of the given sector; [DELTA][D.sub.oci] in the topsoil was computed by using the relationship in Fig. 2.

To get the level of confidence in the values obtained for [DELTA][P.sub.oc], [DELTA][D.sub.oci] was calculated in light of 2 bases, i.e. the administrative regions and soil usage (paddy and upland soils in terms of soil order). [S.sub.i] was accordingly expressed as the area covered by the sampling in the given province and the area of the given soil order, respectively.

Results

Changes in the SOC contents in the Ap horizon

Of the 56 records (Appendix A), ~86% showed an increase, while only 14% described a decrease in the SOC contents in the Ap horizon. The average and standard deviation of the SOC increase was 2.48 [+ or -] 0.41 g/kg. Approximately 71% of this increase occurred in the range 1-5 g/kg and nearly half was 1-3 g/kg over the 2 decades (Fig. 3).

Changes in SOC stocks in the Ap horizon

Changes in SOC stocks on the basis of the administrative region

The computations indicated that the SOC density increased significantly in the provinces of Jiangsu, Jiangxi, and Anhui, with a weaker increase having occurred in the provinces of Fujian and Zhejiang. The area-weighted mean [DELTA][D.sub.oc] ranged from 3.79 to 5.30 t/ha (Table 1).

The area covered by the sampling region, excluding the overlapped areas (Appendix A), was 13.66 Mha, accounting for ~76% of the total cropland in eastern China (Table 1). Assuming that the estimated [DELTA][D.sub.oc] of the published data (Appendix A) were representative of the region, we calculated the changes in the SOC stock for each administrative region [Eqn 4] by extrapolating the area-weighted mean [DELTA][D.sub.oc] (Table 1) to the corresponding region. The calculations suggested that the SOC stock increased by 67.9-95.0 Tg, with an average value of 81.4 Tg over the 2 decades (Table 1).

[FIGURE 3 OMITTED]

Changes in SOC stocks on the basis of soil usage

The areas of paddy soils and upland soils specified by the sampling region (Appendix A) were 447.5 x [10.sup.4] ha and 221.0 x [10.sup.4] ha, accounting for ~48% and 26% of the total paddy soils and upland soils in eastern China, respectively (Table 2). The area-weighted mean [DELTA][D.sub.oc] was estimated to be 4.40-6.15 t/ha for paddy soils and 2.50-3.49 t/ha for upland soils (Table 2). Accordingly, the SOC stock increased by 40.7-56.9 Tg in the paddy soils and 21.6-30.2 Tg in the upland soils when extrapolating [DELTA][D.sub.oc] to the area of the corresponding soil usage in eastern China. A total increase in the SOC stock in the Ap horizon was estimated to range from 62.3 to 87.1 Tg with an average of 74.7 Tg on the basis of soil usage (Table 2).

Changes in topsoil organic carbon stocks

The slopes of the linear relationships shown in Fig. 2a and b were adopted as conversion factors to estimate the changes in SOC density in the topsoil via [DELTA][D.sub.oc] in the Ap horizon (Tables 1 and 2). Statistical analysis indicated that the respective values of the slopes at the lower and upper 95% confidence bands were 1.06 and 1.22 for upland soils (Fig. 2a), 1.29 and 1.53 for paddy soils (Fig. 2b), and 1.26 and 1.41 for all soils (n = 334, data not shown).

The estimation of [DELTA][D.sub.oc_T] was made under 4 scenarios: the lower estimates of [DELTA][D.sub.oc_A] (Tables 1 and 2) with the lower slope value, the lower estimates of [DELTA][D.sub.oc_A] with the upper slope value, the upper estimates of [DELTA][D.sub.oc]_A with the lower slope value, and the upper estimates of[DELTA][D.sub.oc_A] with the upper slope value. The mean and standard deviation in [DELTA][D.sub.oc_T] in the topsoil were determined from these 4 scenarios.

The SOC density in the topsoil was estimated to increase by 6.07 t C/ha with a range of 4.7-7.49 t C/ha. The SOC stock increased accordingly by 108.7 Tg, with a range of 85.4-134.1 Tg, when it was calculated on the basis of the administrative region.

