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.