Non-negligible NO emission hotspots: Rivers impacted by ion-adsorption rare earth mining.

Rare earth mining causes severe riverine nitrogen pollution, but its effect on nitrous oxide (NO) emissions and the associated nitrogen transformation processes remain unclear. Here, we characterized NO fluxes from China's largest ion-adsorption rare earth mining watershed and elucidated the mechanisms that drove NO production and consumption using advanced isotope mapping and molecular biology techniques. Compared to the undisturbed river, the mining-affected river exhibited higher NO fluxes (7.96 ± 10.18 mmol md vs. 2.88 ± 8.27 mmol md, P = 0.002), confirming that mining-affected rivers are NO emission hotspots. Flux variations scaled with high nitrogen supply (resulting from mining activities), and were mainly attributed to changes in water chemistry (i.e., pH, and metal concentrations), sediment property (i.e., particle size), and hydrogeomorphic factors (e.g., river order and slope). Coupled nitrification-denitrification and NO reduction were the dominant processes controlling the NO dynamics. Of these, the contribution of incomplete denitrification to NO production was greater than that of nitrification, especially in the heavily mining-affected reaches. Co-occurrence network analysis identified Thiomonas and Rhodanobacter as the key genus closely associated with NO production, suggesting their potential roles for denitrification. This is the first study to elucidate NO emission and influential mechanisms in mining-affected rivers using combined isotopic and molecular techniques. The discovery of this study enhances our understanding of the distinctive processes driving NO production and consumption in highly anthropogenically disturbed aquatic systems, and also provides the foundation for accurate assessment of NO emissions from mining-affected rivers on regional and global scales.