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Soil organic carbon storage of paddy soils in China using the 1:1,000,000 soil database and their implications for C sequestration

Qing-Hua Liu and Xue-Zheng Shi

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences,
Nanjing, China



Also at Graduate School, Chinese Academy of Sciences,
Beijing, China

D. C. Weindorf

Texas Agricultural Experiment Station,
Stephenville, Texas, USA

Dong-Sheng Yu, Yong-Cun Zhao, Wei-Xia Sun, and Hong-Jie Wang

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences,
Nanjing, China

Abstract

[1]  Organic carbon storage in agricultural soils plays a key role in the terrestrial ecosystem carbon cycle. Paddy soils support important croplands in many parts of the world, especially in Asia. A thorough understanding of organic carbon storage in Chinese paddy soils would be helpful to both greenhouse gases emission and carbon sequestration studies. This paper examines soil organic carbon density (SOCD) and storage (SOCS) of paddy soils in China using the newly compiled 1:1,000,000 digital soil map of China as well as data from 1490 paddy soil profiles. Results show that paddy soils in China cover about 45.7 M ha, nearly 1.5 times more than the results of other studies. In China, the mean SOCD of paddy soils at a depth of 0–100 cm is 111.4 t C ha−1, with a SOCS of 5.1 Pg. These results are 66–75% higher than studies from other scientists. However, the mean SOCD of paddy soils from 0 to 20 cm is 37.6 t C ha−1, with a SOCS of 1.7 Pg, which is 89% higher than studies from other scientists.

Received 31 March 2006; accepted 5 June 2006; published 23 September 2006.

Keywords: soil organic carbon (SOC), soil organic carbon density (SOCD), paddy soils, China.

Index Terms: 1055 Geochemistry: Organic and biogenic geochemistry; 3367 Atmospheric Processes: Theoretical modeling; 3360 Atmospheric Processes: Remote sensing; 4806 Oceanography: Biological and Chemical: Carbon cycling (0428).


全文

1. Introduction
[2]  Organic carbon sequestrated in soils all over the world amounts to approximately 1500 Pg; twice as much as that present in the global atmosphere (750 Pg) and three times as much as that present in terrestrial vegetation (500–600 Pg) [Batjes, 1996; Eswaran et al., 1993; Lal et al., 1995]. Therefore any slight change in SOCS may greatly affect the concentrations of greenhouse gases in the atmosphere and subsequently, global climate change. Compared to natural ecosystems, agro-ecosystems are more profoundly affected by human activities. Consequently, changes in regional organic carbon levels in cultivated soils and, subsequent changes in atmospheric organic carbon may be much more substantial than natural changes [Freibauer et al., 2004; Yuri et al., 2005; Houghton and Hackler, 2003; Marlen et al., 2002]. The promotion of C sequestration in agricultural soils is recognized as one strategy for achieving food security through improvement in soil quality. However, such improvements are considered finite and realizable over relatively short periods of time (20 to 50 years) [Lal, 2004]. China is a country with a long farming history. Croplands in China cover an area of 133 M ha, 14% of the country. Cultivated for over 7000 years, paddy soils represent a major cultivated soil in China, and a unique type of anthropogenic soil recognized by Chinese soil taxonomy. The total area of paddy soils is 30 M ha; 25% of the total cultivated land in China. This area also accounts for 23% of the total aqua-farming area worldwide and produces about 44% of all grain in China [Li, 1992].

