Riverine nitrous oxide dynamics across China: Scale-dependent patterns and environmental controls

Nitrous oxide (N₂O) is a potent greenhouse gas and ozone-depleting substance, yet its riverine emissions remain difficult to assess across large and heterogeneous river networks. Here, we synthesized 2957 riverine N₂O observations across China and integrated them with site-level water-quality data and basin-scale environmental variables. The compiled observations indicate that Chinese rivers are predominantly N₂O sources, with median concentration and flux values of 17.5 nmol L^-1 and 10.2 μmol m^-2 d^-1, respectively. Dissolved N₂O concentration and flux were positively associated but not always proportional. Nitrogen and carbon availability were closely linked to site-level dissolved N₂O accumulation, highlighting the importance of substrate conditions. Environmental associations, however, differed across analytical levels; basin-scale analyses linked concentration primarily to anthropogenic pressure and nutrient conditions, whereas flux was more closely coupled with physicochemical and hydro-geomorphic characteristics, with distinct pathways operating in natural versus human-dominated basins. These findings demonstrate that concentration-based assessments alone may not fully identify riverine N₂O emission hotspots. Riverine N₂O inventories should therefore consider both dissolved accumulation and water-air exchange, while mitigation should prioritize reductions in nutrient and organic-matter inputs in human-dominated basins.