The urban nitrogen (N) cycle is greatly impacted by human behavior, with a direct driver of N availability being land manager fertilization practices. Another source of N in cities is atmospheric deposition, which can also be impacted by aspects of human behavior. More research is needed to better understand how these different sources of N impact urban vegetation. In this study, we assessed the influence of traffic density (a proxy for atmospheric deposition via vehicle emissions) and fertilization on foliar N and stable isotope composition (δ15N). We hypothesized that atmospheric pollution would be a major driver of the leaf chemistry of unfertilized trees and that the chemistry of fertilized trees may be driven by median household income and time since development. We assessed unfertilized street trees along gradients of traffic density that were managed by the Salt Lake City Urban Forestry Department and some homeowners. We also studied fertilized residential trees that were managed by homeowners. Street tree leaf samples (n=59) were collected from 3 species: Acer platanoides, Koelreuteria paniculata and Gleditsia triacanthos. Residential tree leaf samples (n=78) were also collected from 3 species, Acer platanoides, Populus tremuloides, and Pyrus calleryana. We used Student’s t-test, Welch’s t-test and two way ANOVAs to analyze our comparisons.
Results/Conclusions
G. triacanthos leaves on high traffic density roads had more enriched δ15N than leaves on lower traffic density roads (W= 76, p= 0.01). We did not find relationships between δ15N and traffic density for A. platanoides, K. paniculata (p=0.32, p=0.74 respectively). When comparing unfertilized A. platanoides, to its fertilized residential counterpart, foliar % N of fertilized trees were higher (t= 3.15, p=0.003). We did not detect a difference in δ15N between fertilized and unfertilized A. platanoides (p=0.65). Finally, there was no relationship between income and age for fertilized residential trees in regards to δ15N or %N (p=0.22 and p=0.25 respectively).