Background
The authors state that "in the shortgrass steppe of northeastern Colorado [USA], above- and belowground growth were both enhanced by 15-35% (depending on precipitation) during a 5-year open-top chamber (OTC) experiment (Morgan et al., 2004; Milchunas et al., 2005)," and that "grasslands exposed to elevated CO2 have shown increased belowground carbon inputs and may [our italics] have increased soil organic matter storage (Jastrow et al., 2000; Pendall et al., 2004)."

What was done
To see if the tentative "may" in the quoted background material might actually be replaced with a more definitive "will," Pendall and King "conducted a series of laboratory incubation experiments to quantify changes in pool sizes and turnover rates of active and slow organic matter pools in shortgrass steppe soils exposed to elevated CO2 for 1-3 years," while measuring "δ¹³C of respired CO2 to evaluate changes in pool sizes and turnover rates of organic matter added since the beginning of the experiment."

What was learned
The two researchers report that the results of their incubation study showed larger active and slow carbon pool sizes under elevated CO2 (EC = 360 ppm) than under ambient CO2 (AC = 720 ppm), noting that "for active pool carbon in topsoil [5-10 cm depth], the ratio of EC/AC increased systematically with time, suggesting that the active carbon pool was increasing under elevated CO2," which results, in their words, "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 say their isotopic labeling allowed them to compare mineralization of new carbon inputs across the experiment, which data "demonstrated that new carbon turnover was not enhanced by elevated CO2," confirming that "new carbon inputs under elevated CO2 are not simply lost to mineralization."

What it means
Pendall and King state that "the active and slow carbon pools increased in the surface soil under elevated CO2 and that there was not a simultaneous increase in turnover rates." These findings suggest that soil carbon storage likely will increase in semi-arid grasslands under elevated CO2; but ever the circumspect scientists, the two researchers judiciously stick with the verb "may."

References
Jastrow, J., Miller, R. and Owensby, C. 2000. Long-term effects of elevated atmospheric CO2 on below-ground biomass and transformations to soil organic matter in grassland. Plant and Soil 224: 85-97.

Milchunas, D.G., Mosier, A.R., Morgan, J.A., LeCain, D.R., King, J.Y. and Nelson, J.A. 2005. Root production and tissue quality in a shortgrass steppe exposed to elevated CO2: using a new ingrowth method. Plant and Soil 268: 111-122.

Morgan, J., Mosier, A., Milchunas, D., LeCain, D., Nelson, J. and Parton, B. 2004. CO2 enhances productivity, alters species composition and reduces digestibility of shortgrass steppe vegetation. Ecological Applications 14: 208-219.

Pendall, E., Mosier, A.R. and Morgan, J.A. 2004. Rhizodeposition stimulated by elevated CO2 in a semi-arid grassland. New Phytologist 162: 447-458.