Non-negligible N2O emission hotspots: Rivers impacted by ion-adsorption rare earth mining

Rare earth mining causes severe riverine nitrogen pollution, but its effect on nitrous oxide (N₂O) emissions and the associated nitrogen transformation processes remain unclear. Here, we characterized N₂O fluxes from China's largest ion-adsorption rare earth mining watershed and elucidated the mechanisms that drove N₂O production and consumption using advanced isotope mapping and molecular biology techniques. Compared to the undisturbed river, the mining-affected river exhibited higher N₂O fluxes (7.96 ± 10.18 mmol m^-2d^-1 vs. 2.88 ± 8.27 mmol m^-2d^-1, P = 0.002), confirming that mining-affected rivers are N₂O 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 N₂O reduction were the dominant processes controlling the N₂O dynamics. Of these, the contribution of incomplete denitrification to N₂O 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 N₂O production, suggesting their potential roles for denitrification. This is the first study to elucidate N₂O 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 N₂O production and consumption in highly anthropogenically disturbed aquatic systems, and also provides the foundation for accurate assessment of N₂O emissions from mining-affected rivers on regional and global scales.