When calculated on the basis of soil usage, SOC density was estimated to increase by 3.41 t C/ha, with a range of 2.64-4.26 t C/ha for upland soils, and by 7.44 t C/ha with a range of 5.68-9.40 t C/ha for paddy soils. The area-weighted mean SOC density was estimated to increase by 5.49 t C/ha with a range of 4.21-6.92 t C/ha for both upland soils and paddy soils. Accordingly, the SOC stock was estimated to increase by 29.5 Tg with a range of 22.9-36.9 Tg for upland soils, and by 68.8 Tg with a range of 52.5-87.0 Tg for paddy soils. The whole of the increase in the topsoil organic carbon stock was 98.3 Tg with a range of 75.4-123.8 Tg. Paddy soils with 51.7% of the total croplands accounted for ~70% of the total increase.

In light of the results obtained on the basis of the administrative region and the soil usage, we suggest that the SOC density in the topsoil increased by 5.78 t C/ha, ranging from 4.24 to 7.49 t C/ha, and that the SOC storage increased by 75.4-134.1 Tg with an average of 103.5 Tg in eastern China between 1980 and 2000. The SOC increase in the Ap horizon accounted for ~88% (upland soils) and 71% (paddy soils) of that in the topsoil.

Discussion

Agricultural practice and SOC change

Management practices that lead to the depletion of the SOC stock in croplands are the cultivation of upland soils, residue removal, negative nutrient balance, and soil degradation by accelerated soil erosion. Agricultural practices that enhance the SOC stock include the conversion of the uplands to rice paddies, large quantities of residue retainment, integrated nutrient management based on the liberal use of biosolids and compost, and conservation-effective systems (Lal 2002).

The application of fertiliser to crops may improve SOC (Campbell et al. 2001; Hutchinson et al. 2007; Shen et al. 2007). Consumption of synthetic fertiliser in eastern China increased markedly from 3.30 Tg in 1980 to 9.28 Tg in 2000. Accordingly, the crop yield increased from 75.4 to 94.4 Tg (Statistics Bureau of China 2000). The increase in the SOC may be primarily attributed to the consumption of synthetic fertiliser in eastern China. Increasing fertiliser application results in an improvement of crop production, and hence residue (above-ground biomass with the exception of grain yields) retainment. Recent investigations (Huang et al. 2007) indicated that the crop net primary production (NPP) in eastern China increased from 71 [+ or -] 21 Tg C/year in 1980 to l12 [+ or -] 35 Tg C/year in 1999, and the accumulative NPP was 1.56-2.41 Pg C over the 2 decades. Assuming that the ratio of the residue to the grain yield was 1.5, the ratio of the root to shoot was 0.1 (Huang et al. 2007), and that 40% of the above-ground crop residues were retained in the soil (Gao et al. 2002), a total input of residue carbon into the soils is estimated to be ~480-750 TgC over the 2 decades. The present estimate of a 75.4-134.1 Tg increase in the SOC accounts for 16-18% of the crop residue-carbon retainment, which is in accordance with the results of Huang et al. (2007).

It must be noted that manure amendment should also greatly contribute to the SOC in addition to crop residue retainment. According to Li et al. (2003a), the amount of manure carbon incorporation into the croplands in China is ~45% higher than the residue carbon inputs. Manure comes partly from crop residues, and Gao et al. (2002) reported that 22.6% of harvested crop residue was used as animal fodder in China. While estimating the soil C retained, it is not possible from this study, nor is it the intention, to quantify the contribution of residue and manure input to carbon sequestration, because losses of carbon from decomposition offset some of the gain from residue and manure conversion to soil C. Moreover, the carbon loss from decomposition is complex (Chapin et al. 2002), and no simple approach could be applied to estimate the carbon losses from eastern Chinese croplands, which are distributed across a large area with various soils and cropping systems. Nevertheless, the quantification of residue and manure conversion to soil C would be helpful in understanding the contribution of crop NPP to SOC (Huang et al. 2007; Purakayastha et al. 2008).

