Impacts of chronic low level nitrogen deposition along a roadside deposition gradient

Background/Question/Methods   As a result of fossil fuel use and food production human beings are transforming the global nitrogen (N) cycle. In the United States although the majority of the changes to the N cycle are due to agriculture a significat amount is due to emissions from fossile fuel combustion. Mobile sources– highway vehicles (cars and trucks) and off-highway vehicles (construction equipment, planes, boats, etc are the single largest source of NOx emissions in the US and of NH3 emissions in urban areas. The objective of the study was to determine if there was increased N deposition near roads independent of edge effects. This was accomplished by measuring N deposition, forest floor C:N ratios, and N leaching along 150 meter transects away from road and non-road edges.

Results/Conclusions   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. This deposition has likely been occurring for decades causing changes in, forest floor C:N (-5.3 units), leaching (~2x), in sites near the road compared to sites farther away. When near source deposition is taken into account deposition estimates for a small watershed increase by 10.5-12.5 %.