Effects of Urbanization and Forest Fragmentation on Atmospheric Nitrogen Inputs and Ambient Nitrogen Oxide and Ozone Concentrations in Mixed Temperate Forests

Urban ecosystems around the globe experience greater atmospheric nitrogen (N) deposition compared to rural areas and are particularly vulnerable to fragmentation due to land use change. However, while the influences of urbanization and forest fragmentation on atmospheric inputs to temperate forests have been determined separately, the combined effects of the two changes on temperate forest ecosystems have yet to be assessed. To investigate these combined effects, we deployed throughfall collectors to measure atmospheric N inputs and passive samplers to measure nitrogen oxides (NO_x) and ozone (O₃) throughout the 2018 and 2019 growing seasons in seven temperate forest sites along an urbanization gradient from Boston to central Massachusetts. We found a positive relationship between the amount of impervious surface area (% ISA) surrounding each site and throughfall nitrate ((Formula presented.)) inputs at the forest edge, with urban edge (Formula presented.) inputs nearly double the rate at rural edge sites. There were higher rates of (Formula presented.) inputs in the rural forest interior than edge sites, while urban sites experienced significantly higher concentrations of NO_x and O₃ both in the interior and at the edge compared to rural sites. Atmospheric N inputs were significantly elevated in the early (May–July) compared to the late (August–November) growing season and concentrations of NO_x and O₃ were also elevated in the midgrowing season (June–September). Our results demonstrate that together, urbanization and forest fragmentation lead to greater rates of atmospheric N inputs and ambient pollutant concentrations of NO_x and O₃ in temperate forests of the northeastern U.S.