Hot spot of CH₄ production and diffusive flux in rivers with high urbanization

Rivers and streams play a central role in global carbon budget, but our knowledge is limited on the magnitude and extent of urbanization influence on riverine methane (CH₄) dynamics. In this study, we investigated dissolved CH₄ (dCH₄) concentration and CH₄ diffusive fluxes in 27 river segments of two 4^th-order and three 3^rd-order tributary rivers to the Yangtze River in China, which drained land areas with varied urbanization intensities. We found that urban development was the key factor responsible for high fluvial dCH₄ concentration and diffusive flux, exceeding the influence of agricultural farming, and these headwater rivers were over-saturated in CH₄ with respect to atmospheric equilibrium. dCH₄ concentration (3546 ± 6770 nmol L^-1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L^-1 and 764 ± 708 nmol L^-1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). River segments draining higher urban area also acted as important sources of CH₄ to the atmosphere (8.93 ± 14.29 mmol m^-2 d^-1). Total nitrogen (TN) concentration in river water showed the best prediction capacity when compared to other water parameters. Based on urban land use grouping, nutrient elements could predict dCH₄ well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (TN, ammonia nitrogen, and total phosphorus). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH₄ concentration. These findings highlight that combining distal geomorphic and hydrologic drivers can be effective in determining the relationship between riverine CH₄ and the proximal controls (e.g., nutrients, dissolved oxygen, dissolved organic carbon), as well as in identifying their key drivers. Being rapid urbanization a common feature of catchments worldwide, our results suggest riverine CH₄ emissions will increase into the future.