[3]  Chinese scientists have studied SOC storage for many years. However, in the past 10 years, estimates of SOCS in China have varied from 50 Pg to 180 Pg owing to diverse methods and different data sources [Pan, 1999; Wang et al., 2000; Wu et al., 2003; Xie et al., 2004; Yu et al., 2006; Fang, 1996]. For three studies involved in the estimation of SOCS in China's paddy soils, Xie et al. [2004] estimated that the SOCS of paddy soils in China at depths of 0–100 cm was 2.9 Pg and in the upper 0–20 cm was 0.9 Pg. Wang et al. [2000] reported that the SOCS of paddy soils profile in China was 3.1 Pg. Pan et al. [2003] concluded that SOCS from the surface through the plowpan layers of paddy soils amounted to 1.3 Pg. In these three studies, soil attribute data were all cited from Soil Species of China [Office for the Second National Soil Survey of China, 1993]. This includes records of 525 paddy soil profiles in the country. However, new and different sources of paddy soil types have also been identified and adopted. One is identified in the Second National Soil Survey of China [Wang et al., 2000; Pan et al., 2003], and another is noted in the 1:4,000,000 soil map of China [Xie et al., 2004]. All of these constitute a foundation for studies on SOCS of paddy soils in China. However, more detailed studies are needed, because: (1) the 1:4,000,000 soil map of China fails to reflect the actual details of paddy soil distribution in China owing to significant data uncertainty associated with landforms containing paddy soils, (2) data uncertainty has also resulted from errors in the Second National Soil Survey of China due to various inadequacies in administrative or statistical parameters, and (3) a finite number of paddy soil profiles (only 525) was used in estimating SOCS for the entire country.

[4]  In this study, spatial distribution of soil types was obtained from a newly compiled 1:1,000,000 digital soil map of China [Shi et al., 2004a, 2004b]. Soil attribute data was derived from 1490 paddy soil profiles. SOCD and SOCS were estimated for profile (0–100 cm) or surface layer (0–20 cm) depths. Spatial SOC variation patterns in paddy soil subgroups and soil regions are also discussed.

2. Data Source and Research Methods
2.1. Data Source

[5]  The Soil Spatial Database was digitized from the 1:1,000,000 Soil Map of the People's Republic of China [Office for the Second National Soil Survey of China, 1995]. Obviously, the precision of the database is two orders higher than that of the 1:4,000,000 soil map, which was widely used in previous research studies [Shi et al., 2004a, 2004b]. The mapping units for paddy soils in the 1:1,000,000 database are soil families. Correspondingly, 18,162 polygons of paddy soils are given in the 1:1,000,000 soil map; 75 times more than the 238 polygons in the 1:4,000,000 soil map. Soil attribute data of 1490 paddy soil profiles was utilized in this study; double the number of soil profiles used in comparable studies. Of the 1490 profiles, 525 are from the Soil Species of China. The remaining profiles are from Soil Species published by various individual provinces. The Soil Attribute Database includes extensive information such as soil names, profile locations, bulk density, horizon depth, particle composition, organic matter, and others. Thus the more detailed 1:1,000,000 paddy soil database of China was developed by linking the Soil Attribute Database with the Soil Spatial Database using a pedological professional knowledge-based method (PKB) [Zhao et al., 2005, 2006].

2.2. SOCD Calculation

[6]  The SOCD of a profile can be calculated using equation (1) [Wang et al., 2000; Kazuhito et al., 2004],



where SOCD is SOC density (kg m−2) of a profile, i is the gravel (>2 mm) content in horizon i (%) ρi is the soil bulk density of horizon i (g cm−3) Ci is the organic carbon content of horizon i (C g kg−1) Ti is thickness of horizon i (cm) and n is the number of horizons involved. The organic carbon content is calculated by multiplying soil organic matter content by 0.58 (the Bemmelen index) [Wen, 1984]. The SOCD of paddy soils was estimated both at depths of 0–100 cm and 0–20 cm [Sun et al., 2003]. SOCS for each polygon in the map was calculated with SOCD and the surface area of the polygon. Finally, the total SOCS of paddy soils in China was calculated by summing the SOCS of all polygons. The mean SOCD of paddy soils was calculated by dividing total SOCS by total area.