Increase in cropping frequency and specific type of crops in rotation has potential benefit to increasing SOC gains (West and Post 2002; Hutchinson et al. 2007). Triple cropping systems with rice-rice-upland crop (e.g. wheat, rapeseed, barley, alfalfa) rotation in a 1-year circle are dominant in Zhejiang, Jiangxi, and Fujian Provinces, while double cropping systems with rice-upland crop rotation or 2 harvests of upland crops (e.g. wheat-maize, wheat-soybean) per year are prevalent in Jiangsu and Anhui Provinces (Li et al. 2008). According to the datasets from Huang et al. (2007), the crop NPP averaged 7.6 t C/year from 1980 to 1999 in Zhejiang, Jiangxi, and Fujian Province and 6.2 t C/year in Jiangsu and Anhui Province, suggesting the triple cropping system may input more residue carbon into the soils than the double cropping system. Table 1 shows that the increase in SOC density in Jiangxi Province is more significant than that in Zhejiang and Fujian Provinces, which could be due to the different rates of residue retainment. According to Gao et al. (2002), the rates of residue retainment were 65% in Jiangxi Province, and 24% and 36% in Zhejiang and Fujian Province, respectively. As for the double cropping system, the crop NPP averaged 6.9 t C/year in Jiangsu Province and 5.6 t C/year in Anhui Province from 1980 to 1999 (Huang et al. 2007), which may partly explain why the increase in SOC density in Jiangsu Province is greater than that in Anhui Province, although the rates of residue retainment were similar in the 2 provinces (Gao et al. 2002).

Based on the analysis of field survey datasets (Fig. 1), our results suggested an overall increase in SOC (Tables 1 and 2), while Li et al. (2003a) estimated a loss of SOC in Chinese croplands by running their DNDC model. Our results (Tables 1 and 2) could be supported by more recent work conducted by Lu et al. (2009), who reported a carbon sequestration rate of 16.5 Tg C/year in Chinese croplands. The DNDC model gave the crop productivity of 561-557 Tg C/year (Li et al. 2003a), in accordance with the result of Fang et al. (2007), while the residue/grain ratio (0.96) and the root/shoot ratio (0.06) derived from the modelled components of crop productivity (Li et al. 2003a) are much lower than those from other reports. Based on the harvest data of crops from 300 agro-meteorology stations in China, Zhang and Zhu (1990) gave the residue/grain ratio as 1.32 for rice (n = 718), 1.72 for wheat (n = 605), 1.56 for barley (n = 26), 1.27 for maize (n = 387), 1.30 for soybean (n = 92), and 2.99 for rapeseed (n = 174). According to Huang et al. (2007), the root/shoot ratio is 0.10 for rice, 0.11 for wheat, 0.09 for maize, 0.08 for bean, and 0.06 for rapeseed. The inappropriate residue/grain ratio and root/shoot ratio in the DNDC model may underestimate the residue carbon input into the soils, and thus resulted in an estimate of SOC loss in Chinese croplands.

Agricultural soils with low to intermediate organic matter levels often exhibit a linear relationship between carbon input and soil carbon levels (e.g. Larson et al. 1972; Rasmussen et al. 1980; Paustian et al. 1992; Havlin and Kissel 1997), such that soil carbon levels increase in direct proportion to increases in the residue incorporation (Paustian et al. 1998, 2000). Even if the SOC was at a relatively higher level, carbon sequestration was assured when agricultural practices were improved (VandenBygaart et al. 2004). In China, 31.5% of rice paddies and 69.4% of upland soils have lower organic carbon. Topsoil organic carbon in these soils was lower than 8.5 g/kg in the early 1980s (Lin 1998), which suggested that the increase in the SOC in eastern China most probably occurred over the past 2 decades and a great capacity to sequester carbon exists under sound agricultural management.

Uncertainties

Precise estimation of the changes in the SOC stock relies on the thickness ascertained for the given soil horizons when the SOC concentration is determined. Due to the lack of detailed information on the spatial distributions of the thicknesses of the Ap horizon, we simply took the scenarios of H (average thickness)--s.d. and H + s.d. to obtain the lower and upper estimates of [DELTA][D.sub.oc] [Eqns 1 and 2] in the Ap horizon. Such a simplification might induce uncertainties in the estimates.

The great majority of changes in SOC were thought to appear in the top 0.30 m, while some changes did occur below 0.30 m. A long-term experiment conducted in India by Rudrappa et al. (2006) indicated that the SOC concentrations under an integrated use of farm-yard manure with synthetic fertilisers increased by 52%, 34%, and 11% in the soil depths 0-0.15, 0.15-0.30, and 0.30-0.45 m, respectively, in contrast to those that used solely synthetic fertilizers. Zinn et al. (2005) reported that an intensive land use system with annual tillage in Brazil caused a significant SOC loss of 10.3% in the upper 0-0.20 m of soil depth, but a statistically insignificant change over the top 0.40m. Several studies (Zhou et al. 1993; Shi et al. 2002; Gu et al. 2004; Xie et al. 2004; Olson et al. 2005) have suggested that the SOC changes due to intensive cultivation might reach to more than 1.00m deep, especially in the upland soils. The thickness of the topsoil (Ap and P horizons) in eastern China was determined to be 30.1 [ + or -] 7.7 cm (n = 44) for upland soils and 23.7 [ + or -] 4.1 cm (n- 168) for paddy soils, but until now we know little about the changes in the SOC below these depths.

