Net anthropogenic nitrogen accumulation in the Beijing metropolitan region

Background, aim, and scope A rapid increase in anthropogenic nitrogen inputs has a strong impact on terrestrial and aquatic ecosystems. We have estimated net anthropogenic nitrogen accumulation (NANA) as an index of nitrogen (N) pollution potential in the Beijing metropolitan region, China. Our research provides a basis for understanding the potential impact of anthropogenic N inputs on environmental problems, such as nation-wide water quality degradation under the current rapid urban expansion in modern China. Methods The NANA estimation is based on an inventory of atmospheric N deposition, N fertilizer use, consumption of human food and animal feed, N fixation, and riverine N import and export. We calculated N accumulation values for the years 1991, 1997, 2003, and 2007. Results and discussion The average NANA values for the urban and suburban areas from 1991 to 2007 were 24,038 and 13,090 kg N km⁻² year⁻¹, respectively. NANA is higher in eastern and southern areas than in northern and western areas, and higher in the urban area than in the suburban area. The overall average NANA in Beijing has a downward trend from 15,187 kg N km⁻² year⁻¹ in 1991 to 11,606 kg N km⁻² year⁻¹ in 2007, but is still two to five times as that of developed countries. N input from nitrogenous fertilizer is the largest source of NANA, accounting for 44.4% (6,764 kg N km⁻² year⁻¹) of the total N input, followed by atmospheric N deposition and N in human food and animal feed. NANA is closely related to land use, on average 23,140 kg N km⁻² year⁻¹ in densely populated developed land, 17,904 kg N km⁻² year⁻¹ in agricultural land, and 10,445 kg N km⁻² year⁻¹ in forest land. Human population density is the best single predictor of NANA.