Urban leaf nitrogen isotopes are associated with neighborhood age and income in the Los Angeles metropolitan area

Background/Question/Methods

Human activity in cities can greatly alter nitrogen cycling throughout the environment. Many studies have used the nitrogen isotope composition (δ¹⁵N) of leaves as a tracer to better understand nitrogen cycling. However, in cities the nitrogen cycle is very complex and involves interactions between social and biophysical processes. The potential for utilizing natural abundance stable isotopes to evaluate these interactions has not been fully explored. In this project, we evaluated relationships between urban sociodemographic patterns and variations in urban canopy δ¹⁵N. We collected tree leaves of about 70 species in 37 residential neighborhoods in Los Angeles, Orange and Riverside counties, and analyzed δ¹⁵N and nitrogen content. Each neighborhood varied in sociodemographic variables, particularly median household income and average age of built structures, which was used as a proxy for neighborhood age.  We hoped to evaluate the potential for studying the influence of sociodemographic variables on urban nitrogen cycling using δ¹⁵N.

Results/Conclusions

We found a negative relationship between δ¹⁵N and income and a positive relationship between δ¹⁵N and age (r= -0.394, p<0.001, r= 0.365, p<0.001 respectively) for all species combined. Stronger correlations between δ¹⁵N and neighborhood age and income were found at the genus and species level. δ¹⁵N increased with age in both Schinus (r= 0.658, p= 0.039) and Pinus (r= 0.804, p= 0.005) genera; however, there was no relationship between income and δ¹⁵N. Income showed a negative relationship with δ¹⁵N in the species Platanus acerifolia (r= -0.907, p< 0.001). However, there was no relationship between age and δ¹⁵N for this species. We propose two likely explanations for this relationship: 1) variations in atmospheric N pollution with neighborhood income and age, and 2) effects of neighborhood income and age on soil N cycling via differences in soil age and management.  Both of these mechanisms are consistent with our results in that older neighborhoods showed more enriched δ¹⁵N, consistent with increased N losses, and lower incomes were also associated with enriched δ¹⁵N, consistent with higher atmospheric pollution. Spatial variability in urban nitrogen isotopes may highlight possible human-N cycle interactions in cities that can be further explored with detailed, neighborhood and household scale studies.