Seasonal thermal stratification synergizes with allochthonous carbon inputs to amplify priming effects across the sediment profile in reservoir.

Methane (CH) dynamics are significantly influenced by thermal stratification in freshwater reservoirs, yet the mechanistic links between stratification-induced redox shifts and sediment-hosted CH cycling remain poorly understood. This study integrates sediment column experiments with molecular analyses to unravel the direct and indirect effects of thermal stratification on CH production and oxidation in Hongfeng Lake, a subtropical reservoir. The results reveal that thermal stratification leads to the formation of unique redox gradients, resulting in two peaks of CH concentration peaks in surface layer (-6 cm) and deeper sediments (19-22 cm), and the fluxes were observed in summer to be 9 times higher than those observed in winter. Isotopic tracing revealed acetate-driven methane increases across the sediment profile, highlighting the impact of algal-derived carbon on promoting methanogenesis. Methane oxidation in surface sediments was promoted by aerobic conditions following the dissipation of thermal stratification. Microbial community analysis showed that methanogenic archaea, particularly Methanocellales and Methanosarciniales, dominated under both conditions, with higher abundances in the presence of labile carbon substrates. The input of organic matter stimulates the decomposition of carbon and the release of methane within the sediment column, posing the risk of a short-term high-concentration methane release. These findings highlight the importance of thermal stratification, algal-derived carbon, and oxygen availability in regulating CH dynamics, providing new insights into the spatial and temporal variability of methane emissions in freshwater reservoirs. It emphasizes the need to quantify the risk of methane release caused by reservoir turbulence in the future and highlights the management through controlling nutrient input and artificially regulating reservoirs.