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

Drake, John E., Macdonald, Catriona A., Tjoelker, Mark G., Crous, Kristine Y., Gimeno, Teresa E., Singh, Brajesh K., Reich, Peter B., Anderson, Ian C., Ellsworth, David S.
Global change biology 2016 v.22 no.1 pp. 380-390
Eucalyptus, biodegradation, canopy, carbon, carbon dioxide, climate, free air carbon dioxide enrichment, leaves, microbial biomass, photosynthesis, rhizosphere, soil fertility, soil organic matter, soil respiration, soil water, terrestrial ecosystems, uncertainty, woodlands, Australia
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 (eCO₂) 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 150 ppm CO₂ above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO₂ across a range of CO₂ concentrations in an intact ecosystem. Soil CO₂ efflux (i.e., soil respiration or Rₛₒᵢₗ) increased in response to initial enrichment (e.g., +30 and +60 ppm CO₂) but did not continue to increase as the CO₂ enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (Aₛₐₜ) also showed similar stimulation by elevated CO₂ at +60 ppm as at +150 ppm CO₂. The lack of significant effects of eCO₂ on soil moisture, microbial biomass, or activity suggests that the increase in Rₛₒᵢₗ likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in Rₛₒᵢₗ suggests that under eCO₂, 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 eCO₂ are a priority for future research.