Nitrogen inputs and isotopes in precipitation in the North China Plain

Nitrogen (N) deposition in the North China Plain (NCP), an intensive agricultural region undergoing rapid economic development in China, has not been well documented until now. In a 4-year study, using a deposition network (11 sites), we analyzed bulk and wet-only deposition, and the isotopic composition (δ15N) of ammonium and nitrate in precipitation in the NCP. Annual bulk deposition of inorganic N averaged 27 kg N ha−1 (ranging from 15 to 50 kg N ha−1) across sites and years, but with very large variation in both space and time. Bulk and wet deposition of inorganic N were significantly higher at sites in the Beijing area compared with that at rural sites in the NCP, implying more air N pollution around Beijing. Although there was a wide seasonal variation of δ15N for NH4-N (−12.7‰ to +12.9‰) and NO3-N (−10.0‰ to +20.7‰) in precipitation across the sites in the NCP, a significant difference in the mean isotopic composition of NH4-N and NO3-N was measured in precipitation samples from two suburban sites (Dongbeiwang, DBW; Campus experimental farm, CEF) in Beijing (+0.45±4.39‰ for NH4-N and +4.23±4.34‰ for NO3-N on average) compared to two rural sites (Quzhou, QZ; Wuqiao, WQ) in Hebei (−2.8±4.06‰ for NH4-N and −1.19±3.74‰ for NO3-N on average). Both deposition flux and isotopic composition of inorganic N in precipitation suggest that N deposition is most probably influenced by multiple pollution sources in the urban area around Beijing area, while it is mainly derived from local agricultural sources of reactive N species at rural areas of the NCP.