Variation and drivers of nitrogen deposition, cycling, and loss throughout an urban metropolitan area
Stephen Decina1, Pamela H. Templer2, Lucy R. Hutyra3 and Conor K. Gately4
As the world’s urban areas continue to grow in both population and extent, it is essential to understand patterns and drivers of atmospheric nitrogen (N) deposition in these areas. Urban N deposition may have substantial effects on regional N budgets, and geographic areas downwind of urban areas may be directly affected by N deposition in cities. Though urban N deposition has been shown to be elevated in numerous studies around the world, past research has typically examined only one urban sampling location either in a binary urban-rural comparison or as the anchor for an urban-rural spatial gradient. During the growing seasons of 2014 and 2015, we measured throughfall deposition, N leaching, and soil respiration at 24 sites around the greater Boston area. In addition, we established two new urban NADP National Trends Network (NTN) sites established in 2015 and 2016 in the City of Boston. We find that rates of atmospheric N deposition are on average 8.70 ± 0.68 kg N ha-1 yr-1, but vary more than threefold, from 3.84 to 13.82 kg N ha-1 yr-1 within the greater Boston area. The mean deposition rate is almost double the rate of total (wet + dry) N deposition measured at the rural NADP-CASTNET site in Abington, CT, ~90 km southwest of Boston. Ammonium (NH4+) deposition in the greater Boston area makes up about 70% of total N deposition and is highest in late spring. Rates of NH4+ and total inorganic N deposition are strongly correlated with on-road emissions of nitrogen oxides (NOx) and distance to roads, suggesting a significant source of ammonia (NH3) emissions from urban vehicles. In contrast to past studies, we do not find significant relationships between atmospheric N inputs and N losses via leaching within the city, nor do we find a relationship between atmospheric N deposition and soil carbon dioxide (CO2) respiration. The results of our work demonstrate that N deposition can be both high and variable within an urban area and highlight the need for more measurements of atmospheric N deposition across urban areas.
1Boston University, email@example.com 2Boston University, firstname.lastname@example.org 3Boston University, email@example.com 4Boston University, firstname.lastname@example.org