Climate Warming Increases Nitrate Export in High-Elevation Rivers: Divergent Temperature and Hydrological Controls across Basins

The response of fluvial nitrate (NO3–) export to climate warming remains poorly constrained in high-elevation basins. Here, we integrate multi-decadal NO3– flux records with multi-year measurements of NO3– concentrations and dual-isotope signatures (δ15N–NO3– and δ18O–NO3–) from two contrasting rivers on the Qinghai–Tibetan Plateau, the permafrost-rich Jinsha River (JSR) and the permafrost-scarce Yalong River (YLR), to evaluate climate-driven shifts in NO3– dynamics. In the JSR, where sustained warming has driven widespread permafrost thaw, NO3– concentrations and fluxes increased by ∼190 and ∼220%, respectively, from 1993 to 2018. In contrast, the NO3– export in the YLR was primarily controlled by hydrological variability rather than temperature. In-soil nitrification is the dominant source of fluvial NO3–, with warming-enhanced nitrification emerging as the primary driver of increased NO3– flux, while runoff plays a minor role. Using the space-for-time substitution method, we infer that ongoing permafrost degradation may shift NO3– export from warming-driven to hydrology-driven controls. This transition is expected to enhance nitrogen leaching under warming and intensified precipitation, contributing to nitrogen limitation in high-elevation vegetation and weakening of CO2 uptake. These findings reveal a previously underappreciated climate-warming nitrogen–carbon feedback with implications for Earth system models in permafrost-affected regions.