Cities occupy just 2% of terrestrial land cover, but over 50% of the world’s population currently lives in urban areas. Ecosystems downwind of cities are exposed to numerous compounds in the urban atmosphere that can alter ecological properties and processes, including species composition, plant productivity, and biogeochemical cycling. We explored the effects of the urban atmosphere on ecosystem processes in native desert ecosystems within and surrounding Phoenix, AZ, the sixth largest and one of the fastest growing cities in the United States.  We conducted our study in 15 sites within the greater Central Arizona-Phoenix Long Term Ecological Research (CAP LTER) ecosystem, including 5 sites each upwind, within the core, and downwind of the metropolitan area.  We additionally tested the impacts of nutrient enrichment on ecosystem processes through a fertilization study composed of plots amended with nitrogen, phosphorus, and both in combination.  Over multiple years, we measured soil properties and nutrient cycling, seasonal stem elongation of the dominant Sonoran Desert shrub (Larrea tridentata), as well as species composition and biomass of herbaceous annual vegetation.  
Results/Conclusions

Growth of L. tridentata occurred primarily in the spring-summer months and was faster in the urban core compared to native desert sites outside of the city.  Across all sites, warm-season stem elongation was significantly correlated with seasonal rainfall and rate of water infiltration in soils. Contrary to expectations, L. tridentata did not respond to experimental nutrient enrichment, even in wet years.  Annual herbaceous plant communities were less diverse in remnant desert sites within the Phoenix metropolitan area compared to outlying desert sites. Fertilization with nitrogen also decreased diversity and increased spring annual production, particularly in sites downwind of the urban area that received substantial winter rainfall.  Together, these data suggest that the urban atmosphere may be altering fast-cycling components of the Sonoran Desert, including production and composition of annual plant communities and soil nutrient cycling associated with pulsed precipitation events.  However, our results suggest that perennial shrub communities may respond more slowly to nutrient enrichment associated with urbanization.