Hydrological state dependency of ecosystem CO2 sink in floodplain wetlands: Insights from long-term eddy covariance observations.

Floodplain wetlands experience complex hydrological variations that make them hotspots of carbon cycling, yet the characteristics and mechanisms of carbon flux dynamics under varying hydrological states remain poorly understood. Using multi-year eddy covariance measurements in China's Poyang Lake wetland, this study quantified net ecosystem exchange (NEE) of CO2 across four distinct hydrological states: Deep Submerged (DS), Shallow Submerged (SS), Vegetated exposure (VE), and Bare exposure (BE). Results showed that intermediate hydrological states (SS and VE) maximize CO2 uptake (NEE:0.58 to -1.67 g C m⁻² d⁻¹), while DS state shifts the floodplain system toward CO2 neutrality or emission (NEE:0.38 to 0.33 g C m⁻² d⁻¹). Enhanced vegetation index (EVI), inundation water level (IWL), and air temperature (Ta) emerged as dominant NEE drivers across all hydrologic states, with significant IWL × Ta and IWL × EVI interactions. However, the pathways by which these factors regulated NEE differed distinctly among hydrological states, with IWL and climatic drivers controlling DS state, vegetation and three-day variability in IWL controlling SS state, climate indirectly controlling VE state via vegetation, and climate alone controlling BE state. Furthermore, exposure-submergence transition analyses showed asymmetric NEE response. Exposure to submergence transition caused a surge in CO2 uptake for about two weeks, while submergence to exposure transitions triggered a one-week "drought pulse" of increased uptake. These findings highlight the mechanism of NEE response under different hydrological states, and optimizing the hydrological regime to maintain partial submergence and dense vegetation is expected to increase CO2 sinks in floodplain wetlands under climate change.