Abstract: Water-level fluctuations are a key driver of the composition and diversity of soil microbial communities in wetlands. However, few studies have focused on the responses of soil microorganisms to waterlevel changes in lake wetlands with different hydrological connectivity. We examined the soil physicochemical properties, vegetation, and soil microbial communities at different water levels (frequently flooded areas and unflooded area) in these two types of lake wetlands, including Honghu Lake (a shallow river-connected and gate-controlled lake) and Chong Lake (a disconnected lake) in the middle and lower reaches of the Yangtze River. We further elucidated the effects of water level fluctuations on soil microbial communities and their response mechanisms in lakes with different hydrological connectivity based on hydrological indicators including runoff volume, runoff depth, and water level. The results showed that: (1) The vegetation richness index of Honghu Lake increased with rising water levels, while Simpson index showed the opposite trend. In addition, the aboveground biomass of Chong Lake and the microbial richness indices of the two lakes increased as water levels increased. (2) The results of the NMDS analysis showed that water level changes significantly altered the vegetation community in Honghu Lake and microbial community composition in two types of lakes. Meanwhile, the number of nodes and edges in the microbial network of the unflooded area in both lakes was significantly higher than in the frequently flooded areas. (3) Runoff volume, runoff depth, nitrate, available phosphorus content, and the average degree of microbial network edges were the important predictors of bacterial community composition and richness in Honghu Lake. By contrast, daily mean water level, organic carbon content, and ammonium content were important predictors of bacterial community composition and richness in Chong Lake. (4) Structural equation modeling showed that hydrology not only directly affected aboveground vegetation communities and microbial communities, but also indirectly affected microbial communities by changing soil physicochemical properties and vegetation communities. This study can provide a microbial perspective for the scientific management and protection of wetlands with varying connectivity.

Key words: vegetation, soil microorganism, water level fluctuation, lake connectivity