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.
Phosphorus (P) is one of main pollution elements of eutrophication. P emissions from different pathways and sources are a key issue in the protection of water quality and sustainable watershed management practices. We have estimated net anthropogenic P accumulation (NAPA), as an index of P pollution potential in the Beijing metropolitan region, China. The NAPA estimation is based on an inventory of P fertilizer use, consumption of human food and animal feed, non-food P, and riverine P net flux. The overall average NAPA for 1991, 1997, 2003, and 2007 are 777, 943, 1218, and 1084 kg P km -2 y -1 , about two times that reported in developed countries. The Urban unit has the largest NAPA (5526 kg P km -2 y -1 ), whereas Mentougou P was negative, outputting 34 kg P km -2 y -1 . P input of fertilizer is the largest source of NAPA, accounting for 40.7% (455 kg P km -2 y -1 ) of the total P input, followed by non-food P and P in human food and animal feed. NAPA is closely related to land use, on average 5433 kg P km -2 y -1 in densely populated developed land, 503 kg P km -2 y -1 in agricultural land and 84 kg P km -2 y -1 in forest land. Human population density is the best single predictor of NAPA. Our results provide a basis for understanding the potential impact of anthropogenic P inputs on environmental problems, such as nation-wide water quality degradation under the current rapid urban expansion in modern China.
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