Seasonal dynamics of groundwater discharge: Unveiling the complex control over reservoir greenhouse gas emissions

The pronounced topographical differences, giving rise to numerous water bodies, also endow these formations with substantial hydraulic gradients, leading to pronounced groundwater discharge within their low-lying, natural reservoir settings. However, the dynamics of groundwater discharge in reservoirs and their impact on greenhouse gas (GHG) production and emission under different conditions remain unclear. This study focuses on a reservoir in southeastern China, where we conducted seasonal field observations alongside microcosm incubation experiments to elucidate the relationship between greenhouse gas emissions and groundwater discharge. Based on the radon (²²²Rn) mass balance model, groundwater discharge rates were estimated to be 2.14 ± 0.49 cm d^-1 in autumn, 4.04 ± 2.09 cm d^-1 in winter, 2.55 ± 1.32 cm d^-1 in spring, and 2.61 ± 1.93 cm d^-1 in summer. Groundwater discharge contributes on average to 31.23 % of CH₄, 35.65 % of CO₂, and 11.26 % of N₂O emissions across all seasons in the reservoir. Groundwater primarily influences GHG emissions by directly inputting carbon and nitrogen, as well as by altering aquatic chemical conditions and the environment of dissolved organic matter (DOM), exerting significant effects particularly during spring and autumn seasons. Especially, in winter, higher groundwater discharge rates influence microbial activity and environmental conditions in the water body, including the C/N ratio, which somewhat reduces its enhancement of greenhouse gas emissions. This study provides an in-depth exploration of greenhouse gas emissions from reservoirs and examines the impact of groundwater on these emissions, aiming to reduce uncertainties in understanding greenhouse gas emission mechanisms and carbon and nitrogen cycling.