Enzai Du ^{1, 2}, Yuan Jiang ^{1 *}, Jingyun Fang ², Wim de Vries ³

 

Abstract: 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.

 

Keywords: Nitrogen deposition，Ammonium:nitrate ratio，Throughfall，Enrichment ratio，Dry deposition，Urban hotspot hypothesis