Atmospheric transport of urban-derived NHx: Evidence from nitrogen concentration and δ15N in epilithic mosses at Guiyang, SW China

Nitrogen concentration and δ15N in 175 epilithic moss samples were investigated along four directions from urban to rural sites in Guiyang, SW China. The spatial variations of moss N concentration and δ15N revealed that atmospheric N deposition is dominated by NHx-N from two major sources (urban sewage NH3 and agricultural NH3), the deposition of urban-derived NHx followed a point source pattern characterized by an exponential decline with distance from the urban center, while the agricultural-derived NHx was shown to be a non-point source. The relationship between moss N concentration and distance (y = 1.5e−0.13x + 1.26) indicated that the maximum transporting distance of urban-derived NHx averaged 41 km from the urban center, and it could be determined from the relationship between moss δ15N and distance [y = 2.54 ln(x) − 12.227] that urban-derived NHx was proportionally lower than agricultural-derived NHx in N deposition at sites beyond 17.2 km from the urban center. Consequently, the variation of urban-derived NHx with distance from the urban center could be modeled as y = 56.272e−0.116x − 0.481 in the Guiyang area.