Average Age of Soil Carbon Pools Contributing to Soil CO2 in a Forest Exposed to FACE

Soils represent the largest C reservoir in terrestrial ecosystems and have been highlighted as potential sinks for the anthropogenic additions of CO₂ to the atmosphere. Soil organic matter (SOM) consists of several pools with different C turnover rates. Long-term terrestrial C storage with rising atmospheric [CO₂] requires that a substantial proportion of the additional C assimilated by plants is transferred to recalcitrant soil C pools and not quickly returned to the atmosphere through R_S. Soil CO₂ reflects oxidation of SOM occurring in soil C pools of different turnover rates and the age of soil CO₂ reflects the age of the C pools where the CO₂ originated. Soil CO₂ can therefore be used to infer the age of the C pools contributing to the return of soil C to the atmosphere through R_S. At the FACTS-1 research site, an intact forest ecosystem has been exposed to elevated CO₂ for 8 years. The ¹³C composition of the fumigation CO₂ functions as a continuous ecosystem label and the rate of incorporation of this label into SOM can be used to identify the soil C pools contributing to soil CO₂. We used the long-term record of the \delta¹³C of soil CO_{2} to determine the age and relative contribution of the C pools contributing to soil CO_{2}$ at different depths in this forest ecosystem.