3. Results and Discussion
3.1. SOCD of Paddy Soils in China

Figure 1.  Distribution pattern of SOCD (0–100cm) of paddy soils in China. On the basis of a soil regionalization map of China (1:10,000,000) [Editorial Board for Physical Geography of China, 1981] and a map of paddy soil regionalization [Li, 1992], six paddy soil regions were defined as follows. SCSY: South China and Southern Yunnan latosol (Acrisols), lateritic red soil (Acrisols) and paddy soil (Anthrosols) region; SYNT: South of the Yangtze River and Northern Taiwan red soil (Acrisols), yellow soil (Cambisols), and paddy soil (Anthrosols) region; MLY: Middle and lower Yangtze River Valley yellow brown soil (Luvisols) and paddy soil (Anthrosols) region; SBIA: Sichuan basin and its adjacent mountains yellow soil (Cambisols), purple soil (Cambisols), and paddy soil (Anthrosols) region; NC: North China brown soil (Luvisols), black soil (Phaeozems), kastanozem (Kastanozems), and desert soil (Calcisols), region; YGQT: Yunnan-Guizhou Plateau and Qinghai-Tibet Plateau red soil (Acrisols), yellow soil (Cambisols), paddy soil (Anthrosols), and alpine soil (Cryosols) region.


[7]  Statistical results based on the 1:1,000,000 paddy soils database of China show that the area of paddy soils in China is 45.7 M ha, approximately 1.5 times as much as the area (29.9 M ha) calculated from 1:4,000,000 soil map of China [Xie et al., 2004]. The mean SOCD of the 0–100 cm depth in paddy soils is 111.4 t C ha−1, 14% higher than the mean SOCD based on the same thickness in paddy soils (97.8 t C ha−1) reported by Xie et al. [2004]. SOCD (0–100 cm) of polygons in paddy soils show a great variation (Figure 1). The difference is over 800 times between the highest and lowest values (4,462 t C ha−1 and 5.3 t C ha−1). SOCD in the range of 40–80, 80–120 and 120–200 t C ha−1 cover 37.0%, 29.0% and 23.8% of the total paddy soil areas in China, respectively. Those with a SOCD lower than 40 t C ha−1 and greater than 200 t C ha−1 cover a limited area. A fairly significant variation of SOCD (0–100 cm) is found among soil subgroups and soil regions in China. As for soil subgroup, the highest SOCD (213.7 t C ha−1) is found in the subgroup of “gleyed paddy soils,” while the lowest SOCD (77.3 t C ha−1) is found in the subgroup of “bleached paddy soils,” a difference of nearly threefold (Table 1). As for soil region, a twofold difference exists in SOCD between the South China and Southern Yunnan region (SCSY) containing the highest SOCD (140.1 t C ha−1) and the Sichuan Basin and Its Adjacent Mountain regions (SBIA) with the lowest SOCD (79.7 t C ha−1) (Table 2).

[8]  The mean SOCD for the 0–20 cm depth in paddy soils is 37.6 t C ha−1. At this depth, the variation in SOCD is even more pronounced; the difference between the highest and lowest SOCD values being over 300-fold (553.8 t C ha−1 and 1.7 t C ha−1). Paddy soils with a SOCD of 20–40 t C ha−1 cover 56.2% of the total area of paddy soils in China; those with a SOCD 40–60 t C ha−1 follow (26.7%). Paddy soils with less than 20 t C ha−1 or more than 60 t C ha−1 cover comparatively limited areas. Similarly, significant variation of SOCD (0–20 cm) of paddy soils is found among soil subgroups and soil regions in China. As for soil subgroup, the difference between the highest subgroup of “gleyed paddy soils” (52.7 t C ha−1) and the lowest subgroup of “salinized paddy soils” (29.5 t C ha−1) is twofold (Table 1). As for soil region, the highest SOCD is 48.8 t C ha−1 for the Yunnan-Guizhou Plateau and Qinghai-Tibet Plateau region (YGQT). This is roughly double that of the Sichuan basin and its adjacent mountain regions (SBIA) with the lowest SOCD of 27.7 t C ha−1 (Table 2).

