The nitrogen cycle has been radically changed by human activities1. China consumes nearly one third of the world’s nitrogen fertilizers. The excessive application of fertilizers2,3 and increased nitrogen discharge from livestock, domestic and industrial sources have resulted in pervasive water pollution. Quantifying a nitrogen ‘boundary’4 in heterogeneous environments is important for the effective management of local water quality. Here we use a combination of water-quality observations and simulated nitrogen discharge from agricultural and other sources to estimate spatial patterns of nitrogen discharge into water bodies across China from 1955 to 2014. We find that the critical surface-water quality standard (1.0 milligrams of nitrogen per litre) was being exceeded in most provinces by the mid-1980s, and that current rates of anthropogenic nitrogen discharge (14.5 ± 3.1 megatonnes of nitrogen per year) to fresh water are about 2.7 times the estimated ‘safe’ nitrogen discharge threshold (5.2 ± 0.7 megatonnes of nitrogen per year). Current efforts to reduce pollution through wastewater treatment and by improving cropland nitrogen management can partially remedy this situation. Domestic wastewater treatment has helped to reduce net discharge by 0.7 ± 0.1 megatonnes in 2014, but at high monetary and energy costs. Improved cropland nitrogen management could remove another 2.3 ± 0.3 megatonnes of nitrogen per year—about 25 per cent of the excess discharge to fresh water. Successfully restoring a clean water environment in China will further require transformational changes to boost the national nutrient recycling rate from its current average of 36 per cent to about 87 per cent, which is a level typical of traditional Chinese agriculture. Although ambitious, such a high level of nitrogen recycling is technologically achievable at an estimated capital cost of approximately 100 billion US dollars and operating costs of 18–29 billion US dollars per year, and could provide co-benefits such as recycled wastewater for crop irrigation and improved environmental quality and ecosystem services.
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Author contributions CQ.Y. led the research and drafted the manuscript. X.H., H.C. and CQ.Y. performed modelling. CQ.Y., SQ.N., GR.H., SC.Q., YC.X., J.Z. and Z.F. collected and processed the data. H.C.J.G., J.S.W., J.H., P.G., XT.J., P.C., N.C.S., D.O.H., ZL.S., L.Y., WJ.C., HH.F., XM.H., C.Z., HB.L. and J.T. were involved with improving the research design and the manuscript.
Competing interests: The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Part of this work was supported by the Chinese National Basic Research Program (2017YFA0603602 and 2014CB953803). H.C.J.G. and J.H. acknowledge support from the Wellcome Trust, Our Planet Our Health initiative (Livestock, Environment and People project 205212/Z/16/Z). We acknowledge support from the supercomputer team of Sunway TaihuLight.
Nature thanks David Kanter, Xin Zhang and the other anonymous reviewer(s) for their contribution to the peer review of this work.