What was done
The authors constructed open-top chambers in serpentine and sandstone annual grasslands located in California, USA, and exposed the enclosed ecosystems to atmospheric CO2 concentrations of 350 and 700 ppm for four years before sampling ecosystems for soil microbiota analyses.

What was learned
Elevated CO2 increased the biomass of active fungal organisms and the number of flagellated protozoa on both grasslands, particularly during early-season vegetative ecosystem growth, without having any effect on total bacterial biomass.

What it means
Although the overall effects of elevated CO2 on soil microbiota were relatively small, they do suggest, nonetheless, that some of the additional belowground carbon inputs resulting from CO2-induced increases in root growth and exudation are transferred up the soil microbial food web.  Thus, as the air's CO2 content rises, it is likely that metabolic activity within soils will be increased, possibly enhancing the rate of new soil formation and increasing the availability of soil nutrients for plant utilization.