Longitudinal Riverine CH ₄ Emissions and the Impacts of River Damming and Urbanization

Abstract Urbanization and river damming represent profound anthropogenic disturbances that alter hydrological processes and watershed characteristics, thereby influencing riverine methane (CH 4 ) emissions. However, the mechanisms of how these activities drive CH 4 emissions within the watershed scale remain inadequately understood, especially in densely populated regions. To address this, here we measured CH 4 emissions along a 747 km transect in a populated northern China river during late summer. Our results show that CH 4 emissions were 70%–110% higher in reservoir‐affected and lower reaches than in the upper reaches (3.30 and 2.64 vs. 1.58 mmol m −2 d −1 , p < 0.01). CH 4 emission hotspots were closely associated with fine sediments, which were most pronounced in urbanized and impounded reaches. Reduced flow velocity and increased water residence time in these reaches promoted fine sediments and nutrients accumulation, which enhanced organic substrate availability, created anoxic conditions, and reshaped methanogenic communities. Furthermore, urbanization and damming markedly altered carbon metabolic pathways, as evidenced by the increase in the CH 4 : CO 2 ratio from 0.03 in the upperstream to 0.08 in urbanized‐reaches and to 0.16 in reservoir‐affected reaches, along with a higher pmoA / mcrA gene ratio in the upper reaches. Oxygen‐depleted conditions and elevated organic inputs from sewage created a favorable environment for methanogens' growth and activity, ultimately contributing to more CH 4 emissions in urbanized reaches. These findings highlight the critical role of human activities in controlling riverine CH 4 emissions and shifting CH 4 : CO 2 ratios along anthropogenic gradients by changing sediment and hydrological regimes as well as methanogenic communities.