Long-term trends in Net Nitrogen Inputs across the Yangtze River Basin and large-scale management implications

Anthropogenic transformations to river systems are profoundly altering basin-scale nitrogen cycling globally, leading to long-term nitrogen accumulation that poses persistent and episodic threats to downstream water quality. Net nitrogen input (NNI) is a key integrative indicator that quantifies the cumulative influence of human activities on nitrogen budgets. This study constructed a comprehensive spatiotemporal NNI dataset for the Yangtze River Basin (YRB) between 1980-2020 and systematically examined its temporal dynamics, source composition, and landscape-driven controls. Results show that basin-wide NNI in the YRB followed a distinct three-stage trajectory during 1980–2020, characterized by a rapid increase, a high-level plateau, and a subsequent partial decline. Average NNI intensity typically increased along an upstream-downstream gradient, primarily governed by intense nitrogen fertilizer use and dense population pressures. Trend analyses revealed strong spatial asynchrony in NNI evolution, whereby: the downstream basin exhibited the earliest plateauing effect (c. 1994); the midstream basin experienced the longest period of sustained accumulation (1995–2010); and the upstream basin, despite possessing the lowest average NNI overall, displayed the fastest growth rate that highlights emerging nitrogen management challenges in upstream regions. Machine learning analyses demonstrated that these trends were primarily driven by agricultural land cover, whereas urban land and water bodies also exerted strong but nonlinear controls on long-term NNI evolution. This study thus provides novel and unique insights into long-term, large-scale nitrogen fluxes operating across one of the world’s mega-basins. By characterizing anthropogenic pressures governing these trends, this research could help underpin effective nutrient management efforts in the Yangtze River Basin and beyond.