Traffic-related dustfall and NOx, but not NH3, seriously affect nitrogen isotopic compositions in soil and plant tissues near the roadside

Abstract

Ammonia (NH3) emissions from traffic have received particular attention in recent years because of their important contributions to the growth of secondary aerosols and the negative effects on urban air quality. However, few studies have been performed on the impacts of traffic NH3 emissions on adjacent soil and plants. Moreover, doubt remains over whether dry nitrogen (N) deposition still contributes a minor proportion of plant N nutrition compared with wet N deposition in urban road environments. This study investigated the δ15N values of road dustfall, soil, moss, camphor leaf and camphor bark samples collected along a distance gradient from the road, suggesting that samples collected near the road have significantly more positive δ15N values than those of remote sites. According to the SIAR model (Stable Isotope Analysis in R) applied to dustfall and moss samples from the roadside, it was found that NH3 from traffic exhaust (8.8 ± 7.1%) contributed much less than traffic-derived NO2 (52.2 ± 10.0%) and soil N (39.0 ± 13.8%) to dustfall bulk N; additionally, 68.6% and 31.4% of N in mosses near the roadside could be explained by dry N deposition (only 20.4 ± 12.5% for traffic-derived NH3) and wet N deposition, respectively. A two-member mixing model was used to analyse the δ15N in continuously collected mature camphor leaf and camphor bark samples, which revealed a similarity of the δ15N values of plant-available deposited N to 15N-enriched traffic-derived NOx-N. We concluded that a relatively high proportion of N inputs in urban road environments was contributed by traffic-related dustfall and NOx rather than NH3. These information provide useful insights into reducing the impacts of traffic exhaust on adjacent ecosystems and can assist policy makers in determining the reconstruction of a monitoring network for N deposition that reaches the road level.