ISOTOPIC INVESTIGATION OF DRY NITROGEN DEPOSITION ALONG TWO URBAN TO RURAL GRADIENTS

Fossil fuel combustion from point and mobile sources in urban areas is a key contributor to dry nitrogen deposition. While wet deposition makes up the majority of total N deposition in the eastern U.S., dry deposition accounts for 20-50%, and thus can be a significant source of N to ecosystems. However, atmospheric N deposition in urban areas is poorly characterized because N monitoring is primarily conducted at remote locations. For example, national monitoring networks, such as the Clean Air Status and Trends Network (CASTNET), intentionally locate sites in rural areas to capture regional background levels of deposition. This may potentially underestimate the amount of atmospheric N reaching the land surface, as it does not take into account hotspots of N deposition near area sources of pollution, such as roadways and urban areas. Furthermore, vegetation in urban environments represents an important sink for anthropogenic inputs of N, and understanding the nutrient dynamics of vegetation in urban ecosystems is critical to understanding N budgets. However, little is known about how urban ecosystems compensate for higher local N loading.

This study characterizes the amount and sources of dry N deposition along two urban to rural gradients. Passive samplers were deployed at urban, suburban and rural sites in Baltimore, MD and Pittsburgh, PA for five months to collect dry N deposition for analysis of both concentration and stable isotopes of nitrogen (δ¹⁵N). This method provides an easy and inexpensive approach for monitoring N deposition across large spatial gradients for extended time periods. This study also uses stable isotope geochemistry to trace atmospheric nutrients to urban vegetation. We deployed grasses as biomonitors along the urban to rural gradients to assess the influence of excess dry N deposition on isotopic composition and N content of plant tissue.