CO2, CH4, and N2O Fluxes Are Affected by Elevated CO2, Water Availability and Species Composition in a Greenhouse Study.

Rising atmospheric CO₂ concentration alters C and N cycling in terrestrial ecosystems.  These changes in C and N cycling are to a large extent caused by CO₂ effects on water availability and plant species composition, but their separate and interactive effects have rarely been tested simultaneously.  We studied the effects of atmospheric CO₂ concentration (400 vs 750 ppm), water availability (15 vs 20% soil moisture), and species composition (perennial grasses Bouteloua gracilis, Pascopyrum smithii, and Stipa comata; sub-shrub Artemisia frigida; forb Linaria dalmatica grown in monoculture and all five species in competition) on CO₂, CH₄, and N₂O fluxes in a full factorial greenhouse experiment.  Plants were grown in PVC pots capped at the bottom.  We also included pots without plants.  On days 48, 69, and 83 after planting pots were covered with air-tight caps and changes in gas concentrations during a two-hour period were measured.  The CO₂ flux significantly increased with elevated CO₂ (by 11%), with increased water availability (17%), and differed significantly among species.  Methane consumption was significantly higher under elevated CO₂ (19%), particularly with low water availability (significant CO₂*water availability interaction).  Methane consumption did not differ among species, but CH₄ consumption was significantly higher in planted pots than in pots without plants (on average 98% higher).  The N₂O flux was not significantly affected by elevated CO₂, but was significantly higher with high water availability (67%) and differed significantly among species.  The N₂O flux was particularly high in pots grown with S. comata in combination with high water availability (significant species*water availability interaction).  Our results suggest that elevated CO₂ can alter greenhouse gas fluxes through its effect on plant growth, but that the magnitude and direction of changes in these fluxes strongly depend on how atmospheric CO₂ affects water availability and species composition.