What was done
White clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were grown in FACE plots maintained at atmospheric CO2 concentrations of 350 and 600 ppm for three years before the soil was sampled to determine the effects of elevated CO2 on its bacterial populations.

What was learned
Although elevated CO2 increased the total number of bacteria and respiring bacteria in the bulk soil beneath white clover by 40 and 70%, respectively, it had no significant impact on bulk-soil bacterial numbers beneath perennial ryegrass. When the total bacterial numbers in rhizosphere soil were expressed on a per unit land area basis, however, it was found that elevated CO2 increased the total number of bacteria and respiring bacteria beneath white clover by about 100 and 250%, respectively, while it increased the total number of bacteria and respiring bacteria beneath perennial ryegrass by approximately 85 and 125%, respectively.

What it means
As the air's CO2 concentration continues to increase, it will likely boost rates of net photosynthesis in nearly all plant species, including white clover and perennial ryegrass.  These increases in photosynthesis, in turn, will likely lead to greater root production and enhanced exudation of organic compounds belowground into the soil rhizosphere, which should stimulate the growth and productivity of microbial organisms living there.

The present study demonstrated that this reasoning is indeed correct: greater microbial populations existed under CO2-enriched plots of white clover and perennial ryegrass than under ambiently-grown plots.  Consequently, one would expect earth's plants to increase their productivity even more with greater microbial populations existing beneath them, as bacterial microbes often help to make certain soil nutrients more available to plants.