Combined effects of urbanization and forest fragmentation on nitrogen cycling in New England forests

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Stephen Caron and Kyle Schultz, Biology, Boston University, Boston, MA, Pamela H. Templer, Department of Biology, Boston University, Boston, MA, Lucy R. Hutyra, Department of Earth & Environment, Boston University, Boston, MA, Jonathan Gewirtzman, School of the Environment, Yale University, New Haven, CT

Background/Question/Methods
New England is one of the most heavily forested regions in the United States, and its temperate hardwood forests provide essential ecosystem services such as carbon sequestration, air pollution mitigation, and water purification. However, this region has also experienced rapid urban expansion in recent decades, particularly in the greater Boston area. The impacts of this urbanization and resulting forest fragmentation on biogeochemical the cycles of biogeochemically important elements such as carbon and nitrogen remain underexplored. The Urban New England (UNE) project was established in 2018 to examine the combined effects of urbanization and fragmentation along an urban to rural gradient from Boston to central Massachusetts. Within each of eight study sites, plant and soil measurements were taken along a 90 meter transect from a developed forest edge (i.e., adjacent to a road or open field), to the intact forest interior. Rates of soil net nitrogen mineralization were measured at all eight sites from the forest edge to interior using the buried bags method. Two paired soil samples were taken from the organic layer during the growing seasons of 2018 and 2019 at 0, 15, 30, 60, and 90 meters from the forest edge at each site, with one sample left in the field for approximately one month, and the other returned immediately to the lab, followed by colorimetric analysis of ammonium and nitrate content in both samples. The difference in ammonium and nitrate between initial and field-incubated samples were used to calculate net ammonification, nitrification, and total mineralization. Sunlit foliage from oak (Quercus spp.) trees was also sampled from all plots at the same transect distances, and analyzed for total nitrogen and carbon content.
Results/Conclusions
Rates of net ammonification and net mineralization were significantly higher in urban than rural sites (P < 0.01), while net nitrification showed no difference between urban and rural forests. At urban sites, net ammonification and mineralization were higher at forest interiors compared to edges (P < 0.01). Net nitrification was higher at rural forest edges than interiors (P < 0.01). Mean foliar nitrogen was higher, while mean foliar C:N ratio was lower, at urban sites compared to rural sites, though neither difference was significant (P = 0.098 and P = 0.26). These findings demonstrate the impacts of urbanization and forest fragmentation on nitrogen cycling in northeastern U.S. forests, and suggest that despite widespread disturbance, urban forests are dynamic, nitrogen rich ecosystems.