Traffic exposure increases natural 15N and heavy metal concentrations in mosses

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO(x) emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope 15N/14N content (δ15N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ15N. Plants collected near motorways or busy urban roads had δ15N values ranging between +6 and -1‰, while in rural areas with hardly any traffic these ranged from -2 to -12‰. In a separate survey of mosses, the average δ15N of shoots from busy roadsides in London was + 3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was -7.8‰. This indicates that the two main atmospheric N sources, NO(x) and NH(x), have different δ15N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ15N as an aid to distinguish between urban (NO(x)) and rural (NH(x)) forms of N pollution