Miquel A Gonzalez-Meler¹, Roser Matamala², Jeff Andrews³, Evan DeLucia⁴, Gaby Katul³, Yqi Luo⁵ and William H Schlesinger^3
1University of Illinois at Chicago, Chicago IL-60607, ²Argonne National Laboratory Argonne IL-60439, ³Duke University Durham NC-27708, ⁴University of Illinois at Urbana-Champain, Urbana IL-61801, ⁵University of Oklahoma, Norman OK-73019

Terrestrial ecosystems exchange about 120Gt of carbon (C) with the atmosphere, through the processes of photosynthesis (leading to gross primary productivity, GPP) and ecosystem respiration (Re). Increasing evidence indicates that raising atmospheric CO₂ enhances carbon uptake in most ecosystems, however, responses of Re and its components to elevated CO₂ are still unresolved due, largely, to our inability to reliably determine Re. We used an isotope approach combined with measurements of soil respiration to estimate the components of Re in an intact loblolly pine-dominated forest exposed to elevated CO₂ using Free-air CO₂ Enrichment (FACE). We have used the depleted ¹³C signature of the fumigation CO₂ to distinguish and evaluate the effects of CO₂ on rhizosphere autotrophic respiration and microbial heterotrophic respiration and its components. Results from this study indicate a rapid carbon cycling in forests exposed to elevated CO₂ largely due to increased root and rhizosphere activity limiting the ability of this forest to sequester atmospheric carbon.