Contrasting greenhouse gas fluxes in a city river-lake continuum: CH₄ diffusive emissions offset by N₂O sequestration

Urban aquatic systems are critical yet poorly constrained contributors to global greenhouse gas (GHG) budgets. This is particularly true for semi-natural river-lake continuum undergoing rapid urbanization. This study investigated the spatiotemporal variations and influencing factors of methane (CH₄) and nitrous oxide (N₂O) diffusive fluxes across water-atmosphere boundary in an urban river-lake system of China. Dissolved CH₄ and N₂O were collected using a headspace equilibrium method, and the patterns and governing factors of these gases were then unraveled through partial least squares path modeling. We found significant spatiotemporal heterogeneity in CH₄ and N₂O fluxes. CH₄ fluxes were 3.2-fold higher in the river Qingshangang (QSG) (32.81 ± 34.84 μmol m^-2 d^-1) than in the river Lihu (LH) (10.15 ± 8.15 μmol m^-2 d^-1), while N₂O fluxes demonstrated a clear source-sink transition, with QSG acting as a net source (3.23 ± 4.14 μmol m^-2 d^-1), and LH as a net sink (-2.66 ± 1.99 μmol m^-2 d^-1). CH₄ flux was significantly higher in the rainy season (QSG: 42.34 ± 37.61 μmol m^-2 d^-1; LH: 17.39 ± 8.48 μmol m^-2 d^-1) than in the dry season (QSG: 21.28 ± 55.93 μmol m^-2 d^-1; LH: 2.47 ± 1.39 μmol m^-2 d^-1). N₂O flux was also higher in the rainy season (4.22 ± 4.70 μmol m^-2 d^-1) than in the dry season (2.23 ± 3.57 μmol m^-2 d^-1) in QSG, in contrast, an opposite pattern occurred in LH, where rainy season flux (-4.09 ± 2.86 μmol m^-2 d^-1) was significantly more negative than dry season flux (-1.24 ± 0.52 μmol m^-2 d^-1). CH₄ flux was mainly regulated by nutrient species, whereas N₂O flux was more strongly influenced by basic water quality parameters. Aquatic vegetation significantly reduced CH₄ emissions, however, had negligible effects on N₂O flux. The study shows an overall net carbon sink for the studied urban landscape lake over a 100-year global warming potential timeframe. These findings underscore the importance of high-resolution, site-specific assessments for accurate GHG accounting and effective mitigation strategy.