What was done
The authors grew Plantago lanceolata and Trifolium repens inoculated with the arbuscular mycorrhizal fungus Glomus mosseae for 75 days in open-top chambers fumigated with atmospheric CO2 concentrations of 400 and 650 ppm to study the effects of elevated CO2 on mycorrhizal formation and phosphorus (P) uptake as a function of plant growth and development.

What was learned
Elevated CO2 stimulated the growth of both plant species by significantly increasing shoot and root dry weights.  At any given time over six sequential harvests, atmospheric CO2 enrichment substantially increased the percentage root length colonized and total root length colonized by the fungus for both plant species.  In addition, elevated CO2 also increased the external mycorrhizal hyphal density associated with the roots of both species at any given time.  However, when the larger plant sizes were accounted for, there was only slight evidence showing a direct effect of elevated CO2 on mycorrhizal colonization.  Nonetheless, because elevated CO2 did increase plant size, total mycorrhizal biomass per plant was greater for CO2-enriched plants at any given time.

Total plant P content was significantly enhanced with elevated CO2, due to the fact that larger plants resulted from atmospheric CO2 enrichment.  Whereas elevated CO2 had no effect on root P concentrations, shoot P concentrations decreased in both species over the last two harvests  This observation was consistent with a reported decrease in P uptake during those periods, and may be explained by a reduced need for P in shoots due to photosynthetic acclimation to elevated CO2.  Thus, just as acclimation allows for the optimization of nitrogen within most plants, it may be optimizing P within these species.

What it means
As the CO2 content of the air continues to rise, it is likely that most plants will exhibit increases in photosynthesis and growth.  This common response should lead to greater above- and belowground biomass production.  With larger root systems belowground, it is likely that greater mycorrhizal colonization will occur due to greater root surface area availability.  Thus, in the future, plants should possess greater amounts of mycorrhizal cohorts to help them retrieve important mineral elements from the soil.  Additionally, for plant species that exhibit photosynthetic acclimation, it is likely that resource optimization for nitrogen, and possibly P, may occur.  Because most soil P is taken up by plants with the aid of mycorrhizal fungi, a decrease in P uptake could allow more energy to be used in creating additional and enhanced mycorrhizal networks, that ultimately could benefit their host plants to an even greater degree.