The main focus of this study was: (1) to determine the residence time of photosynthetic C in a forest exposed to elevated [CO₂]; (2) to determine the sources of forest soil respiration under elevated [CO₂] and to characterize their temporal variability under field conditions; and (3) to determine if and how soil organic matter decomposition is altered by ecosystem exposure to elevated [CO₂]. To address these objectives, I used the depleted signature of the fumigation CO ₂ at the Duke Forest Free-Air CO₂ Enrichment (FACE) experimental site as a continuous ecosystem tracer in a warm-temperate loblolly pine forest in North Carolina.

In this study, I found that up to 70% of photosynthetic carbon is returned to the atmosphere within a year of assimilation. The average turnover time of oxidized soil C was less than 5 years for soil depths down to 200 cm and there were pools that turn over in less than a year detected below the rooting zone in this forest. The fast turnover time of these carbon pools, as detected in soil CO₂, did not match the turnover times reported for several soil organic matter standing stocks.

An examination of the diurnal variability of four components of soil respiration revealed that the presence or absence of changes in the rate of total soil respiration cannot be interpreted as changes in the rate of its components. Soil respiration components varied diurnally and did not follow a simple soil temperature/moisture response function. Interestingly, the contribution of an older soil organic matter pool (> 8 years old) to growing season soil respiration was detected and this pool had a very pronounced diurnal pattern, with higher rates at night. Despite higher soil respiration rates under elevated [CO₂], there were no intrinsic differences found in the decomposition kinetics of soil organic matter from the FACE and control plots of this forest.

The results presented in this thesis suggest that estimating the turnover time of soil carbon by measuring that of soil organic matter stocks may lead to underestimation of decomposition rates and overestimates of ecosystem carbon storage capacity. Furthermore, our estimates of the contribution of recently assimilated carbon to soil respiration were lower than those found in the literature, suggesting that the contribution of current and recent photosynthate to soil respiration may be currently overestimated.