Plant and Soil 240: 201–211, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 201 Soil-atmosphere exchange of CH4, CO2, NOx, and N2O in the Colorado

In late March 1997, an open-top-chamber (OTC) CO2 enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO2 (∼720 µmol mol−1) on plant production, photosynthesis, and water use of this mixed C3/C4 plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO2 on trace gas exchange. From this study, we report here our weekly measurements of CO2, CH4, NOx and N2O fluxes within control (unchambered), ambient CO2 and elevated CO2 OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C3 and C4 plant biomass increased under elevated CO2 and soil moisture content was typically higher than under ambient CO2 conditions, none of the trace gas fluxes were significantly altered by CO2 enrichment. Over the 43 month period of observation NOx and N2O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 µg N m−2 hr −1 in elevated CO2 OTCs, respectively. NOx flux was negatively correlated to plant biomass production. Methane oxidation rates averaged −31 and −34 µg C m−2 hr−1 and ecosystem respiration averaged 43 and 44 mg C m−2 hr−1 under ambient and elevated CO2, respectively, over the same time period