Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 3 2003 10.5194/acpd-3-2691-2003 http://www.atmos-chem-phys-discuss.net/3/2691/2003/ http://www.atmos-chem-phys-discuss.net/3/2691/2003/acpd-3-2691-2003.html http://www.atmos-chem-phys-discuss.net/3/2691/2003/acpd-3-2691-2003.pdf 2691 2706 2003-05-20 Soil-atmosphere exchange of CH₄, CO₂, NO_x, and N₂O in the Colorado Shortgrass Steppe following five years of elevated CO₂ and N fertilization A. R. Mosier P. Pendall J. A. Morgan USDA/ARS, Fort Collins, CO, USA Department of Botany, University of Wyoming, Laramie, WY, USA An open-top-chamber (OTC) CO₂ enrichment study was conducted in the Colorado shortgrass steppe to determine the effect of elevated CO₂ (~720 mmol 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. Weekly measurements of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ 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 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 55 month period of observation. During early summer of 2002, following the removal of the open-top-chambers from the CO₂ 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 CO₂ 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^−2 of ammonium nitrate-N was applied to the soil surface. Fluxes of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ OTCs soils at one to three day intervals for the next month. With water addition alone, CO₂ and NO emission did not differ between ambient and elevated CO₂ soils, while CH₄ uptake rates were higher and N₂O fluxes lower in elevated CO₂ soils. Adding water and mineral N resulted in increased CO₂ emissions, increased CH₄ uptake and decreased NO emissions in elevated CO₂ soils. The N addition study confirmed previous observations that soil respiration is enhanced under elevated CO₂ and N immobilization is increased, thereby decreasing NO emission.