Soil Respiration, CH4 and N2O Fluxes in a Semi-Arid Grassland Affected by Elevated CO2 Warming, and Water Availability

As the global climate changes, information on how those changes will affect the land-atmosphere exchange of greenhouse gases is needed to understand and predict future exchanges. While considerable research has been done on the effects of single climate factors, almost no work has been done evaluating how multiple climate factors affect greenhouse gas exchanges. We studied the effects of a factorial combination of two CO₂ (present ambient [385 ppm], and elevated [600 ppm] CO₂), and two temperature (present ambient, and elevated temperature [1.5/3.0 ºC warmer day/night]) regimes on soil respiration, CH₄, and N₂O fluxes during the growing season of 2007 in a semi-arid grassland, Wyoming, USA (Prairie Heating And CO₂ Enrichment, PHACE). Because biological activity in this system is highly constrained by water availability, we also studied the effects of water additions (4 additions of 20 mm each during the growing season) in separate plots. Water addition plots helped determine the extent to which CO₂ and warming effects on trace gas fluxes were due to indirect effects on soil water. Flux rates were determined by monitoring temporal changes in gas concentrations in static chambers placed over the grassland. Elevated CO₂ significantly increased soil respiration (by 22%) and warming significantly reduced CH₄ consumption (by 23%). Water additions marginally increased soil respiration, and had no effect on CH₄ consumption. CO₂ and warming treatment effects on trace gas fluxes remained significant after correcting for their effects on soil moisture. These results indicate that mechanisms other than soil moisture were largely responsible for the elevated CO₂ and warming effects on soil respiration and CH₄ consumption. We observed no significant treatment effects on N₂O and no significant CO₂*warming interactions. Our results highlight the need for a more mechanistic understanding of greenhouse gas exchange responses to climate change.