An open-top-chamber (OTC) CO<sub>2</sub> enrichment study was conducted in the Colorado shortgrass steppe to determine the effect of elevated CO<sub>2</sub> (~720 mmol mol<sup>−1</sup>) on plant production, photosynthesis, and water use of this mixed C<sub>3</sub>/C<sub>4</sub> plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by \CO<sub>2</sub> on trace gas exchange. Weekly measurements of CO<sub>2</sub>, CH<sub>4</sub>, NO<sub>x</sub> and N<sub>2</sub>O fluxes within control (unchambered), ambient CO<sub>2</sub> and elevated CO<sub>2</sub> OTCs and soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2001. Even though both aboveground plant biomass increased under elevated CO<sub>2</sub> and soil moisture content was typically higher than under ambient CO<sub>2</sub> conditions, none of the trace gas fluxes were significantly altered by CO<sub>2</sub> enrichment over the 55 month period of observation. During early summer of 2002, following the removal of the open-top-chambers from the CO<sub>2</sub> enrichment sites in October, we conducted a short term study to determine if soil microbial processes were altered in soils that had been exposed to double ambient CO<sub>2</sub> concentrations during the growing season for the past five years. Microplots were established within each experimental site and 10 mm of water or 10 mm of water containing the equivalent of 10 g m<sup>−2</sup> of ammonium nitrate-N was applied to the soil surface. Fluxes of CO<sub>2</sub>, CH<sub>4</sub>, NO<sub>x</sub> and N<sub>2</sub>O fluxes within control (unchambered), ambient CO<sub>2</sub> and elevated CO<sub>2</sub> OTCs soils at one to three day intervals for the next month. With water addition alone, CO<sub>2</sub> and NO emission did not differ between ambient and elevated CO<sub>2</sub> soils, while CH<sub>4</sub> uptake rates were higher and N<sub>2</sub>O fluxes lower in elevated CO<sub>2</sub> soils. Adding water and mineral N resulted in increased CO<sub>2</sub> emissions, increased CH<sub>4</sub> uptake and decreased NO emissions in elevated CO<sub>2</sub> soils. The N addition study confirmed previous observations that soil respiration is enhanced under elevated CO<sub>2</sub> and N immobilization is increased, thereby decreasing NO emission.