3.2. SOCS of Paddy Soils in China

[9]  On the basis of the SOCD of 18,162 paddy soil polygons and their corresponding areas, the calculated total SOCS (0–100 cm) of paddy soils in China is 5.1 Pg. which accounts for 5.2% of the total SOCS of all soils (89.1 Pg) in China [Yu et al., 2006]. The total SOCS (0–20 cm) of paddy soils amounts to 1.7 Pg. This indicates that a remarkable concentration of SOCS (33.7%) resides in the surface layer.

[10]  Subgroup of “hydeomophic, submergenic, percogenic and gleyed paddy soils” account for 92.3% of the total paddy soil area. Thus SOCS (0–100 cm) in these four subgroups is 91.7%, accounting for a majority of the total SOCS of paddy soil in China. For the 0–100 cm depth, the SOCS of the subgroup “hydeomophic paddy soils” is the highest (2722 Tg) and accounts for 53.5% of the total SOCS of paddy soils in China. The subgroup of “acid sulfate paddy soils” is the lowest (16.6 Tg); only 0.33% of the total. The same subgroups were identified at 0–20 cm depths, with the highest and lowest values being 943.3 Tg and 4.3 Tg, respectively.

[11]  Most paddy soils in China are distributed in the South China and Southern Yunnan region (SCSY), the South of the Yangtze River and Northern Taiwan region (SYNT), and the Middle and Lower Yangtze River Valley region (MLY). Paddy soils areas in these three regions account for 75.5% of paddy soils in China and the SOCS of these same areas accounts for 77.2% of the total storage in China. For the 0–100 cm depth, the SOCS of paddy soils in the South of Yangtze River and Northern Taiwan region (SYNT) is the highest (1683 Tg) among six regions. The SOCS of paddy soils in the North China region (NC) is the lowest (253.0 Tg). The same regions were identified based on 0–20 cm depths, with the highest and lowest values being 581.7 Tg and 88.6 Tg, respectively.

[12]  Compared to the SOCS estimates of paddy soils in China by other scientists, results of this study (5.1 Pg) are 66% higher than those of Wang et al. [2000] at 3.1 Pg and 75% higher than those of Xie et al. [2004] at 2.9 Pg. This is likely due to the increased accuracy of data sources used in this study (1:1,000,000 soil database of China), a larger-profile data set, and more precise linkage methods for soil attribute data and spatial data. For example, the total paddy soil area in China derived from the 1:1,000,000 paddy soil database is 45.7 M ha, considerably larger than areas delineated by other studies, i.e., 29.8 M ha [Wang et al., 2000, Pan et al., 2003] and 29.9 M ha [Xie et al., 2004]. These other studies appear to have less statistical certainty owing to the adoption of less accurate or detailed data from the Second National Soil Survey and 1:4,000,000 soil map of China. It is quite clear that fewer paddy soil polygons would be recognizable in a digital map derived from an original map with smaller scale, such as the 1:4,000,000 soil map. In such an instance, tiny spot-like, belt-like and branch-like paddy soil polygons are blended into larger polygons of other soil types, thus becoming unrecognizable. Consequently, the surface area of paddy soils would be underestimated.

