Physicochemical and microbial regulators of contrasting diffusive greenhouse gas fluxes in Danjiangkou Reservoir

Reservoirs play a key role in the global greenhouse gas (GHG) balance through carbon and nitrogen cycling, yet the spatiotemporal variation in GHG fluxes from large drinking-water reservoirs and the underlying regulatory mechanisms remain unclear. This study investigated the seasonal dynamics of CO2, CH4, and N2O diffusive fluxes in Danjiangkou Reservoir and examined their physicochemical and microbial associations. In August, when water levels were low and water temperatures were high, the mean CO2 flux was close to zero (5.59 ± 10.74 mg·m-2·h-1), with both source and sink fluxes observed among sites; the reservoir showed net CO2 emission during the other sampling periods (40.89 ± 8.88 mg·m-2·h-1). CH4 and N2O exhibited both source and sink fluxes without consistent seasonal patterns. Mixed-effects models and random-forest analyses consistently identified total dissolved solids (TDS) as a major predictor of all three GHG fluxes, whereas pH, dissolved oxygen, and inorganic nitrogen showed gas-specific associations. Metagenomic analyses indicated a reorganization of microbial functional potential with changing water levels: under high-water conditions, the relative contributions of nitrogen-transformation markers, including nitrate reduction, NO-to-N2O conversion, and dissimilatory nitrate reduction to ammonium, were higher; under low-water conditions, carbon-fixation markers, including those associated with the Calvin cycle and the 3-hydroxypropionate cycle, accounted for larger proportions of the target functional profile. Genus-level annotations identified Polynucleobacter as a prominent CO2-associated taxon, Methylibium and Methylocystis as prominent CH4-associated taxa, and Nitrospira and Haliscomenobacter as candidate N2O-associated taxa. Together, these findings describe spatiotemporal GHG flux patterns and their physicochemical and microbial associations, providing a basis for further investigation of reservoir GHG budgets and potential mitigation strategies.