Temporal-spatial dynamics of anthropogenic nitrogen inputs and hotspots in a large river basin.

Environmental pollution caused by human activities in the Yangtze River Basin (YRB), especially nitrogen pollution, has always been a hot topic. High-intensity anthropogenic nitrogen (AN) inputs have undergone some changes on account of environmental management practices in the YRB. We used the latest statistical data (2000-2017) to estimate spatiotemporal heterogeneity of AN inputs across the YRB, characterize hotspots of AN inputs, and predict the future trend, which is critical to meet nitrogen management challenges. We found agricultural sources were major contributors to nitrogen inputs (more than 70%) in the YRB. Due to the reduction in agricultural fertilizers use in China, AN inputs had gradually decreased from a peak of 19.0 Tg/yr in 2014 after a rapid growth period. Additionally, the nitrogen flux in sub-catchments and from various sources indicated an increasing distribution characteristic from the upper reaches to the lower reaches. Hotspots of AN inputs were mainly concentrated in the Sichuan Basin and the Middle-Lower Yangtze Plain (more than 50 tons/km2), however, growth rates were relatively low or even negative. STIRPAT model showed population size was the most important factor affecting AN loads. Although the growth rate would slow down in the future, AN loads would be maintained at a high level. Besides, aquaculture had become an important source of potential nitrogen growth in the whole basin, although the contribution was relatively small at present. Controlling nitrogen loads in hotspots and avoiding high inputs of new nitrogen sources should be the focus of future nitrogen environmental management.