The only way to know for sure what the ultimate outcome would be in this situation is to measure it, which requires that one replicates the future, as it were, and that it be done as accurately as possible.  In the case of the atmosphere and its CO2 concentration, the most natural way of accomplishing this feat is to utilize Free-Air CO2 Enrichment or FACE technology.  This approach to maintaining an elevated atmospheric CO2 concentration utilizes computer-controlled vertical vent pipes arranged in circular arrays out-of-doors in the natural environment that are programmed to respond to real-time changes in wind speed and direction in such a way that they continuously release CO2-enriched air from the appropriate (upwind) pipes, so that the plants growing within the circular arrays are always supplied with air of the desired atmospheric CO2 concentration.

To date, a total of only thirteen FACE experiments have explored the impact of elevated atmospheric CO2 concentrations on carbon sequestration in soils; and every one of these studies has presented its investigators with a major scientific challenge.  The difficulty they face is the fact that annual organic carbon additions to soils are typically very small compared to the sizes of the soil organic carbon (SOC) pools already there.  In addition, the difference between the SOC additions to soils in ambient and CO2-enriched FACE arrays is even smaller, so small, in fact, that SOC variability from one place to another in an agricultural field is often considerably greater than this tiny differential, which makes the CO2-induced SOC difference extremely difficult to detect, especially with a reasonable degree of statistical significance.

In their recent review of FACE studies conducted on agricultural crops, Kimball et al. confronted this challenge.  In all but a couple of rare instances, they could find no statistically significant differences in the amounts of new organic carbon that made its way into the soils of the ambient and CO2-enriched FACE arrays.  Nevertheless, they noted that in every single study "there was an increase in SOC due to the FACE treatment," as long as soil nitrogen supply was ample and about half the time when it wasn't.  Hence, they treated each of these study results as an individual observation; and when this was done, the mean CO2-induced increase in SOC storage under ample soil nitrogen supply was determined to be significant.

Kimball et al.'s broader-based calculations indicated that a 195 ppm increase in the air's CO2 concentration produced a statistically significant 11% increase in SOC when soil nitrogen supply was ample, as well as a non-significant 2% increase when soil nitrogen supply was limiting to plant growth.  For the more common 300-ppm increase in atmospheric CO2 concentration employed in most other types of CO2 enrichment studies, these results scale linearly to 17% and 3%, respectively.  "Looking at the combined experience from these several experiments," say Kimball and his co-authors, "it appears that significant increases in SOC have occurred under the FACE plots, at least when nitrogen was nonlimiting."

Kimball et al.'s ultimate conclusion after studying the results of all agricultural FACE studies conducted to date - including some based on carbon isotopic measurements - is that "some increase in the SOC should happen due to the increasing atmospheric CO2 concentration, at least where nitrogen is nonlimiting."  Clearly, however, we can not yet say we have a good handle on the magnitude of the effect.  More highly replicated measurements would help in this regard, as would experiments of longer duration.  Due to the great importance of the knowledge to be gained from such experiments, we would highly encourage the USDA's Agricultural Research Service to support a number of pertinent long-term studies of this phenomenon.

Reference
Kimball, B.A., Kobayashi, K. and Bindi, M.  2002.  Responses of agricultural crops to free-air CO2 enrichment.  Advances in Agronomy 77: 293-368.