Wednesday, 8 October 2008: 10:45 AM
George R. Brown Convention Center, 370C
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.