Tissue N content and 15N natural abundance in epilithic mosses for indicating atmospheric N deposition in the Guiyang area, SW China

Tissue N contents and delta N-15 signatures in 175 epilithic mosses were investigated from urban to rural sites in Guiyang (SW China) to determine atmospheric N deposition. Moss N contents (0.85-2.97%) showed a significant decrease from the urban area (mean = 2.24 +/- 0.32%, 0-5 km) to the rural area (mean = 1.27 +/- 0.13%, 20-25 km), indicating that the level of N deposition decreased away from the urban environment, while slightly higher N contents re-occurred at sites beyond 30 km, suggesting higher N deposition in more remote rural areas. MOSS delta N-15 ranged from -12.50 parts per thousand to -1.3 parts per thousand and showed a clear bimodal distribution (-12 parts per thousand to -6 parts per thousand and -5 parts per thousand to -2 parts per thousand), suggesting that there are two main sources for N deposition in the Guiyang area. More negative delta N-15 (mean = -8.87 +/- 1.65 parts per thousand) of urban mosses mainly indicated NH3 released from excretory wastes and sewage, while the less negative delta N-15 (from -3.83 +/- 0.82 parts per thousand to -2.48 +/- 0.95 parts per thousand) of rural mosses were mainly influenced by agricultural NH3. With more negative values in the urban area than in the rural area, the pattern of moss delta N-15 variation in Guiyang was found to be opposite to cities where N deposition is dominated by NOx-N. Therefore, NHx-N is the dominant N form deposited in the Guiyang area, which is supported by higher NHx-N than NOx-N in local atmospheric deposition. From the data showing that moss is responding to NHx-N/NOx-N in deposition it can be further demonstrated that the variation of moss delta N-15 from the Guiyang urban to rural area was more likely controlled by the ratio of urban-NHx/agriculture-NHx than the ratio of NHx-N/NOx-N. The results of this study have extended knowledge of atmospheric N sources in city areas, showing that urban sewage discharge could be important in cities co-generic to Guiyang. (c) 2008 Elsevier Ltd. All rights reserved.