What was done
Open-top chambers were built on two different, but adjacent, grassland communities located near Stanford, California, USA, and subjected to atmospheric CO2 concentrations of about 350 and 700 ppm for six growing seasons to determine whether short-term CO2 responses of fungal root colonization can persist in the field after several years of atmospheric CO2 enrichment.

What was learned
Elevated CO2 caused significant increases and decreases in various fungal root colonization parameters and fungal composition that depended upon plant species.  Total percentage of root length colonized, for example, was not significantly affected by elevated CO2 in either the serpentine or sandstone grassland.  However, the percentage infection by arbuscules, which are relatively short-lived fungal structures involved in carbon and nutrient exchange with host roots, increased significantly in both grassland communities.  This single observation is most noteworthy, as it indicates a persistent increase in the functioning of arbuscular mycorrhizal fungi to long-term atmospheric CO2 enrichment.

What it means
This study suggests that some of the short-term CO2-induced stimulations of root colonization by arbuscular mycorrhizal fungi can indeed persist for years under natural field conditions.  Thus, as the CO2 content of the air continues to rise, it is likely that most plants will display increases in photosynthesis, thereby allowing greater carbon supplies to be transferred belowground to symbiotic mycorrhizal fungi.  In turn, these fungal organisms should enhance their mycorrhizal networks and increase the benefits received by host plants involved in these mutualistic relationships.