Ecosystem response to the urban atmosphere in the Sonoran desert

Urban areas occupy just 2% of terrestrial land cover globally, but house 50% of the world's population. Ecosystems positioned within and downwind of cities are exposed to numerous compounds derived from human activity that can alter ecological function, including plant productivity and soil biogeochemical cycling. Over the course of two years, we have measured seasonal atmospheric N deposition, soil nutrient availability, foliar tissue chemistry, and primary productivity of perennial shrubs at 15 desert sites located to the west (upwind), east (downwind), and within the urban matrix (core) of the Phoenix metropolitan area, the fifth largest and one of the fastest growing cities in the US. Our results show that N deposition is lower than expected across the gradient but elevated in the urban core, primarily beneath shrubs (11 and 7.2 kg N ha-1 y-1 under L. tridentata and in inter-plant spaces, respectively). Elevated rates of N deposition in the urban core are related to faster plant growth and potentially higher N concentrations in plant tissue relative to upwind or downwind sites. Pools of organic matter were also highest in the urban core, especially under plants, which exaggerates soil resource islands. Atmospheric N deposition in the urban center has the potential to alter ecosystem pools and processes; however, additional factors such as temperature, ozone exposure, and soil texture may also contribute to our measured patterns. Ongoing and future studies will focus on a combination of manipulative and natural experiments to explore the relative importance of these interacting variables.