Inorganic nitrogen deposition in China's forests: Status and characteristics

Nitrogen (N) deposition in China has been dramatically enhanced by anthropogenic emissions and has aroused great concerns of its impacts on forest ecosystems. This study synthesized data on ammonium (NH4+) and nitrate (NO3−) contents in bulk precipitation and throughfall from 38 forest stands in published literature to assess the status and characteristics of N deposition to typical forests in China between 1995 and 2010. Our results showed that ammonium dominated N deposition in this period, with a mean NH4+-N:NO3−-N ratio of ∼2.5 in bulk deposition and throughfall. Mean throughfall N deposition in China's forests was as high as 14.0 kg N ha−1 yr−1 for ammonium, 5.5 kg N ha−1 yr−1 for nitrate and 21.5 kg N ha−1 yr−1 for total inorganic N (TIN), respectively. Mean bulk deposition was 9.4 kg N ha−1 yr−1 for ammonium, 3.9 kg N ha−1 yr−1 for nitrate and 14.0 kg N ha−1 yr−1 for TIN, respectively. Canopy captured dry deposition, calculated as the difference between throughfall and bulk deposition, was thus approximately half of the bulk deposition. Spatial patterns of N deposition were in accordance with our urban hotspot hypothesis, showing a strong power-law reduction of ammonium with increasing distance to large cities but only slightly lower nitrate deposition. Our results suggest that high N deposition, especially of ammonium, exceeds critical N loads for large areas of China's forests.