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

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 similar to 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. (C) 2009 Elsevier B.V. All rights reserved.