What was done
Open-top chambers were established on a tallgrass prairie located in Kansas, USA.  Chambers containing mixtures of C3 and C4 grasses were fumigated with ambient air and air enriched to twice the ambient concentration of atmospheric CO2 for a period of eight years to determine the effects of atmospheric CO2 enrichment on soil carbon dynamics in this prairie ecosystem.

What was learned
Ecosystems enriched with CO2 had average soil water contents that were 15% greater than those of ecosystems exposed to ambient CO2 within the first 15 cm of their soil profiles.  Increased soil moisture in these CO2-enriched plots was positively correlated with soil microbial activity, which averaged 14% greater than what was observed in ambient plots across the final five years of the study.  Elevated CO2 increased both microbial biomass carbon and nitrogen by an average of 4% and contributed to an 8% enhancement of total soil carbon and nitrogen.

What it means
As the CO2 content of the air increases, it is likely that tallgrass prairie ecosystems will experience an increase in their soil water contents.  These increases in soil moisture will likely lead to greater soil microbial activity, which can produce a host of positive phenomena, including the enhancement of microbial carbon and nitrogen biomass.  Ultimately, the increase in the air's CO2 content should increase soil carbon sequestration in tallgrass prairies, as indicated by the 8% increase in the total soil carbon of this study.  If extrapolated to all of earth's temperate grasslands, which make up 10% of the land area of the globe, an additional 1.3 Pg of carbon could be sequestered in the top 15 cm of their soil horizons within the next century.