Highway contributions to reactive nitrogen deposition: tracing the fate of vehicular NOx using stable isotopes and plant biomonitors

Abstract

While epidemiologists have long acknowledged that automobile emissions create corridors of increased NOx concentrations near highways, the influence of these emissions on dry nitrogen (N) deposition and effects on surrounding ecosystems are not well-characterized. This study used stable isotopes in plant tissue and dry N deposition to examine the extent of N deposition from automobile emissions along a roadside transect spanning 400 m perpendicular to a moderately trafficked highway (33,000 vehicles per day). Passive samplers were deployed monthly for four months at six stations to collect dry deposition of nitric acid (HNO3) and nitrogen dioxide (NO2), analyzed for concentration and natural abundance isotopic composition (δ15N). Agrostis perennans (bentgrass) and Panicum virgatum (switchgrass) were deployed as biomonitors to examine relative sources of N to plant tissue. Both NO2 flux and δ15N–NO2 values were significantly higher close to the road indicating a high proportion of automobile-sourced N is deposited near-road. Further, this near-road deposition occurred primarily as NO2 prior to oxidation to HNO3, as HNO3 fluxes were an order of magnitude lower than NO2 fluxes and were highest midway through the transect. Plant tissue δ15N values were higher near the road, signifying the influence of automobile emissions on plant tissue composition. Importantly, N flux near the road was four times higher than background N flux measured at the nearest regional dry deposition monitoring locations. We extrapolated these results to demonstrate that the observed spatial patterns of concentrated N deposition impact our understanding of regional N deposition to watersheds when applied to a metropolitan area.