[Review of CO₂ and CH₄ Emissions from Rivers]

Streams and rivers play a major biogeochemical role in the global carbon cycle and act as hot spots for carbon dioxide (CO₂) and methane (CH₄) emissions to the atmosphere, excepting their roles of transporting the water and carbon from the terrestrial environment to the ocean. While carbon gases have been of great global concern, systematic reviews are still scarce. Given recent recognition of the pervasiveness of CO₂ and CH₄ in streams and rivers, this study synthesized existing research and discoveries to identify patterns and controls for riverine CO₂ and CH₄, knowledge gaps, and research opportunities. This study presented a conceptual framework for sources and the fates of CO₂/CH₄ from streams and rivers and used this framework to understand the dynamic processes of fluvial carbon evasion and potential anthropogenic disturbances. Multiple environmental influences combined with different contributions of endogenous metabolism and terrigenous input, and the CO₂ and CH₄ in streams and rivers showed significant spatial and temporal variability on a global scale, regional scale, and watershed scale, which indicates a substantial challenge for understanding the larger-scale dynamics. For a clearer recognition of how the changing environment and human activities may modify fluvial CO₂ and CH₄ dynamics, this study constructed a system framework of controls on CO₂ and CH₄ production and persistence in streams and rivers. The framework of controls can be viewed in terms of endogenous environmental controls that influence river metabolism (organic matter, temperature, nutrients, pH, and alternative electron acceptors) and external factors, including geomorphic and hydrologic drivers and human activities (agriculture, damming, and urbanization). We point out that the carbon emissions from rivers should be integrated into the terrestrial carbon budget, and in the future, more attention should be given to research on the sources of CO₂ and CH₄ in rivers, the generation and diffusion of CO₂ and CH₄ at different interfaces, the spatiotemporal variability of riverine carbon emissions, and the response of riverine CO₂ and CH₄ dynamics to the changing environment and human activities.