Unveiling riverine NO dynamics along urbanization gradients by integrating hydrological, biogeochemical and microbial processes.

Human-disturbed rivers are globally significant sources of atmospheric nitrous oxide (NO). Yet, the underlying mechanisms of urbanization impact on riverine NO dynamics are not well understood. This study unveiled the effects of urbanization on NO dynamics by integrating hydrological, biogeochemical and microbial processes in a river with various urbanization intensities. Riverine NO concentration enhanced with increasing urban land percentage, primarily because of the increased proportional contribution of sewage & manure source. The N site preference and relevant isotopic evidences revealed that the proportion of denitrification derived NO increased from 60 % to 76 %, with the urban land percentage increasing from 〈 5 % to 〉 22 %, which was caused by decreases in flow velocity and dissolved oxygen saturation, increases in NO concentration and NO-denitrifying genes. The non-negligible contribution of nitrification to NO production (∼ 40 %) in lower-urbanized river stretches may be attributed to aerobic conditions and lower impermeable riparian zone facilitating the occurrence of in-river nitrification and the access of in-soil nitrification to river. Urbanization-mediated decreases in flow velocity and dissolved oxygen and increases in nitrogen availability and denitrification process resulted in an increase in NO concentration and flux, with NO concentration approximately four times higher in higher-urbanized river reaches (50.7 ± 26.3 nmol/L) than in lower-urbanized river reaches (14.4 ± 2.5 nmol/L). In addition, increased proportional contribution of sewage & manure source also provides the possibility for exogenous NO inputs with urban expansion. These findings contribute to deepening our understanding of how urbanization drives NO dynamics in river systems.