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Urban ecosystems are expanding globally, and assessing the ecological consequences of urbanization is critical to understanding the biology of local and global change related to land use. We measured carbon (C) fluxes, nitrogen (N) cycling, and soil microbial community structure in a replicated (n=3) field experiment comparing urban lawns to corn, wheat-fallow, and unmanaged shortgrass steppe ecosystems in northern Colorado. The urban and corn sites were irrigated and fertilized. Wheat and shortgrass steppe sites were not fertilized or irrigated. Aboveground net primary productivity (ANPP) in urban ecosystems (383 +/- 11 C m(-2) yr(-1)) was four to five times greater than wheat or shortgrass steppe but significantly less than corn (537 +/- 44 C m(-2) yr(-1)). Soil respiration (2777 +/- 273 g C m(-2) yr(-1)) and total belowground C allocation (2602 +/- 269 g C m(-2) yr(-1)) in urban ecosystems were both 2.5 to five times greater than any other land-use type. We estimate that for a large (1578 km(2)) portion of Larimer County, Colorado, urban lawns occupying 6.4% of the land area account for up to 30% of regional ANPP and 24% of regional soil respiration from land-use types that we sampled. The rate of N cycling from urban lawn mower clippings to the soil surface was comparable with the rate of N export in harvested corn (both similar to 12-15 g N m(-2) yr(-1)). A one-time measurement of microbial community structure via phospholipid fatty acid analysis suggested that land-use type had a large impact on microbial biomass and a small impact on the relative abundance of broad taxonomic groups of microorganisms. Our data are consistent with several other studies suggesting that urbanization of arid and semiarid ecosystems leads to enhanced C cycling rates that alter regional C budgets.