Drivers of spatial variability in urban plant and soil isotopic composition in the Los Angeles basin

Aims We explored drivers of the spatial variability of plant carbon (C) and nitrogen (N) isotopes in the Los Angeles Basin and the implications of such plant isotopes for alterations to urban environments. Methods We made simultaneous measurements of plant and soil isotopes, air pollutant concentrations, and soil N cycling. The common winter annual Bromus (B. hordeaceus and B. madritensis) and 0–10 cm soil were sampled at 13–15 sites located near air quality monitoring stations in 2008 and 2009. Results The N isotopic composition (δ15N) of plants and soils were significantly correlated in both years. The plant-soil δ15N enrichment factor (EF), or the deviation of plant δ15N from soil δ15N, was positively correlated with nitrogen dioxide (NO2) concentrations in 2008 but not in 2009. However, in 2009 plant EF decreased as the relative percentage of net nitrification in the soil (% net nitrification) increased up to 90%. Plant C isotopic composition (δ13C) was significantly and negatively correlated with soil moisture and with concentrations of atmospheric carbon monoxide (CO) and NO2. Conclusion Urban plant isotopes may be used as indicators of altered environmental factors such as air pollution and also of soil N cycling.