Mean residence times of active and slow soil organic carbon pools in croplands across China

Using active (C-a) and slow (C-s) soil organic carbon (SOC) pools to simulate changes in SOC stocks has been determined to be a major improvement over treating SOC as a single, homogeneous pool. However, regional patterns and primary controls of the mean residence times of the C-a (MRTa) and C-s (MRTs) pools in China's croplands are largely unknown. In this study, a total of 373 samples from six regions were analysed based on a 100-d incubation experiment. The data from acid hydrolysis-incubation experiments were fitted to a three-pool first-order kinetics model that estimated MRTa = 1/K-a and MRTs = 1/K-s (K-a and K-s represent the decomposition rate constants of the C-a and C-s pools, respectively). The results showed that significant regional differences were found in the MRTa and MRTs in the surface upland soils (0-20 cm) across China, in which the longest MRTa and MRTs values were 72 d in the north China and 24.0 yr in the northeast China, the shortest MRTa and MRTs values were 12 d and 4.7 yr, respectively, both in the south-central China, while relatively small regional differences were observed in the values of the surface paddy soils (0-20 cm). Both the MRTa and the MRTs in upland soils (average: 30 d and 13.5 yr) were longer than the corresponding values in the paddy soils (average: 11 d and 5.4 yr), especially in the southwest China. Thus, the carbon sequestration potential of paddy soils needs to be revalued. The climate could partly explain the regional distribution of the MRTa and MRTs in the surface upland and paddy soils in China; however, as Chinese agricuture intensified from 1980 to 2010, the regional variabilities of MRTa and MRTs in the surface upland and paddy soils were primarily dominated by the soil texture. Therefore, more attention should be paid to the destruction of mechanical composition caused by land-use change or conventional tillage methods when faced with future carbon sequestration in croplands.