What was done
By means of stable carbon isotope analyses of air, plants and soil in the last year of a three-year free-air carbon dioxide enrichment (FACE) experiment on cotton (Gossypium hirsutum L. cv. Deltapine 77) conducted at the Maricopa Agricultural Center south of Phoenix, Arizona, where cotton crops were grown at both 370 and 550 ppm CO2, the authors sought to determine whether or not any of the carbon removed from the air by the CO2-enriched plants made its way into the soil and was stored there.

What was learned
It was found that about 10% of the organic carbon present in the soil below the CO2-enriched plants at the conclusion of the three-year experiment came from the extra CO2 supplied to the FACE plants.  It was additionally determined that some of this carbon had made its way into a very recalcitrant portion of the soil organic matter that has an average age of 2200 years.

What it means
The authors conclude that "at a minimum, the results indicate that about 10% of the soil carbon was replaced with 'fresh' carbon in 3 years, including the more recalcitrant fractions," and that these findings "provide some indication that the soils may be acting as an enhanced carbon sink under high ambient CO2."  Consequently, there can be little doubt that the ongoing rise in the air's CO2 content holds within it the seeds of a powerful negative feedback mechanism that helps to limit its rate of rise.