What was done
The authors conducted a series of long-term (170-330 days) laboratory incubation experiments to examine changes in soil organic matter pool sizes and turnover rates in soil collected from an open-top chamber (OTC) atmospheric CO2 enrichment study in the shortgrass steppe of northeastern Colorado, USA, where the air in the ambient CO2 chambers (ACs) and elevated CO2 chambers (ECs) had atmospheric CO2 concentrations of 360 and 720 ppm, respectively, and where this degree of CO2 enrichment enhanced both above- and below-ground plant growth by 15-35%.

What was learned
Pendall and King report that "active pool carbon increased in EC relative to AC treatments systematically over the first 3 years of exposure to elevated CO2 in topsoils and to a lesser degree in subsoils," noting that "these results are consistent with independent results from the same OTC study showing that rhizodeposition rates doubled (Pendall et al., 2004) and root production increased under elevated CO2 (Milchunas et al., 2005)." In addition, they determined that "new carbon turnover was not [our italics] enhanced by elevated CO2," confirming that "new carbon inputs under elevated CO2 are not simply lost to mineralization" and that "pool sizes may continue to increase under elevated CO2."

What it means
In the words of the two researchers, "these results suggest that soil carbon storage may increase in semi-arid grasslands under elevated CO2," and this phenomenon, in turn, would tend to mitigate the degree of global warming thought by many to accompany increases in the air's CO2 content.