Impacts Of Chronic Low Level Nitrogen Deposition Along A Roadside Deposition Gradient On Forest And Estuarine N Loading

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Despite large reductions in the emission rates of individual vehicles, mobile source NOx emissions, which have increased by 33% since the Clean Air Act amendments of 1970, are the single largest source of nitrogen (N) emissions in the US. It is likely that a portion of these emissions are deposited adjacent to roads and thus not measured by traditional monitoring networks, which were designed to measure long-term and regional trends in precipitation chemistry and dry deposition. With this research, I address whether there is increased deposition near roadways. If so, is it due to near-source deposition of mobile source emissions or is it due to an edge effect? How much additional N is deposited, and how long is the gradient away from the roadway? What is the impact of this chronic N deposition on forest ecosystem processes such as decomposition and N retention, and what is the potential flux to downstream aquatic ecosystems? Near-source deposition of mobile source emissions results in the formation of deposition gradients away from roadways. These gradients are 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) is 1.5-2 x that of areas farther away (150 meters) from the road. This increased deposition was not due to an edge effect and is due to near-source deposition of mobile source emissions. 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 with sites farther away.

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