Methane oxidation driven by multiple electron acceptors in the water level fluctuation zone of the Three Gorges Reservoir area, China

Water level fluctuations in China's Three Gorges Reservoir (TGR) area are typical of many reservoirs and significantly impact water level fluctuation zones (WLFZ), including upstream rivers. Understanding methane oxidation in the TGR-WLFZ is crucial for evaluating the impact of large-scale reservoir construction on global climate change. In this study, we investigated methane oxidation rates in the TGR-WLFZ, focusing on periods of drying and flooding. The highest methane oxidation rates were observed during the drying period, ranging from 35.69 to 56.32 nmol/(g soil)/d, while the lowest rates were recorded during the flooding period, at 11.58 to 11.98 nmol/(g soil)/d, in lab-scale simulated columns. Using ¹³CH₄ labeling experiments, we measured CH₄ oxidation potentials for aerobic methane oxidation (AMO) using oxygen and anaerobic oxidation of methane (AOM) using nitrite, nitrate, sulfate, ferric iron, and manganese oxide as electron acceptors at varying concentrations. AMO was the dominant process across all experiments, with potentials ranging from 145.71 to 180.77 nmol ¹³CO₂/(g soil)/d. For AOM, metal-dependent oxidation, particularly with Fe (III) and Mn(IV), was predominant (12.64-17.59 and 3.91-12.69 nmol ¹³CO₂/(g soil)/d, respectively), followed by nitrite and nitrate-dependent pathways (1.49-9.10 nmol ¹³CO₂/(g soil)/d). Sulfate-dependent AOM was limited (1.33-3.27 nmol ¹³CO₂/(g soil)/d). Metagenomic analysis identified key microorganisms responsible for AMO, such as unclassified_f_Methylobacteriaeae and Methylobacterium sp., and for AOM are Ca. Methylomirabilis oxyfera, Ca. Methanoperedens nitroreducens and Ca. Methylomirabilis sp. Complete functional genes and enzymes for the methane oxidation and reverse methanogenesis pathways were obtained in each hydrological period, with the highest content during the drying period and the lowest during flooding. Our study shows that reservoirs, traditionally considered significant sources of methane, may also act as methane sinks. This finding raises new questions: How do different methane oxidation pathways respond to water level fluctuations in reservoirs, and are some pathways more resilient to changes in hydrological conditions?