Mechanisms of Groundwater–Surface Water Interactions on Ecosystem Greenhouse Gas Emissions

Greenhouse gas (GHG) emissions from inland waters exhibit pronounced spatial and seasonal variability, yet the role of groundwater discharge in regulating these dynamics remains insufficiently constrained. In reservoirs located in topographically complex regions, strong hydraulic gradients can induce substantial groundwater–surface water exchange, potentially altering carbon and nitrogen inputs as well as biogeochemical conditions that govern GHG production and emission.
Here, we investigate the seasonal influence of groundwater discharge on CH₄, CO₂, and N₂O emissions in a subtropical reservoir using an integrated approach combining multi-season field observations and controlled microcosm experiments. Groundwater discharge rates were quantified using a radon-222(²²²Rn) mass balance framework, revealing marked seasonal variability, with enhanced discharge during winter and moderate but persistent inputs during spring and autumn. Dissolved GHG concentrations in groundwater were consistently elevated relative to surface water, indicating groundwater as a direct source of atmospheric GHGs.
Across seasons, groundwater discharge contributed substantially to reservoir-scale emissions, accounting for approximately one-third of CH₄ and CO₂ fluxes and a smaller but non-negligible fraction of N₂O emissions. However, the relationship between discharge intensity and GHG fluxes was non-linear. Field observations and incubation experiments demonstrate that moderate groundwater inputs during transitional seasons enhanced CH₄ and CO₂ production by increasing carbon availability, modifying dissolved organic matter composition, and reducing oxygen availability at the water–sediment interface. In contrast, higher discharge rates in winter altered C/N ratios and microbial activity in ways that partially constrained GHG production despite increased groundwater inflow.
Our results highlight groundwater discharge as a dynamic regulator of aquatic GHG emissions rather than a simple source term. By linking seasonal hydrological exchange to biogeochemical responses, this study provides process-based constraints on groundwater-driven GHG emissions from reservoirs and underscores the importance of incorporating groundwater–surface water interactions into regional and global assessments of inland-water GHG budgets.