What was done
In August of 1996, circular FACE plots (30 meters in diameter) exposed to air of either 360 or 560 ppm CO2 were established in a 15-year-old loblolly pine plantation in North Carolina, USA.  Although the plantation is dominated by loblolly pine trees, several hardwood species are present in the understory beneath the primary coniferous canopy.  This paper describes the impact of elevated CO2 on several soil properties and functions following two years of atmospheric CO2 enrichment.

What was learned
The modest 200 ppm increase in atmospheric CO2 concentration increased the CO2 concentration within the soil pore space by an average of 20%, resulting in an increase in soil CO2 efflux that was approximately 27% greater in the CO2-enriched plots.  Nevertheless, because the increase in soil CO2 efflux was less than the CO2-induced increase in plant photosynthesis, net rates of carbon sequestration were significantly stimulated by atmospheric CO2 enrichment.

On another note, the elevated CO2 increased the weathering rate of parent rock material, as indicated by a 271% increase in soil mineral cation concentration and a 162% increase in soil alkalinity after the second year of CO2 enrichment.  In addition, the elevated CO2 increased the flux of dissolved inorganic carbon compounds to the groundwater by 33%.

What it means
As the CO2 content of the air rises, it is likely that ecosystems composed of loblolly pine trees and their associated understory species will exhibit significant increases in net photosynthesis and belowground biomass production.  Concurrently, some of the extra CO2 within the soil pore space will be converted into carbonic acid, likely increasing soil weathering rates, which should make more nutrients available to the ecosystem and help to further increase plant growth and development.  Extrapolating this phenomenon to the global land area covered by forests, the authors say the observed increase in the efflux of dissolved inorganic carbon compounds to groundwater "may act to buffer the rate of CO2 increase in the atmosphere over geologic time periods."

Isn't carbon dioxide wonderful?  It sets in motion and intensifies a whole host of beneficial phenomena; but by providing another brake on the rate of rise of the air's CO2 content, it tempers it's own success!