Eastern China has developed much faster than China's other regions. Changes in soil usage have occurred over the last 2 decades. For example, the rice planted area in Jiangsu Province was 38.6%, 39.1%, and 41.5% of the grain crops in 1985, 1990, and 2000, respectively (Statistics Bureau of Jiangsu 2000). The soil usage might induce uncertainties in the estimates of SOC. Unfortunately, detailed information on soil usage change in eastern China is not available, which presents difficulty in quantifying the uncertainties.

Conclusions

Topsoil organic carbon of croplands in eastern China increased from 1980 to 2000. The SOC density in the topsoil increased by 5.78 t C/ha, ranging from 4.24 to 7.49 t C/ha. The SOC storage increase was then estimated to be 103.5 Tg with a range of 75.4-134.1 Tg during this period. The increase in SOC may be attributed to an increased biomass (e.g. residue retainment) input into soils due to increased crop net primary production. Approximately 88% (upland soils) and 71% (paddy soils) of the increase occurred in the Ap horizon. Paddy soils with 51.7% of the total croplands in this region accounted for -70% of the total increase in SOC, implying the great importance of paddy soils in mitigating atmospheric C[O.sub.2].

Acknowledgments

This work was jointly supported by the National Natural Science Foundation of China (Grant No. 40431001, 40675075) and the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX2-YW-432-4). We thank Dr Su YH at the Institute of Atmospheric Physics for her assistance in pooling data. Thanks are also dedicated to 2 anonymous referees for their helpful comments.
Appendix A. Summary of the changes in cropland soil organic carbon
(SOC) concentrations in eastern China

Values in parenthesis represent the numbers of monitoring sites,
counties, or soil species

 No. of Area Soil order
Province/ samples ([10.sup.4] in Soil
city Locality or sites ha) Taxonomy

Shanghai City 152 (6) 31.50 Inceptisols
 Baoshan District 73 (12) 1.52

Jiangsu Province 80 506.17 Inceptisols
 Province
 Wuxi City (4) 17.54 Inceptisols
 Danyang City 79 (4) 5.58 Inceptisols
 Yixing City 83 6.58 Inceptisols
 Jintan City 40 4.63 Inceptisols
 Nantong City 204 48.32 Inceptisols;
 Ustochrepts;
 Aridisols
 Nantong City
 Jiangyin City 5.18
 Rudong City 1239 11.03
 Xincao Farm 181 Aridisols
 Huaihai Farm 0.50 Aridisols
 Ganlu Township 0.13 Inceptisols
 Kunshan City 133 4.70
 Qidong City 7.05 Ustochrepts;
 Aridisols
 Haian County 590 5.45 Ustochrepts
 361 Inceptisols

Zhejiang Jiaxing Plain 729
 Xiaoshan City 234 5.57 Inceptisols
 Cangnan, 90 23.75 Inceptisols
 Chengzhou,
 Jinhua City
 Jinhua City 270 18.00 Inceptisols
 Jinhua City
 Haiyan County 2.38
 Leqing City 11.79 Inceptisols
 Cixi City (6) 2.72 Ustochrepts;
 Inceptisols
 Shaoxing City 50 16.75 Inceptisols
 Ningbo City 159 21.64 Inceptisols
 Deqing County 141 2.00 Inceptisols

Anhui Hanshan County (3) 4.80 Inceptisols
 Alfisols
 Lingbi County (3) 12.07 Inceptisols
 Ustochrepts
 Udert
 Shouxian County 192 (4) 11.70 Inceptisols
 Liuan City 66 (12) 46.60 Inceptisols
 Udalf
 Qimen County (3) 1.33 Inceptisols
 Guichi District (5) 2.96 Inceptisols
 Ustochrepts
 Changfeng County 9.00 Ustochrepts
 Fuyang City 291 133.30 Ustochrepts
 Chuzhou City 40.70
 Qianmiao Township 0.35 Ustochrepts;
 Inceptisols;
 Udalf
 Juchao District 200 Ustochrepts

