Housing Age and Affluence Influence Plant and Soil Nitrogen and Carbon Cycles in Two Semiarid Cities

While human activities have altered the urban nitrogen (N) and carbon (C) cycles, the relationships between social and biophysical processes in cities are not well understood. Here we evaluated relationships between sociodemographic variables (median household income and housing age) and N and C contents and stable isotope ratios of vegetation in the Los Angeles Metropolitan Area (LA), California, and the Salt Lake Valley, Utah. We hypothesized that (1) N content and stable isotope ratios would be negatively related to income via increased N deposition in lower-income areas; (2) N and C content and N stable isotope ratios would have a positive relationship with age due to soil organic matter accumulation and increased exposure to N losses over time, respectively; and (3) C-13 would increase with income as a result of increased fossil fuel-derived CO2 concentrations in lower-income areas. We found that N-15 values decreased with median household income in both cities and N content decreased with income in LA. In addition, atmospheric NO2 concentrations decreased with income in LA. Soil N and C content increased with housing age. However, N-15 had opposing relationships with age in the two cities. Finally, foliar C-13 values were more isotopically depleted with both increasing household income and increasing NO2 concentrations in LA. These results show that urban foliar and soil isotopic composition are associated with sociodemographic variables and that affluence, as well as housing age, influences urban plant and soil function.