Organic matter is an important soil component that influences the physical, chemical and biological properties of soils.
Maintenance of its content in the soil has been the crucial measure for improving productivity and stabilizing ecosystems. Stabilization of the organic matter also plays a pivotal role in controlling global warming through an increasing sequestration of atmospheric CO 2 to the plant-soil system. In this paper, dynamics and processing mechanisms of
soil organic matter accumulation for a
cultivation chronosequence in paddy field were studied in subtropical China by sampling in field, and determining steady-state soil C contents. Under paddy cultivation, soil organic C and N increased rapidly for 30 years, leveling off thereafter. After 30 years of rice planting with irrigation, 0～20 cm soil organic C and N increased to 20 g kg -1 and 1.6 g kg -1, respectively, and then insignificantly changed even through 80 years of utilization. During 20 days' incubation, the mineralization rates of 0～10 cm soil organic C and N in a cultivation chronosequence of paddy field ranged from 2.2%～3.3% and 2.8%～6.7%, which in general increased with soil organic C and N contents. Soil microbial biomass C increased proportionately with cultivation years, in comparison, the soil microbial biomass N increased in the first 30 years and then leveled off. 0～10 cm soil cultivated as paddy field for 30 years has a 332.8 mg kg -1 of microbial biomass C and 23.85 mg kg -1 microbial biomass N, which were 111% and 47% higher than that for paddy field cultivated for 3 years. The numbers of bacteria in the soil cultivated as paddy field for 30 years were 1.1-fold (0～10 cm) and 3.8-fold (10～20 cm) higher than that for 3 years, and the corresponding values in soils cultivated for 80 years were 19-fold (0～10 cm) and 12-fold (10～20 cm). The number of fungi was also increased in the first 30 years' cultivation and then leveled off. Furthermore, the increasing community of bacteria from 4 in wasteland to 8 in paddy soil under 30～80 years of cultivation indicated the improvement of soil fertility.