Fujian Province 517 143.47
 Longyan City 420 (4) 13.03 Inceptisols
 Fuan City 1221 2.35 Inceptisols
 Guangze County 1.23 Inceptisols
 Jianyang City 2.85
 Ansha Township 50 Inceptisols

Jiangsxi Province 207 299.34 Inceptisols
 Nanchang County 142 (9) 13.90 Inceptisols
 etc.
 Nanchang County 60 7.21 Inceptisols;
 Ultisols
 Xingguo County 3.12 Inceptisols;
 Ultisols

 Initial

 Bulk
Province/ SOC density
city Locality Year (g/kg) (g/[cm.sup.3])

Shanghai City 1982 14.2 1.29
 Baoshan District 1980 11.7 1.30

Jiangsu Province 1980 13.0 1.29
 Province 1985 9.5 1.32
 Wuxi City 1982 12.7 1.30
 Danyang City 1983 10.9 1.31
 Yixing City 1982 -- --
 Jintan City 1984 -- --
 Nantong City 1982 7.9 1.33
 Nantong City 1982 8.0 1.33
 Jiangyin City 1981 13.3 1.29
 Rudong City 1982 8.2 1.32
 Xincao Farm 1980 7.0 1.33
 Huaihai Farm 1980 6.5 1.33
 Ganlu Township 1981 15.5 1.28
 Kunshan City 1981 17.2 1.27
 Qidong City 1980 9.2 1.32
 Haian County 1982 6.8 1.33
 1982 10.7 1.31

Zhejiang Jiaxing Plain 1982 17.3 1.27
 Xiaoshan City 1982 19.7 1.25
 Cangnan, 1982 18.5 1.26
 Chengzhou,
 Jinhua City
 Jinhua City 1981 13.9 1.29
 Jinhua City 1980 14.6 1.28
 Haiyan County 1981 15.9 1.28
 Leqing City 1982 26.3 1.21
 Cixi City 1981 6.7 1.33
 Shaoxing City 1983 20.9 1.25
 Ningbo City 1983 24.0 1.23
 Deqing County 1984 18.2 1.26

Anhui Hanshan County 1984 14.3 1.29
 1984 9.6 1.31
 Lingbi County 1984 6.6 1.33
 1984 8.9 1.32
 1984 5.4 1.34
 Shouxian County 1984 7.4 1.33
 Liuan City 1984 10.6 1.31
 1984 6.0 1.34
 Qimen County 1984 17.0 1.27
 Guichi District 1984 11.6 1.30
 1984 14.2 1.29
 Changfeng County 1981 8.2 1.32
 Fuyang City 1984 7.1 1.33
 Chuzhou City 1982 8.5 1.32
 Qianmiao Township 1982 7.0 1.33
 Juchao District 1985 9.4 1.32

Fujian Province 1984
 Longyan City 1984 17.4 1.27
 Fuan City 1984
 Guangze County 1984 17.7 1.26
 Jianyang City 1984
 Ansha Township 1982 17.4 1.27

Jiangsxi Province 1981 14.4 1.28
 Nanchang County 1981 13.7 1.29
 etc.
 Nanchang County 1981 13.1 1.29
 Xingguo County 1981 11.5 1.30

 Final

 Bulk
Province/ SOC density
city Locality Year (g/kg) (g/[cm.sup.3])

Shanghai City 2000 16.0 1.28
 Baoshan District 1995 12.8 1.30

Jiangsu Province 2000 16.8 1.27
 Province 1996 11.8 1.30
 Wuxi City 1996 15.1 1.28
 Danyang City 1999 13.1 1.29
 Yixing City 1999 0.34 (A)
 Jintan City 2000 0.24 (A)
 Nantong City 1997 9.2 1.32
 Nantong City 2002 9.63 1.31
 Jiangyin City 1996 15.9 1.28
 Rudong City 1997 9.4 1.32
 Xincao Farm 2000 7.3 1.33
 Huaihai Farm 1997 9.2 1.32
 Ganlu Township 2000 17.3 1.27
 Kunshan City 1997 18.2 1.26
 Qidong City 1996 8.4 1.32
 Haian County 2001 10.0 1.31
 2001 14.7 1.28

