Land use and urbanization indirectly control riverine CH₄ and CO₂ emissions by altering nutrient input

Urban rivers are recognized as significant sources of methane (CH₄) and carbon dioxide (CO₂) emissions. Despite this, the influence of land use and urbanization on carbon emissions across rural-urban rivers at the watershed scale has been insufficiently explored. This study utilized in-situ surveys of the Liao River in northern China to investigate the spatial and temporal variations of CH₄ and CO₂ emissions and their relationship with urbanization and its potential controlling factors. The findings revealed that CH₄ emissions peaked in fall, whereas CO₂ emissions were highest in summer. The average fluxes of CH₄ and CO₂ at the water-gas interface were 1387.22 ± 2474.98 µmol·m^-2·d^-1 and 52.78 ± 54.44 mmol·m^-2·d^-1, respectively. Water quality parameters accounted for 80.49 % of the total variation in CH₄ and CO₂ concentrations and fluxes. Structural equation modeling indicated that TN, TP, DTC, and conductivity had direct effects on riverine CH₄ and CO₂ emissions, with standardized direct effects of 0.50 and 0.49, respectively. Nutrient input emerged as the primary driver, increasing CH₄ and CO₂ concentrations and fluxes, particularly in urban-adjacent river sections likely receiving higher nutrient loads. This study underscores that land use and urbanization indirectly influence riverine CH₄ and CO₂ emissions by modifying nutrient inputs. Effective land use management and nutrient input control are recommended strategies to mitigate riverine CH₄ and CO₂ emissions.