Inundation depth affects ecosystem CO2 and CH4 exchange by changing plant productivity in a freshwater wetland in the Yellow River Estuary

Climate change (extreme rainfall) and water management activities have led to variation in hydrological regimes, especially inundation, which may alter the function and structure of wetlands as well as wetland-atmosphere carbon (C) exchange. However, the degree to which different inundation depths (standing water depth above the soil surface) affect ecosystem CH4 fluxes, ecosystem respiration (Reco) and net ecosystem CO2 exchange (NEE) remains uncertain in wetland ecosystems. We conducted a field inundation depth manipulation experiment (no inundation, i.e. only natural precipitation; 0, water-saturated; 5, 10, 20, 30 and 40 cm inundation depth) in a freshwater wetland of the Yellow River Delta, China. The CH4 fluxes, Reco and NEE were measured with a static chamber technique during the growing seasons (May–October) of 2018 and 2019. Inundation depth significantly increased plant shoot density, above-water level leaf area index (WLAI), above-water level plant shoot height (WHeight), aboveground and belowground biomass of the dominant grass Phragmites australis in both years. Meanwhile, inundation depth increased the CH4 fluxes, Reco (except for 0 cm) and NEE compared to no inundation, which could be attributed partly to the increased plant productivity (shoot density, WLAI, WHeight, biomass). Additionally, the CH4 fluxes, Reco or NEE exhibited parabolic responses to inundation depth. Furthermore, global warming potential (GWP) was significantly decreased under different inundation depths during the growing season, especially from 5 to 40 cm inundation depth in 2019. NEE was the largest contributor to the seasonal GWP, which indicates that the inundated wetlands are a net sink of C and have a cooling climate effect in the Yellow River Delta. Inundation depth substantially affects the magnitude of CH4 fluxes, Reco and NEE, which were correlated with altered plant traits in wetland ecosystems. Inundation depth could mitigate greenhouse gas emissions in the P. australis wetlands during the growing season. Inundation depth-induced ecosystem C exchange should be considered when estimating C sequestration capacity of wetlands due to climate change and water management activities, which will assist to accurately predict the impact of hydrological regimes on C cycles in future climate change scenarios.