Abstract: Soil organic carbon is composed of active, slow and passive fractions. Separating different organic carbon components could help to improve modeling dynamic of soil organic carbon pool and its feedback to global climate change. Poyang Lake is the largest freshwater lake in China. It is a typical subtropical shallow water lake characterized by drastic annual and interannual water level fluctuation. In recent years, climate change and the operation of the Three Gorges Dam have largely altered the hydrological regime of Poyang Lake, resulting in reduction of summer flooding and extension of winter dry season. As a result, the submerged plant communities shrank, whereas the emergent plant communities expanded. In this study, soil samples at 0-15 cm and 15-30 cm depths under seven dominant plant communities were collected along a water table gradient in a typical Poyang Lake wetland. Subsequently, contents of total soil organic carbon (SOC), heavy fraction organic carbon (HFOC), light fraction organic carbon (LFOC), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) were measured. The objectives of this study were to clarify the characteristics of various soil organic carbon components and their affecting factors, and explore the potential impact of community succession on soil organic carbon and its components. Our results showed that the ranges of SOC, HFOC, LFOC, MBC and DOC were 4.92-48.14  mg·g^-1, 4.66-41.42 mg·g^-1, 0.46-6.52 mg·g^-1, 124.22-419.23  mg·kg^-1 and 33.17-153.63 mg·kg^-1, respectively. The organic carbon components varied significantly with plant communities and soil layers. The content of each soil organic carbon component under various plant communities decreased from the top soil to the deeper profile. Soil layers and communities types as well as their interaction all significantly affected the spatial pattern of various organic carbon components. The ratios of HFOC/SOC, LFOC/SOC, MBC/SOC and DOC/SOC were 92.9%, 7.1%, 1.44% and 0.74%, respectively. The distribution patterns of LFOC, MBC and DOC were not spatially consistent, suggesting that these active organic carbon components were not controlled by the same environmental factors. In the short term, the shrink of submerged plant communities and the expansion of emergent plant communities in Poyang Lake wetlands could increase soil organic stock; however, the stability of carbon pool decreased.

Key words: precipitation concentrated index, alpine steppe-meadow, precipitation centroid, precipitation use efficiency, aboveground productivity.