Multiple Isotopes Reveal a Hydrology Dominated Control on the Nitrogen Cycling in the Nujiang River Basin, the Last Undammed Large River Basin on the Tibetan Plateau

The Tibetan Plateau is sensitive to climate change, but the feedbacks of nitrogen (N) cycling to climate conditions on this plateau are not well-understood, especially under varying degrees of anthropogenic disturbances. The Nujiang River Basin, the last undammed large river basin on the Tibetan Plateau, provides an opportunity to reveal the feedbacks at a broad river basin scale. The isotopic compositions revealed that the conservative mixing of multiple sources controlled the nitrate (NO3–) loadings during the low-flow season, while biological removal processes (assimilation and denitrification) occurred in the high-flow season. During the high-flow season, soil sources, sewage, and atmospheric precipitation contributed 76.3%, 15.6%, and 8.1% to the riverine NO3–. In the low-flow season, the contribution of soil sources decreased while that of sewage increased. The relationship between d-excess and δ15N-NO3– suggests that the hydrological conditions largely controlled the N cycling dynamics in the basin, causing the high spatiotemporal heterogeneity of the riverine NO3– sources and transformation mechanisms. During the high-flow season, the precipitation and evaporation patterns controlled the in-soil processes and soil leaching. In contrast, in-stream nitrification became more evident during the low-flow season, which was related to the long water residence time. This study illustrates hydrology dominated control on N cycling over a large basin scale, which has implications for understanding the N cycling dynamics in the Tibetan Plateau.