Short-term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

Projections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free-Air CO2 Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO2 concentrations (eCO(2)) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO2 at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150ppm CO2 above ambient of similar to 400ppm), with each step lasting approximately 5weeks. This provided a unique opportunity to study the short-term (weeks to months) response of C cycle flux components to eCO(2) across a range of CO2 concentrations in an intact ecosystem. Soil CO2 efflux (i.e., soil respiration or R-soil) increased in response to initial enrichment (e.g., +30 and +60ppm CO2) but did not continue to increase as the CO2 enrichment was stepped up to higher concentrations. Light-saturated photosynthesis of canopy leaves (A(sat)) also showed similar stimulation by elevated CO2 at +60ppm as at +150ppm CO2. The lack of significant effects of eCO(2) on soil moisture, microbial biomass, or activity suggests that the increase in R-soil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in R-soil suggests that under eCO(2,) additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO(2) are a priority for future research.