3.3. Implications for Soil C Sequestration

[13]  Paddy soils are quite different from upland soils of in their physical, chemical and biological properties owing to impacts of human management. Activities such as frequent alteration of oxidation (when drained) and reduction (when irrigated), clay accumulation caused by special irrigation and drainage arrangements, the large amount of crop residues returned to the soil and large volumes of manure application all markedly affect soil properties. Some results show that the SOCD of the paddy topsoil is higher than the SOCD of the corresponding upland soils [Pan et al., 2003; Yu et al., 2004; Wang et al., 2005; Tian et al., 2006]. This could be attributed to surface waterlogging in paddy soils, where the decomposition rate of SOC was slower than upland soils. Furthermore silt and clay contents in paddy soils were generally higher than those in upland soils [Wang et al., 2005], which also led to larger SOC accumulation. Lal [2002] also considered C pool enhancement in paddy soils as a potential C sequestrator. Accordingly, paddy soil C sequestration may play a major role in China's climate-change policy. Paddy soils produce about 44% of all grain in China [Li, 1992]. Paddy soil C sequestration is one strategy toward achieving food security through improvements in soil quality. Thus paddy soils are important for both food production and economic development in China.

[14]  The results estimated with 1:1,000,000 digital soil map of China and 1490 paddy soil profiles in this study are the most accurate and reliable for paddy soils in China. This knowledge of SOC stocks in paddy soils could help to identify areas or soil subgroups which are of particular interest for SOC gains and losses. It is also critical for evaluating the C sequestration potential in paddy soils and for modeling carbon cycling in the agro-ecosystems.

4. Conclusion
[15]  Statistical results based on the l:1,000,000 paddy soil database of China show that paddy soils in China cover an area of 45.7 M ha, nearly 1.5 times as much as previously published by other researchers. The mean SOCD (0–100 cm) of paddy soils in China is 111.4 t C ha−1, which is 14% higher than one (97.8 t C ha−1) reported by Xie et al. [2004]. SOCS (0–100 cm) of paddy soils in China is 5.1 Pg, which is 66–75% higher than results reported by other scientists. The mean SOCD and SOCS (0–20 cm) of paddy soils are 37.6 t C ha−1 and 1.7 Pg, respectively. While using a newer, more accurate soil database (1:1,000,000 as compared to 1:4,000,000) only caused a 14% difference in the mean SOCD (0–100 cm) of paddy soils, SOCS at the same depths increased from 2.9 Pg to 5.1 Pg. This disparity between old and new estimates of SOCS underscores the importance of using data with the highest accuracy and highest resolution available. For polygons of tiny spot-like, belt-like and branch-like paddy soils, the use of high-resolution digital soil maps is more effective and accurate than simply increasing the number of soil profiles used to estimate the mean SOCD and SOCS. The use of detailed soil data sets with high-resolution digital soil maps and robust soil profile data is essential in creating accurate models of the soil carbon cycle at national or regional scales.

Acknowledgment
[16]  We gratefully acknowledge support for this research from the Key Innovation Project of Chinese Academy of Sciences (KZCX3-SW-427) and National Natural Science Foundation of China (30390080, 40471081) and Knowledge Innovation Program of CAS (INF105-S).

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Figure

Figure 1. Distribution pattern of SOCD (0–100cm) of paddy soils in China. On the basis of a soil regionalization map of China (1:10,000,000) [Editorial Board for Physical Geography of China, 1981] and a map of paddy soil regionalization [Li, 1992], six paddy soil regions were defined as follows. SCSY: South China and Southern Yunnan latosol (Acrisols), lateritic red soil (Acrisols) and paddy soil (Anthrosols) region; SYNT: South of the Yangtze River and Northern Taiwan red soil (Acrisols), yellow soil (Cambisols), and paddy soil (Anthrosols) region; MLY: Middle and lower Yangtze River Valley yellow brown soil (Luvisols) and paddy soil (Anthrosols) region; SBIA: Sichuan basin and its adjacent mountains yellow soil (Cambisols), purple soil (Cambisols), and paddy soil (Anthrosols) region; NC: North China brown soil (Luvisols), black soil (Phaeozems), kastanozem (Kastanozems), and desert soil (Calcisols), region; YGQT: Yunnan-Guizhou Plateau and Qinghai-Tibet Plateau red soil (Acrisols), yellow soil (Cambisols), paddy soil (Anthrosols), and alpine soil (Cryosols) region. Enhanced TIF [3.6 MB]