Isotopic investigation of nitrogen deposition along a highway road gradient

Background/Question/Methods   Though it is understood that mobile NOx emissions on highways create “hotspots” of air pollution, there is limited understanding of the effects of these emissions on the surrounding environment. EPA legislation now requires monitoring of NO2 near major roadways because of human health effects. However, little is known about how excess N deposition can affect roadside ecology. For example, it may have adverse affects on near-road plant communities. In addition, storm water infrastructure is often located near roads and can channel N deposition to surface waters. Because vehicle emissions constitute about half of Eastern U.S. NOx emissions, it is important to assess the impact of automobile emissions on near-road ecosystems. This study uses stable isotopes of nitrogen in plant tissue and dry nitrogen deposition to examine the extent and sources of N deposition along a gradient perpendicular to a major highway, I-76, in Rector, Pennsylvania. Passive samplers were deployed for five months at six sites along the gradient to collect dry deposition of NH3, HNO3 and NO2 and then analyzed for concentration and isotopic composition (d15N). Additionally, we used two types of grasses as biomonitors to examine the relative sources of nitrogen to plant tissue. These were sampled once a month for five months for isotopic analysis (d15N and d13C).

Results/Conclusions   Fluxes of nitrogen deposition were two to four times higher near the roadway than at the control site in each month. Excess deposition was also reflected in plant tissue, where lower C: N ratios near the roadway indicated additional nitrogen assimilation by plants. Furthermore, carbon and nitrogen isotopic signatures of plant tissue suggest that plants were taking up nitrogen and carbon from an automobile source near the highway and from biogenic sources far from the road. Additional work includes measuring the isotopic composition of gaseous reactive nitrogen from the passive samplers to determine the fate of automobile exhaust across the gradient.