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We measured soil nitrogen (N) cycling and fluxes of N2O and NO in three land-use types across the metropolitan area of Phoenix, Arizona. Urbanization increased N2O emissions compared to native landscapes, primarily due to the expansion of fertilized and irrigated lawns. Fluxes of N2O from lawns ranged from 18 to 80 mu g N m(-2) h(-1) and were significantly larger than managed xeric landscapes (2.5-22 mu g N m(-2) h(-1)) and remnant desert sites within the urban core (3.7-14 mu g N m(-2) h(-1)). In contrast, average NO fluxes from lawns were not significantly different from native desert when dry (6-80 mu g N m(-2) h(-1) lawn; 5-16 mu g N m(-2) h(-1) desert) and were lower than fluxes from deserts after wetting events. Furthermore, urbanization has significantly altered the temporal dynamics of NO emissions by replacing pulse-driven desert ecosystems with year-round irrigated, managed lawns. Short-term, pulse-driven emissions of NO from wetting of dry desert soils may reach 35% of anthropogenic emissions within a day after summer monsoon storms. If regional O-3 production is NOx-limited during the monsoon season, fluxes from warm, recently wet arid soils may contribute to summer O-3 episodes.