Ecosystem carbon cycling is sensitive to elevated atmospheric CO₂ concentration ([CO₂]) and climate warming, but the combined effects of these global changes on ecosystem carbon uptake and loss is uncertain.  Northern mixed-grass prairie is expected to be among the most responsive ecosystems to the effects of elevated [CO₂] because it is co-dominated by C₃ and C₄ plants and highly sensitive to changes in soil water content.  Effects of elevated [CO₂] may be enhanced or diminished when combined with rising temperatures, and responses are predicted to vary seasonally due to plant phenology and the distribution of rainfall events. We measured net ecosystem exchange (NEE), ecosystem respiration (Re), and evapotranspiration (ET) during the 2007-2008 growing seasons at the Prairie Heating and CO₂ Enrichment (PHACE) experiment established near Cheyenne, WY USA. This factorial experiment combined FACE (ambient and elevated [600 ppm] CO₂ concentration), experimental warming (1.5°C daytime, 3°C nighttime) and irrigation to evaluate effects of multiple global change drivers on semi-arid grassland. Our objective was to characterize ecosystem carbon and water flux dynamics in response to the independent and interactive effects of elevated [CO₂] and warming. Frequent diurnal measurements allowed us to estimate daily rates and to quantify seasonal gross and net fluxes.

Results/Conclusions

Elevated [CO₂] stimulated GPP and Re, but the effects of elevated [CO₂] on the balance of these fluxes (NEE) varied seasonally. The combination of warming and elevated [CO₂] reduced Re and GPP (on average) in a non-linear fashion. The strongest effect of elevated [CO₂] on GPP was observed in late May and at the end of the growing season in August, when GPP was highest under elevated [CO2] (-5.3 ±0.6 g C m^-2 day^-1) and strongly reduced under all other treatments (-2.0 ±0.5 g C m^-2 day^-1).  At the end of the growing season, Re remained highest in elevated [CO₂] + warming plots (2.7 ±0.1 g C m^-2 day^-1) compared to all other treatment combinations (1.8 ± 0.5 g C m^-2 day^-1). Elevated [CO₂] plots were a net carbon sink (-54 g C m^-2) and elevated [CO₂] + warming plots were a net carbon source (62 g C m^-2) over the entire growing season as a result of these late season differences in Re and GPP. These results highlight the strong interactive effects of elevated [CO₂] and warming in a semi-arid ecosystem and have important implications for predicting carbon uptake and loss under future global change scenarios.