Spatial and temporal variation of atmospheric nitrogen deposition in the North China Plain

A monitoring network of nine sites was established to determine the spatial and temporal variation of atmospheric nitrogen (N) deposition in the North China Plain (NCP) over a two-year period. The annual bulk deposition of inorganic N in the North China Plain ranged from 18.4 to 38.5 kg/hm2 and averaged 28.0 kg/hm2. The concentration of NH4+-N and NO3−-N in rainwater averaged 3.76 and 1.85 mg/L, respectively, which were significantly higher than the values at background sites in China (normally less than 0.5 mg/L). Annual bulk deposition of inorganic N in the Beijing area (32.5 kg/hm2) was higher than that in Shandong and Hebei provinces (21.2 kg/hm2 on an average). Also bulk N deposition was much greater in Dongbeiwang and Fangshan than in Yanqing and Shunyi counties. Significant spatial variation of bulk deposition was observed in the Beijing area because of variation of precipitation, and 60% of bulk deposition occurred from June to September. Bulk deposition of NH4+-N was 2.0 times that of NO3−-N deposition at the rural monitoring sites. However, the situation was reversed at the Beijing Academy of Agricultural-Forestry Sciences (BAAFS), the unique urban monitoring site. The results suggest that reduced N in precipitation is dominant in rural regions, but oxidized N is the major form in urban regions. The positive relationship between inorganic N deposition and precipitation can be fitted by a power equation (r2= 0.67), showing an increase of NH4+-N and NO3−-N inputs with increased precipitation. Wet deposition of N accounted for 73% of the bulk deposition, implying that dry deposition of N, particularly NH4+-N from dust, is important in the North China Plain.