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

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