Tissue N contents and δ 15 N 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 δ 15 N ranged from −12.50‰ to −1.39‰ and showed a clear bimodal distribution (−12‰ to −6‰ and −5‰ to −2‰), suggesting that there are two main sources for N deposition in the Guiyang area. More negative δ 15 N (mean = −8.87 ± 1.65‰) of urban mosses mainly indicated NH 3 released from excretory wastes and sewage, while the less negative δ 15 N (from −3.83 ± 0.82‰ to −2.48 ± 0.95‰) of rural mosses were mainly influenced by agricultural NH 3 . With more negative values in the urban area than in the rural area, the pattern of moss δ 15 N variation in Guiyang was found to be opposite to cities where N deposition is dominated by NO x –N. Therefore, NH x –N is the dominant N form deposited in the Guiyang area, which is supported by higher NH x –N than NO x –N in local atmospheric deposition. From the data showing that moss is responding to NH x –N/NO x –N in deposition it can be further demonstrated that the variation of moss δ 15 N from the Guiyang urban to rural area was more likely controlled by the ratio of urban-NH x /agriculture-NH x than the ratio of NH x –N/NO x –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.
