Effect of water addition and nitrogen fertilization on the fluxes of CH₄, CO₂, NO_x, and N₂O following five years of elevated CO₂ in the Colorado Shortgrass Steppe

Abstract. An open-top-chamber (OTC) CO₂ enrichment (~720 mmol mol^-1) study was conducted in the Colorado shortgrass steppe from April 1997 through October 2001. Aboveground plant biomass increased under elevated CO₂ and soil moisture content was typically higher than under ambient CO₂ conditions. Fluxes of CH₄, CO₂, NO_x and N₂O, measured weekly year round were not 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 2001, 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 OTC soils were measured 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.