Greenhouse gas emissions in water-level fluctuation zones (WLFZs) of the Three Gorges Reservoir after 15 years of operation.

The water-level fluctuation zones (WLFZs) of reservoirs are potential hotspots of greenhouse gas (GHG) emissions. The Three Gorges Reservoir (TGR), the largest hydropower dam globally, contains extensive WLFZs that undergo winter flooding and gradual summer exposure, resulting in periodic anti-seasonal water-level changes. Nevertheless, the spatial variability and long-term dynamics of GHGs emissions from these zones remain poorly understood. This study investigated the spatial variations of soil GHGs fluxes in WLFZs at longitudinal, vertical and local scales across the entire TGR after 15 years of operation, and identified the effect of soil environment, water level, microtopography, and plant community. Results demonstrated that the WLFZs serve as a significant and persistent source of GHGs during the summer drawdown period, with emissions substantially higher than adjacent non-flooding zones but comparable to or lower than natural riparian zones. Long-term flooding has led to spatial homogenization of soil carbon, resulting in weak longitudinal variation of CO2 fluxes, while the longitudinal differentiations of CH4 and N2O fluxes were obvious and meanly regulated by the spatial distribution of soil dissolved organic carbon (DOC), pH and available nitrogen. Critically, water-level fluctuation dominated the vertical zonation of GHGs emissions by driving the zonal differentiation of soil environments and plant communities. Furthermore, microtopography played a critical role, with gentle slope areas being identified as emission hotspots. The total CO2-equivalent of GHGs emissions from WLFZs in the TGR area were re-estimated to be 751.53-811.49 Gg CO2-eq yr-1, contributing over 53 %-63 % to the previously estimated GHGs inventory for the entire reservoir. This study highlights that WLFZs are an enduring and metastable source of GHGs emissions, offering a holistic framework to access their spatially integrated variability in super-large artificial reservoir with anti-seasonal water-level fluctuations.