Impacts of Chronic Low Level Nitrogen Deposition along a Roadside Deposition Gradient

Neil D. Bettez1, Robert W. Howarth2, Roxanne Marino2 and Eric A. Davidson3

Despite large reductions in the emission rates of individual vehicles, mobile source NOx emissions have increased by 33% since the clean air act amendments of 1970, and are the single largest source of nitrogen (N) emissions in the US. A portion of these emissions are deposited adjacent to roads. In order to assess the contribution of these sources to local N budgets on Cape Cod, we use methods similar to Lovett et al (2000) in which bulk and through-fall precipitation collectors were placed at 10, 50,100, and 150 meters away from a moderately heavy traveled road of ~18,800 vehicles day-1, as well as a third transect of through-fall collectors adjacent to a road less clearing (power line right of way) to control for edge effects. Increased deposition was not due solely to an edge effect. Deposition along roadside edges was higher than along non-road edges. These gradients were short, less than 150 meters, and steep, with most of the elevated deposition occurring within the first 10-50 meters. Deposition near the road (10 meters) was 1.5-2 x that of areas farther away (150 meters) from the road. This increased deposition was likely due to near-source deposition of mobile source emissions, and correlated well with data from passive samplers. The percent of TDN in throughfall that is NH4+ decreased with distance from the road. This deposition has likely been occurring for decades causing changes in decomposition (-15%), leaching (~2x), forest floor C:N (-5.3 units), and pH (- 0.41 units) in sites near the road compared to sites farther away. When near source deposition is taken into account deposition estimates for a small watershed are 10.5-12.5 %.

1 Cary Institute of Ecosystem Studies, Millbrook, NY; 845-677-7600 x 238
2 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY; 607-255-6175
3 Woods Hole Research Center, Woods Hole, MA, 508-540-9900