What was done
Two dominant C4 tallgrass prairie species (Schizachyrium scoparium and Andropogon gerardii) were grown for eight weeks in controlled environment chambers at ambient and subambient (200 ppm) atmospheric CO2 concentrations in combination with high and low soil water contents to study the interactive effects of these variables on shoot and root growth in these grassland species.

What was learned
High soil water content increased aboveground biomass in the two grasses by an average of 82%, while exposure to ambient CO2 increased this parameter by 57% over that displayed by plants grown at 200 ppm CO2.  Interestingly, there were no significant CO2 x soil moisture interactions for any aboveground growth parameter.  Similarly, high soil water content increased root length, mass, surface area and volume in these two species by 40 to 51%, while ambient CO2 exposure increased these parameters by 15 to 27%.  Thus, soil moisture content had a stronger effect on growth in these C4 grasses than did atmospheric CO2 concentration.

What it means
As the air's CO2 concentration increases, the direct effect of elevated CO2 in stimulating photosynthetic carbon fixation in C4 plants will likely lead to increases in their dry mass production.  However, the indirect effects of elevated CO2 on soil water conservation may lead to even greater increases in biomass in prairie ecosystems.  Thus, carbon sequestration in grasslands will likely be larger than what is commonly predicted on the basis of carbon modeling programs that do not account for the additional soil moisture that should be present beneath CO2-enriched plants.