An increase in topsoil SOC stock of Chinaʹs 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 Chinaʹs 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 Chinaʹs 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 Chinaʹs 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 Chinaʹs croplands, especially rice paddies, for C sequestration to mitigate climate change.