Importance of carbon and nitrogen availability to microbial necromass carbon accumulation in the drawdown area

Lake drawdown areas -where sediment is exposed to the atmosphere due to water level fluctuations -have a nonnegligible impact on the carbon cycle. However, our knowledge of microbial necromass carbon (MNC) in the drawdown area remains to be seen, which has been considered an essential precursor to soil organic carbon (SOC). Here, we examined the content of MNC and its contribution to SOC in three typical habitats (mudflat, grass, and reed area) from the water edge to the uplands in the seasonal drawdown area of Dongting Lake. MNC content in the drawdown area ranged from 1.31 to 3.73 g kg -1, accounting for 27.39 %, 24.51 %, and 27.91 % of SOC in mudflat, grass, and reed areas, respectively. Compared to mudflat areas (1.83 +/- 0.11 g kg -1), vegetated areas (grass: 2.15 +/- 0.18 g kg -1; reed: 2.51 +/- 0.18 g kg -1) showed a higher MNC content. MNC variation was mainly related to carbon and nitrogen availability, plant biomass, clay content, and soil moisture. Our results showed that microbial properties were not essential in regulating MNC accumulation. Instead, external factors such as plant and soil properties played a more critical role in the long-term iterative process of MNC accumulation. Carbon and nitrogen availability, as regulated by plant biomass, clay content, and soil moisture, was dominant in MNC accumulation in drawdown areas. In conclusion, these findings highlight the critical role of external factors in MNC accumulation, which would enhance our understanding of MNC stability in drawdown areas.