Zhejiang Jiaxing Plain 1995 18.8 1.26
 Xiaoshan City 1995 21.2 1.24
 Cangnan, 2002 18.8 1.26
 Chengzhou,
 Jinhua City
 Jinhua City 1999 14.6 1.28
 Jinhua City 1999 14.3 1.29
 Haiyan County 2000 20.5 1.25
 Leqing City 1997 30.5 1.19
 Cixi City 2002 7.9 1.33
 Shaoxing City 2002 20.5 1.25
 Ningbo City 2000 24.9 1.22
 Deqing County 2003 19.8 1.25

Anhui Hanshan County 1998 18.9 1.26
 1998 8.2 1.32
 Lingbi County 2002 8.2 1.32
 2002 8.5 1.32
 2002 7.3 1.33
 Shouxian County 2002 11.1 1.31
 Liuan City 1998 11.6 1.30
 1998 8.0 1.33
 Qimen County 1999 15.4 1.28
 Guichi District 1999 20.6 1.25
 1999 8.1 1.32
 Changfeng County 2003 9.3 1.32
 Fuyang City 1994 7.8 1.33
 Chuzhou City 1997 10.3 1.31
 Qianmiao Township 1998 8.4 1.32
 Juchao District 2000 8.5 1.32

Fujian Province 1998 0.0 (A)
 Longyan City 1998 21.1 1.24
 Fuan City 1993 0.45 (A)
 Guangze County 2000 18.9 1.26
 Jianyang City 1998 0.05 (A)
 Ansha Township 2001 20.8 1.25

Jiangsxi Province 1997 18.0 1.26
 Nanchang County 1997 18.4 1.26
 etc.
 Nanchang County 1997 18.9 1.26

 Xingguo County 1997 17.7 1.27

Province/
city Locality References

Shanghai City Mao (2001)
 Baoshan District Jin and Bo (1998)

Jiangsu Province Yu et al. (2003)
 Province Liu and Wang (1998)
 Wuxi City Li et al. (2003b)
 Danyang City Xie et al. (2002)
 Yixing City Gao et al. (2001)
 Jintan City Zhang et al. (2004b)
 Nantong City Huang and Gu (1999)
 Nantong City Wang et al. (2005)
 Jiangyin City Liu et al. (1999)
 Rudong City Yao et al. (2001)
 Xincao Farm Zhou et al. (2002)
 Huaihai Farm Zhang (1998)
 Ganlu Township Chen et al. (2002)
 Kunshan City Gao and Yao (2000)
 Qidong City Zhu et al. (1998)
 Haian County Chen et al. (2005)

Zhejiang Jiaxing Plain Wang and Huang (1999)
 Xiaoshan City He et al. (1997)
 Cangnan, Xie (2003)
 Chengzhou,
 Jinhua City
 Jinhua City Xu and Wu (2002)
 Jinhua City Jin and Hong (2001)
 Haiyan County Jiang et al. (2003)
 Leqing City Zhao et al. (2000)
 Cixi City Zhang et al. (2004a)
 Shaoxing City Zhou et al. (2004)
 Ningbo City Zhang (2002)
 Deqing County Li (2005)

Anhui Hanshan County Wang et al. (2001)
 Lingbi County Zhang and Zhang (2004)
 Shouxian County Hu et al. (2004)
 Liuan City Duan (2004)
 Qimen County Chen and Gao (2001)
 Guichi District He et al. (2002)
 Changfeng County Dai et al. (2004)
 Fuyang City Zhang et al. (1996)
 Chuzhou City Liu et al. (2002)
 Qianmiao Township Zhang et al. (2000)
 Juchao District Yin et al. (2002)

Fujian Province Zhou and Chen (2000)
 Longyan City Huang and Guo (2002)
 Fuan City Zhang (1994)
 Guangze County Qiu (2004)
 Jianyang City Yan (2000)
 Ansha Township Zhou et al. (2003)

Jiangsxi Province Luo et al. (2004)
 Nanchang County Ye et al. (2000)
 etc.
 Nanchang County Ye et al. (1999)
 Xingguo County Ye and Liu (2000)

SOC change per year.

