Driving forces of nitrogen cycling and the climate feedback loops in the Yarlung Tsangpo River Basin, the highest-altitude large river basin in the world

Abstract

• NO 3 − sources in the Yarlung Tsangpo River, the world highest river, were quantified for the first time. • Both climate and anthropogenic activity regulated the nitrogen cycling in the basin. • In-soil nitrification is of pivotal importance in the basin-scale nitrogen cycling. • A nitrification-driven feedback loop on warming was conceptualized. Under the impacts of climate change and the expansion of anthropogenic activities, changes in global nitrogen cycle may be most dramatic at high altitudes. However, a large basin scale understanding of nitrogen cycling is still lacking for the Tibetan Plateau. Taking advantage of the multiple-isotopic approach, we explored the nitrogen cycling processes, the driving forces, and the impacts on climate in the Yarlung Tsangpo River Basin, the highest river basin in the world. We showed that in-soil nitrification dominated the basin-scale nitrogen cycle and that nitrate (NO 3 – ) removal was insignificant. For the first time, the NO 3 – source contributions were estimated by an isotope-mixing model. In the high-flow season, animal manure (AM) and soil organic nitrogen and chemical fertilizer (SON&CF) contributed significant amounts of NO 3 – (34–36% and 30–36%, respectively), followed by domestic sewage (DS; 20–27%). In the low-flow season, the contributions from SON&CF decreased (at 27–34%) while those from DS increased (at 21–37%). The annual NO 3 – flux of the Yarlung Tsangpo River was 27.4 × 10 3 t·yr −1 , of which 32.0%, 32.6%, 26.3%, and 9.1% were from SON&CF, AM, DS, and atmospheric precipitation (AP), respectively. The significant spatiotemporal variations in the nitrate sources were regulated by both anthropogenic activities (e.g., sewage and grazing) and climate (e.g., temperature and precipitation). Based on the findings, a nitrification-driven feedback loop on climate was proposed, in which both positive and negative feedback mechanisms were hypothesized. This study adds important basin-scale understanding of the nitrogen cycling patterns and environmental effect implications on the Tibetan Plateau and other sensitive area in the context of global change.