What was done
A FACE experiment, located in Switzerland, was began in 1993 to study the effects of atmospheric CO2 enrichment on monocultures of ryegrass (Lolium perenne) and white clover (Trifolium repens).  After two years of differential CO2 fumigation, soil samples were taken beneath experimental plots and analyzed to determine the effects of elevated CO2 on bacterial populations and community structure.

What was learned
Elevated CO2 did not impact the total number of bacteria in the bulk soil beneath the swards of ryegrass or white clover.  However, it tended to increase bacterial numbers in the rhizosphere, which consists of soil in closer proximity to plant roots than the bulk soil, beneath both species.  Thus, it appears that enhanced nutrient exudation from plant roots, resulting from atmospheric CO2 enrichment, allowed greater bacterial populations to live within the rhizosphere.

In addition, atmospheric CO2 enrichment altered the profile of bacterial communities in a plant species-dependent manner.  In ryegrass, for example, elevated CO2 increased the dominance of Pseudomonas species, which enhance plant growth by many different mechanisms, while in white clover, it increased the dominance of Rhizobium species, which enhance plant growth by making atmospheric nitrogen available for their utilization.

What it means
As the atmospheric CO2 concentration rises, it is likely that most plants will exhibit increases in photosynthesis and growth.  As a consequence of these phenomena, greater amounts of organic carbon compounds should be input into the soil via root exudation and biomass turnover.  Thus, bacterial numbers can be expected to increase as a result of this CO2-induced enhancement of soil carbon content.

Within the rhizosphere, it is likely that shifts in bacterial communities will occur in such a way as to optimize nutrient exchange between plants in a species-dependent manner.  In the case of the leguminous white clover, for example, elevated CO2 favored a shift towards Rhizobium bacterial species, which likely increased their nitrogen-fixing activities and made more nitrogen available to support enhanced plant biomass production.  In contrast, the non-leguminous ryegrass, which does not form symbiotic relationships with Rhizobium species, exhibited greater dependence upon Pseudomonas bacterial species to increase its acquisition of various soil minerals to support its CO2-induced growth enhancement.

Thus, as the CO2 content of the air rises, it is likely that swards of white clover and ryegrass will both exhibit increased biomass production, due to CO2-induced shifts in bacterial populations that optimize nutrient acquisition for each plant species.