What was done
The authors grew Plantago lanceolata plants that either were or were not inoculated with fungi in glasshouses having atmospheric CO2 concentrations of 350 ppm (ambient) or 540 ppm (elevated) to determine the interactive effects of CO2 and symbiotic soil fungi on plant growth.  Sequential harvests were conducted roughly one, two and three months into the study to obtain various types of data.

What was learned
At ambient CO2, the percentage of total root length occupied by arbuscular mycorrhizal fungi did not increase after the initial harvest.  At the initial harvest in the elevated CO2 treatment, however, this parameter was 50% greater than it was under ambient conditions; and it increased to 75% greater for the remaining two harvests.  This increased mycorrhizal colonization resulting from atmospheric CO2 enrichment led to increased uptake of phosphorus and greater phosphorus tissue content at the final harvest for plants grown in this treatment.  Furthermore, the inoculated plants grown at elevated CO2 produced more than twice the biomass of their ambient treatment counterparts.

What it means
The results of this work suggest that this perennial herb will develop a more extensive root system as the CO2 content of the air continues to rise.  This enhanced root system, in turn, will likely provide increased carbon allocations to the rhizosphere, thereby stimulating greater mycorrhizal colonization by fungi that can increase the availability of certain mineral nutrients to the plant.  This phenomenon should consequently lead to an overall increase in plant biomass, both above- and belowground.