Rates of nitrogen deposition have increased in arid and semi-arid ecosystems, but few studies have examined the impacts of long-term nutrient enrichment on ecological processes in deserts. We are monitoring nitrogen deposition and conducting a multi-year, nutrient-addition study within 15 Sonoran Desert sites across the rapidly growing metropolitan area of Phoenix, Arizona (USA). We hypothesize that desert plants and soils are sensitive to the urban atmosphere and nutrient enrichment, but that these effects will vary among types of organisms that differ in terms of physiological responsiveness, proximity to surface nitrogen sources, and magnitude of carbon or water limitation.