What was done
The authors grew three-year-old Gutierrezia sarothrae shrubs in controlled environment chambers for four months with ambient or enriched (750 ppm) concentrations of atmospheric CO2 and low or high levels of soil nitrogen to determine the effects of these variables on the growth of arbuscular mycorrhizal fungi associated with the roots of this particular shrub.

What was learned
Although elevated CO2 did not significantly alter percent fungal infection in roots of Gutierrezia sarothrae, it did, nonetheless, increase this parameter by approximately 19 and 9% at low and high levels of soil nitrogen, respectively.  The authors were quick to point out, however, that percent fungal infection of roots, which is often the main characteristic studied and reported in this type of research, can be "misleading in evaluating the symbiotic response [of plants and fungi] to elevated CO2."  Consequently, they measured a number of other parameters to better understand the complete growth response of arbuscular mycorrhizal fungi to elevated CO2.

With respect to arbuscules, which are the main structures involved in the symbiotic exchange of carbon and nutrients between a host plant and its associated fungi, Rillig and Allen found that percent arbuscular infection increased more than 14-fold with atmospheric CO2 enrichment in the low nitrogen regime.  In addition, the length of fungal hyphae more than doubled in the low nitrogen treatment with elevated CO2; and in the high nitrogen regime, the percent root infection with vesicles, which are the organs used by the arbuscular mycorrhizal fungi for carbon storage, increased by approximately 2.5-fold.  Moreover, the fungal infection intensity, which qualitatively assesses the amount of root cross-sectional area infected and the density of that infection, increased with atmospheric CO2 enrichment regardless of soil nitrogen content.

All of these observations demonstrate that growth in elevated CO2 increases carbon allocation to arbuscular mycorrhizal fungi.  They also show that arbuscular mycorrhizal fungi invest the additional carbon acquired from plants grown in elevated CO2 in different morphological structures based on soil nitrogen availability.

What it means
As the CO2 content of the air steadily rises, most plants will exhibit increases in photosynthesis and carbohydrate production.  Greater levels of carbohydrates in plant tissues will likely lead to increased exudation of carbon compounds to the soil and to a number of organisms that associate with plant roots, including arbuscular mycorrhizal fungi.  Because these fungi obtain all of their carbon from plant roots, this increased exudation should positively influence their growth in a number of ways that ultimately should result in better and more comprehensive associations with plant roots.  In turn, these associations can provide plants greater access to certain soil nutrients and additional sinks for photosynthetic carbon, both of which can result in even greater increases in plant growth as the CO2 concentration of the air continues to rise.