Insights into the critical processes of methane biogeochemical cycling in inland freshwater ecosystems: a systematic review.

Methane (CH₄) is a potent greenhouse gas with a 100-year global warming potential approximately 27.9 times that of carbon dioxide, contributing to approximately 19% of historical global warming. Inland freshwater ecosystems are significant natural sources of CH₄ emissions and constitute the greatest uncertainty in the global methane budget. This review systematically synthesizes recent advances in understanding the biogeochemical cycling processes and environmental drivers of CH₄ in inland freshwater systems. Special emphasis is placed on the "methane paradox", which is the phenomenon of CH₄ supersaturation in oxic waters, and highlights oxic methane production as a nonclassical pathway for CH₄ production. Oxic methane production occurs via multiple enzymatic and nonenzymatic mechanisms under oxygen-rich conditions and is driven primarily by methyl- and hydrogen-coupled reactions. This process is synergistically regulated by nutrient status, light conditions, thermal stratification, and functional microbial communities. Furthermore, this review integrates key processes governing CH₄ dynamics from production to emission, including diffusive flux, ebullition, and plant-mediated transport, as well as consumption by both aerobic and anaerobic methane oxidation. These findings underscore how environmental factors and hydrological dynamics collectively regulate microbial metabolic activity and substrate availability, thereby determining net CH₄ emissions. Future research should prioritize multiscale observational efforts, integrate advanced technologies with mechanistic models, and quantify the contribution of oxic methane production and multifactor interactions to refine estimates of the global methane budget and support climate change mitigation strategies.

© 2026. The Author(s), under exclusive licence to Springer Nature B.V.