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

• China's upland soils present significant regional differences in the MRT a and MRT s. • The upland soils showed longer MRT a and MRT s than the paddy soils. • The soil texture dominated the regional variations of MRT a and MRT s in cropland soils. 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 (MRT a) and C s (MRT s) 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 MRT a = 1/K a and MRT s = 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 MRT a and MRT s in the surface upland soils (0–20 cm) across China, in which the longest MRT a and MRT s values were 72 d in the north China and 24.0 yr in the northeast China, the shortest MRT a and MRT s 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 MRT a and the MRT s 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 MRT a and MRT s in the surface upland and paddy soils in China; however, as Chinese agricuture intensified from 1980 to 2010, the regional variabilities of MRT a and MRT s 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. [ABSTRACT FROM AUTHOR]