Hydrologic Processes Drive Methane Fluctuations in a Large Subtropical Floodplain

Floodplain methane (CH₄) emissions constitute a substantial component of the global CH₄ budget. Nevertheless, their response to the increasing frequency of extreme drought events remains insufficiently understood, largely owing to the pronounced variability associated with alternating wet–dry hydrological regimes. To address this research gap, we conducted two years of in-situ CH₄ flux measurements across alternating hydrological cycles (2022–2023) in the Poyang Lake floodplain wetland, a period during which the region experienced an extended drought. Our results reveal that CH₄ emissions during non-flooding periods (1.82 ± 1.36 mg CH₄ m⁻² h⁻¹, mean ± standard deviation) were significantly higher than those during flooding periods (1.26 ± 0.96 mg CH₄ m⁻² h⁻¹). Notably, under drought conditions, CH₄ fluxes during the autumn growing season (2.04 ± 1.43 mg CH₄ m⁻² h⁻¹) were 35% greater than those observed in the spring growing season (1.51 ± 1.21 mg CH₄ m⁻² h⁻¹). Further analysis indicates that, apart from air temperature, CH₄ fluxes were primarily regulated by vegetation during non-flooding periods, whereas during flooding periods they were predominantly governed by water level fluctuations and inundation duration, factors that modulate key biogeochemical processes. The enhanced temperature sensitivity of CH₄ emissions emerged as a critical mechanism underlying the elevated autumn emissions relative to spring, a pattern directly attributable to the shortened flooding duration in the Poyang Lake floodplain. These findings highlight the pivotal role of extreme drought in altering hydrological regimes and CH₄ emission dynamics in floodplain wetlands, with important implications for predicting wetland responses under future climate change scenarios.