Sediment properties control riverine methane emissions: A case study of the Liao river in northern China

River and stream sediments act as biogeochemical reactors for greenhouse gases, particularly methane. However, understanding how riverbed sediment properties influence river carbon emissions remains relatively unclear. The Liao River in northern China is a typical watershed with heterogeneous water and sediment sources, characterized by varying sediment properties. In this study, we surveyed CH₄ and CO₂ emissions from its mainstem and tributaries during flood and dry seasons. We found consistent seasonal patterns in CH₄ and CO₂ emissions, with peaks occurring during the flood season. The average CH₄ and CO₂ fluxes were 1.64 ± 1.80 mmol m^-2 d^-1 and 59.66 ± 44.60 mmol m^-2 d^-1, respectively. Notably, the percentage of sediment silt was significantly correlated with CH₄ concentration and flux (R² = 0.12-0.30, p < 0.05). Fine particles dominated the availability of sediment organic matter and redox conditions, which were related to riverine CH₄ production and emissions. Structural equation modeling revealed that both grain size and the percentage of TOC (total organic carbon) directly influenced riverine CH₄ and CO₂ emissions. The organic content and redox conditions of the riverbed sediment collectively explained 65% of riverine CH₄ emissions, while grain size composition indirectly controlled CH₄ emissions by altering sediment substrate quality and redox conditions. In contrast, river CO₂ emissions were only weakly dependent on anaerobic metabolism in riverbed sediments. These findings enhance our understanding of the sources and metabolic mechanisms of riverine CH₄ and CO₂ emissions and offer potential improvements for estimating carbon fluxes in regional or global riverine networks by considering riverbed sediment properties.