Dynamics Variation of Soil Labile Organic Carbon Fractions in Different Wetland Types of Dongting Lake under Seasonal Water Level Fluctuation

Soil labile organic carbon (LOC) fractions are very sensitive to environmental change and closely related to soil quality. They play an important role in the study of terrestrial carbon cycles. This study aimed to explore the sensitivity of soil LOC fractions to environmental changes and analyze their main influencing factors during three seasonal water level periods for scientific management of Dongting Lake wetlands. Soil under three typical wetland types (Carextristachya wetland (CTW), Phragmites australis wetland (PAW) and Salix babylonica (SBW)) in East Dongting Lake in China were collected during the normal season (May), rainy season (August) and dry season (December). Seasonal dynamics of soil LOC fractions (i.e., dissolved organic carbon (DOC), microbial biomass carbon (MBC) and easily oxidized carbon (EOC)) within these wetlands and their relationship to soil nutrients and carbon-cycle enzyme activity were analyzed. The results showed that the soil DOC contents of the three wetlands first increased and then decreased, with the exception of CTW from the normal season to the dry season, while the seasonal changes of soil MBC and EOC for all wetlands followed an opposite pattern. CTW had the largest DOC concentration (228.29 mg·kg−1) during dry season, while the highest contents of soil DOC, MBC and EOC were found in PAW during the three observed seasons, which ranged from 82.05 to 203.60 mg·kg−1, 262.54 to 325.74 mg·kg−1 and 3.30 to 4.61 g·kg−1, respectively. However, the contents of soil DOC and their proportions to soil organic carbon (SOC) of all wetlands during the normal season were 56.58~82.05 mg·kg−1 and 0.41~0.47%, respectively, which were the lowest among the three seasons. Nevertheless, the contents of both MBC and EOC as well as their ratios to SOC in these wetlands showed similar seasonal dynamics, with the lowest values recorded in the rainy season. From the normal season to the dry season, invertase activity in all wetlands increased, while cellulase activity decreased by 12.5–31.3%. The seasonal variation of catalase activity for all wetlands was less distinctive, and the highest enzyme activity was during the rainy season. Correlation analysis revealed that soil LOC fractions for all wetlands were closely related to SOC, TN, TP and invertase for the three seasons, especially during the rainy season, but were negatively correlated with TK, cellulase and catalase activity. Generally, soil LOC fractions of the three wetlands were affected by the seasonal fluctuations of water levels and presented different distribution characteristics.