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

Du Enzai[1,2];Jiang Yuan[1];Fang Jingyun[2];De vries Wim[3];

1. Beijing Normal Univ, Coll Resources Sci & Technol, State Key Lab Earth Surface Proc & Resource Ecol, Beijing 100875, Peoples R China.

2. Peking Univ, Minist Educ, Key Lab Earth Surface Proc, Coll Urban & Environm Sci,Dept Ecol, Beijing 100871, Peoples R China.

3. Wageningen Univ, Environm Syst Anal Grp, NL-6700 AA Wageningen, Netherlands.

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 similar to 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. (C) 2014 Elsevier Ltd. All rights reserved.

Keywords: Nitrogen deposition; Ammonium:nitrate ratio; Throughfall; Enrichment

ratio; Dry deposition; Urban hotspot hypothesis

Published in ATMOSPHERIC ENVIRONMENT, 2014 98:474-482. (DOI: 10.1016/j.atmosenv.2014.09.005 )