[Characteristics of distribution and composition of organic carbon in Dongting Lake floodplain].

Distribution and composition of organic carbon (OC) at nine soil profiles of three types of wetlands in Dongting Lake floodplain were analyzed. Results show that the OC content at top layer (0-10 cm) in Carex spp-dominated floodplain was much higher (>40 g/kg) than that in Phragmites-dominated floodplain (20 +/- 2.8) g/kg and paddy soil (28 +/- 8.6) g/kg. The OC content decreased with increasing depth from 0 to 30 cm, while it was relatively stable (around 15 g/kg) at depths deeper than 30 cm in Carex spp-dominated and Phragmites-dominated floodplain. However, there was a substantial variability in OC content in paddy soil profiles. In terms of the composition of OC, at top layer (0-10 cm), light fraction carbon in Carex spp-dominated floodplain accounted for more than 20% of the total organic carbon, whereas more than 90% of total organic carbon were heavy fraction that were much more difficult to be decomposed in Phragmites-dominated floodplain and paddy soils. The ratio of light fraction to total organic carbon at the soil profiles was greatly affected by the origin of organic matter. Statistical analyses indicate that there were significant correlations between heavy fraction and total organic carbon, between bulk density and total organic carbon, and between OC and nitrogen in heavy fraction (p<0.01).