The effects of elevated atmospheric CO2 (CO2e) on soil respiration were evaluated using inverse models and static chamber measurements collected over 4.5 years in a maturing loblolly pine forest. The chamber measurements of forest floor CO2 efflux showed that the flux enhancement increased progressively, reaching 1.4 after 3 years, but declined thereafter to 1.10 at the end of the study. The subsurface CO2 concentration measurements show that the difference in mean annual concentration gradient between ambient and enriched plots initially increased, and then stabilized after 2 years of enrichment. However, concomitant soil moisture increases in the enriched plots reduced the soil diffusivity, especially over the last 2 years. These two observations, in combination with inverse model calculations, suggest a decline in subsurface CO2 production over the last 2 years. Enhancement in photosynthesis has declined after 4.5 years of enrichment (remaining ∼40% higher than ambient photosynthesis), and the increase in production and respiration alone cannot account for the excess carbon uptake. Thus this modeling approach would suggest that non-gaseous carbon is accumulating belowground. However, we have not been able to measure significant changes in soil organic carbon or fine root biomass. These results, in the context of other forest floor flux studies and the carbon budget at the site, suggest a lack of closure in the carbon budget under elevated atmospheric CO2.