Quantification of groundwater-borne greenhouse gases (CH4, CO2, N2O) fluxes to an oxbow lake in a subtropical alluvial-lacustrine plain

Aquatic ecosystems are an important source of greenhouse gases (GHGs) released to the atmosphere. However, studies on GHGs fluxes from lacustrine groundwater discharge (LGD) remain limited, particularly for subtropical alluvial-lacustrine plains. This study used the radon (²²²Rn) mass balance model to quantify seasonal variations in LGD rates and fluxes of groundwater-borne GHGs (CH₄, CO₂, N₂O) to the Tian-E-Zhou oxbow lake in Jianghan Plain, central Yangtze. The results showed that the LGD rate in winter was 57.67 ± 28.37 mm/d which was higher than that in summer (24.72 ± 12.16 mm/d). The groundwater-borne fluxes of CH₄, CO₂, and N₂O into the lake in winter were 7.84 ± 6.81 mmol m⁻² d⁻¹, 1.47 ± 1.07 mmol m⁻² d⁻¹, and 3.50 ± 1.90 × 10⁻⁵ mmol m⁻² d⁻¹, respectively, whereas that in summer were 1.48 ± 2.36 mmol m⁻² d⁻¹, 0.72 ± 0.47 mmol m⁻² d⁻¹, and 1.53 ± 1.00 × 10⁻⁵ mmol m⁻² d⁻¹, respectively. High groundwater-borne fluxes of CH₄ across both winter and summer could be attributed to abundant buried organic carbon and strong groundwater reducing environment in this subtropical alluvial-lacustrine plain. Seasonally, fluctuations in water levels mainly affected LGD rates, further resulting in greater groundwater-borne GHGs fluxes in winter than in summer. This study can act as an important reference for future studies on the role of groundwater as an emission pathway for GHGs in lakes of subtropical alluvial-lacustrine plains.