Identification of carbon dioxide and methane emission characteristic across Heihe River, China

Rivers are major sources of methane (CH₄) and carbon dioxide (CO₂) emissions into the atmosphere. However, limited research on CH₄ and CO₂ fluxes and production mechanisms in high-altitude and arid-region rivers constrains a comprehensive understanding of riverine carbon cycle. Heihe River is the second- largest inland basin in China, which originates from the Tibetan Plateau and flows through the arid regions of northwestern China. This study focused on CH₄ and CO₂ emissions along the Heihe River using the headspace technique and a gas chromatograph method. A total of 31 sampling sites were selected along the upstream and downstream stretches of the Heihe River, with 18 sites from the upstream and 13 from the downstream. The results indicate that the Heihe River is the sources of both CH₄ and CO₂ emissions, with higher concentrations and fluxes in the downstream arid region. The average diffusion fluxes of CH₄ and CO₂ in the downstream region are 80.1 μmol·m^-2·d^-1 and 63.6 mmol·m^-2·d^-1, respectively, while they are 35.9 μmol·m^-2·d^-1 and 31.1 mmol·m^-2·d^-1 in the upstream region. CH₄ and CO₂ fluxes in the upstream region were primarily controlled by the physical and chemical parameters (salinity, conductivity, air temperature, pH) and groundwater recharge (²²²Rn), while downstream emissions were influenced by DIC (Dissolved Inorganic Carbon) concentration and groundwater recharge. CH₄ primarily originates from thermogenic sources (δ¹³C-CH₄ > -50 ‰) in the upstream region, while CO₂ was primarily derived from atmospheric input and CH₄ oxidation (α_c < 1.04). In contrast, CH₄ was produced via acetoclastic methanogenesis (1.04 <α_c < 1.06) and anthropogenic activities in the downstream region, while CO₂ sources were influenced by both anthropogenic activities and CH₄ oxidation. This study has enhanced our understanding of the carbon cycle in inland rivers of arid regions through the analysis of CH₄ and CO₂ fluxes and production mechanisms along the Heihe River.