What was done
The authors grew monocultures of three grasses and two herbs that co-occur in mediterranean annual grasslands near Stanford, California, in pots placed within open-top chambers that received atmospheric concentrations of ambient or ambient plus 350 ppm CO2 for four months.  In addition, half of the pots in each CO2 treatment received extra soil nutrients, so the interactive effects of CO2 and nutrition on root-colonizing fungal organisms could be investigated.

What was learned
Elevated CO2 increased percent root infection by arbuscular mycorrhizal fungi in all five annual grassland species.  The added effects of high nutrition, however, reversed this trend in four of the five species.  For non-mycorrhizal fungi, elevated CO2 increased percent root infection in two species and decreased it in the other three, regardless of soil nutrient status.

What it means
As the CO2 content of the air rises, these five annual grassland species will likely display complex changes in root colonization by mycorrhizal and non-mycorrhizal soil fungi that may be affected still further by soil fertility.  In general, mycorrhizal root colonization will likely increase with atmospheric CO2 enrichment in all five species at low soil fertility, while non-mycorrhizal root colonization will probably only increase for two of them, irrespective of soil nutrient status.  Much more work will be needed to clarify this picture and to determine if these results from monoculture studies are further corroborated when the plants grow together in a community setting.