Time-series monitoring of river hydrochemistry and multiple isotope signals in the Yarlung Tsangpo River reveals a hydrological domination of fluvial nitrate fluxes in the Tibetan Plateau

• Middle reaches contributed the largest amount of NO 3 – to the Yarlung Tsangpo River (YTR) , of which 74% sourced from soil. • The increased NO 3 – leaching from soil counterbalanced the diluting effect of riverine NO 3 − in YTR in the high flow season. • The controlling mechanism for fluvial NO 3 – fluxes can be interpreted by transport- and generation-limited regimes. • Hydrological condition is the primary factor controlling the fluvial NO 3 − fluxes of the Tibetan Plateau. Nutrient element cycling in the Tibetan Plateau, the highest and largest plateau in the world, is sensitive to anthropogenic disturbances and climate change. Studying the spatiotemporal dynamics of reactive nitrogen (N) – predominantly in the form of nitrate (NO 3 – ) – in the plateau is crucial to understand the regional and global N cycles and their feedbacks with climate change. We conducted the first weekly frequency hydro-geochemical monitoring (i.e., discharge, water chemistry, and multiple isotopes) from the upper to the lower reaches of the Yarlung Tsangpo River, the largest river in the plateau, in pronounced wet/dry cycles to reveal the biogeochemical transformations and fluvial fluxes of NO 3 – response to hydrologic condition. Relative stable NO 3 – concentration and significant linear correlations between the fluvial NO 3 – fluxes and the discharge were observed, suggesting that a significant potential NO 3 – source counterbalanced the diluting effects during the rainy season. The negative correlations between δ 15 N-NO 3 − and discharge/NO 3 − fluxes suggested that the increasing NO 3 − flux respond to the increasing discharge was mainly from water leaching of 15 N-depleted soil sources, rather than 15 N-enriched sewage. The isotopic mixing model calculation showed that NO 3 – fluxes were largely generated in the relatively densely populated middle reaches (56%), of which 74% were from soil sources. The fluxes of the soil sources showed large seasonal variation and peaked in August, with hydrological condition as the primary driver. Based on the critical findings, we put forward a NO 3 − export conceptual model that integrated anthropogenic and climatic forcings and classified NO 3 – export mechanisms in river basins into transport-limited and generation-limited regimes. In a transport-limited regime that characterized most river basins in the Tibetan Plateau, fluvial NO 3 − flux presented a linearly relationship in response to runoff variation. In contrast, in a generation-limited regime, the flux would be largely dependent on the thermodynamic of nitrification.