Appendix B. Summary of field experiments focusing on changes in
nutrients in the soil profile under various agricultural practices

Country Location Soil type Crop rotation

China 116.14[degrees]E, Ustochrepts, Wheat-corn
 40.13[degrees]N Alfisols
China 108.67[degrees]E, Alfisols Wheat-corn
 34.30[degrees]N
China 111.17[degrees]E, Alfisols Cereal-potato
 39.38[degrees]N
China 108.09[degrees]E, Alfisols Wheat-maize
 35.04[degrees]N
China 115.12[degrees]E, Ustochrepts Wheat-corn
 37.54[degrees]N
China 116.26[degrees]E, Ultisols Double corn-fallow
 28.37[degrees]N
Brazil 24.6[degrees]S, Dark red Soybean, oat, corn,
 50.4[degrees]W, latosol wheat, lupin, Lolium,
 25.3[degrees]S, bean
 50.3[degrees]W
USA NA (A) Albic luvisol Maize-soybean

India 77.17[degrees]E, Haplustept Millet-wheat-cowpea,
 28.63[degrees] corn-wheat-cowpea

 Years of
Country Location experiment Treatment

China 116.14[degrees]E, 10 Synthetic fertiliser,
 40.13[degrees]N manure, crop residue
 incorporation
China 108.67[degrees]E, 12 Synthetic fertiliser,
 34.30[degrees]N manure, crop residue
 incorporation
China 111.17[degrees]E, 13 Synthetic fertiliser,
 39.38[degrees]N manure
China 108.09[degrees]E, 23 Synthetic fertiliser,
 35.04[degrees]N manure, crop residue
 incorporation
China 115.12[degrees]E, 16 Synthetic fertiliser,
 37.54[degrees]N manure
China 116.26[degrees]E, 10 Synthetic fertiliser,
 28.37[degrees]N manure
Brazil 24.6[degrees]S, 22 Tillage management
 50.4[degrees]W,
 25.3[degrees]S,
 50.3[degrees]W
USA NA (A) 12 No-till, chisel plough
 and mouldboard plough
India 77.17[degrees]E, 12, 19 Synthetic fertiliser,
 28.63[degrees] manure

Country Location n References

China 116.14[degrees]E, 56 Song et al. (2002)
 40.13[degrees]N
China 108.67[degrees]E, 6 Gu et al. (2004)
 34.30[degrees]N
China 111.17[degrees]E, 40 Wang et al. (2003)
 39.38[degrees]N
China 108.09[degrees]E, 6 Xie et al. (2004)
 35.04[degrees]N
China 115.12[degrees]E, 3 Shi et al. (2002)
 37.54[degrees]N
China 116.26[degrees]E, 3 Shi et al. (2002)
 28.37[degrees]N
Brazil 24.6[degrees]S, 5 Sa et al. (2001)
 50.4[degrees]W,
 25.3[degrees]S,
 50.3[degrees]W
USA NA (A) 3 Olson et al. (2005)

India 77.17[degrees]E, 6 Rudrappa et al. (2006)
 28.63[degrees]

(A) Not available.


Manuscript received 9 June 2008, accepted 4 December 2008

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Wenjuan Sun (A), *, Yao Huang (A,B,C),*, Wen Zhang (A), and Yongqiang Yu (A)

(A) LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, P. R. China.

(B) College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, P. R. China.

* The first two authors contributed equally to this work.

(C) Corresponding author. Email: huangy@mail.iap.ac.cn
Table 1. Changes in the SOC in the Ap horizon on the basis of
administrative regions

 Area
 ([10.sup.4] ha)
 [DELTA] [DELTA]
 Sampling [D.sub.OC] [P.sub.OC]
Province/city Total area (t/ha) (Tg)

Shanghai 31.5 31.5 2.66-3.72 0.8-1.2
Anhui 597.2 262.8 2.73-3.81 16.3-22.8
Jiangsu 506.2 506.2 5.53-7.74 28.0-39.2
Zhejiang 212.5 122.6 1.33-1.87 2.8-4.0
Fujian 143.5 143.5 0.58-0.81 0.8-1.2
Jiangxi 299.3 299.3 6.39-8.93 19.1-26.7

Total/Mean 1790.2 1365.9 3.79-5.30 67.9-95.0

Table 2. Changes in the SOC in the Ap horizon on the basis of
soil usage

 Area
 ([10.sup.4] ha)
 [DELTA] [DELTA]
 Sampling [D.sub.OC] [P.sub.OC]
Soil type Total area (t/ha) (Tg)

Paddy soils 924.9 447.5 4.40-6.15 40.7-56.9
Upland soils 865.3 221.0 2.50-3.49 21.6-30.2

Total/Mean 1790.2 668.5 3.48-4.87 62.3-87.1
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Title Annotation:soil organic carbon
Author:Sun, Wenjuan; Huang, Yao; Zhang, Wen; Yu, Yongqiang
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:9CHIN
Date:May 1, 2009
Words:11079
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