Agricultural soils are among the planets largest carbon (C) reservoirs and hold great potential for expanded C sequestration. As China has approximately 22％ of the worlds rice paddies, an accurate estimation of the size and dynamics in paddy soil organic carbon (SOC) stocks is critical for quantifying national C budgets as well as for mitigating global climate change through C sequestration in soils. Recently, the DeNitrification-DeComposition (DNDC) model has been widely used to determine SOC inventory or sequestration in China at regional and national scale. However, such studies usually take county as the basic geographic simulation unit and use coarsescale soil maps (e.g. 1:14 000 000 soil map of China) to obtain soil properties required by the model. Since SOC dynamics are affected by a series of spatially differentiated factors (e.g. climate, soil and management practices), the county-scale based model simulations are likely to produce great uncertainty and thereby lower the simulation precision. In the current study, stocks and changes in SOC of paddy soils in Chinas Jiangsu Province, one of Chinas source areas for paddy rice cultivation and paddy soils covered approximately 36％ of the total soil area of the province, were simulated from 1980 to 2008 by coupling the DNDC model with a detailed soil database. Being different from the default county scale database, a polygonbased soil database at a scale of 1:1 000 000 was firstly constructed to reduce the uncertainties from the up-scaling process. Two types of regional simulations were then carried out using the polygon- and county-based databases. Model simulation with the former indicated that Jiangsu Provinces paddy soils covering 3.7 Mha sequestrated 208.9 ± 7.3 Tg (ITg = 1012 g) C from 1980 to 2008 with an average sequestration rate of 0.5 Tg C year-1. Approximately 80％ of the paddy soils sequestrated carbon, while 19.9％ lost carbon and 0.1％ kept balance over the past 29 years. Significant variations of SOC exist between different paddy soil subgroups. The highest SOC sequestration (72.5 Tg C) occurred in the hydromorphic paddy soils with an average SOC density of 51.8 t C ha-1, whereas the lowest (1.2 Tg C) appeared in the salinized paddy soils with an average SOC density of 47.4tC ha1. Moreover, the difference between the highest and lowest total SOC stock modeled with the county-based database was achieved 2.4 times for 1980 and 1.8 times for 2008, while the average total SOC stock was about 1.1 times as large as that with the polygon-based database. The average SOC density modeled with the county-based database slightly increased from 61.01C ha-1 in 1980 to 64.21C ha-1 in 2008. However, when the soil polygon-based database was applied, the average SOC density with a single value increased slightly from 54.5 to 58.41C ha-1 during the 29 years period. This was apparently that the county-based database simulations slightly overestimated the SOC stock in paddy soils of Jiangsu Province. Therefore, in order to obtain a more accurate estimation of SOC, soil datasets with finer spatial resolution are more practicable and necessary to successfully run the DNDC model at regional scale.
Shengxiang Xu Xuezheng Shi Yongcun Zhao Dongsheng Yu Shihang Wang Liming Zhang C.S.Li Manzhi Tan
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy State KeyLaboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy College of Environmental and Resource Sciences, Fujian Agriculture and Forestry University, Fujian 3 Institute for the Study of Earth, Oceans and Space, Complex Systems Research Center, University of N State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy