| Summary: | Methane (CH<sub>4</sub>) emission and environmental controls of CH<sub>4</sub>-cycling microorganisms are unclear in inland floodplains. Here, we examined soil CH<sub>4</sub> emissions and the community composition of CH<sub>4</sub>-cycling microorganisms under three vegetation types—mudflat (MF, no vegetation cover), <i>Carex</i> meadow (CM, mainly <i>Carex brevicuspis</i>), and reed land (RL, mainly <i>Miscanthus sacchariflorus</i>)—from water-adjacent areas to higher-elevation land in the Dongting Lake floodplain, China. The results showed that CH<sub>4</sub> emission is the highest in CM, while significant absorption was observed in the RL site. The abundance ratio of methanogen/methanotroph was the highest in CM, intermediate in MF, and lowest in RL. The <i>Methanosarcinaceae</i> family represented the dominant methanogens in the three sampling sites (41.32–75.25%). The genus <i>Methylocystis</i> (60.85%, type II methanotrophs) was dominant in CM, while <i>Methylobacter</i> and <i>Methylosarcina</i> (type I methanotrophs) were the dominant genera in MF (51.00%) and RL (50.24%), respectively. Structural equation model analysis showed that methanogen and methanotroph abundance were affected by water table depth, soil water content, and pH indirectly through soil organic content, total nitrogen, microbial biomass carbon, and microbial biomass nitrogen. These results indicated that the Dongting Lake floodplain may change from a CH<sub>4</sub> source to a CH<sub>4</sub> sink with vegetation succession with an increase in elevation, and the methanogen/methanotroph ratio can be used as a proxy for CH<sub>4</sub> emission in wetland soils. The continuous increase in reed area combined with the decrease in <i>Carex</i> meadow may mitigate CH<sub>4</sub> emission and enhance the CH<sub>4</sub> sink function during the non-flood season in the Dongting Lake floodplain.
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