It has been suggested that enrichment of atmospheric CO2 should alter mycorrhizal function by simultaneously increasing nutrient-uptake benefits and decreasing net C costs for host plants. However, this hypothesis has not been sufficiently tested. We conducted three experiments to examine the impacts of CO2 enrichment on the function of different combinations of plants and arbuscular mycorrhizal (AM) fungi grown under high and low soil nutrient availability. Across the three experiments, AM function was measured in 14 plant species, including forbs, C3 and C4 grasses, and plant species that are typically nonmycorrhizal. Five different AM fungal communities were used for inoculum, including mixtures of Glomus spp. and mixtures of Gigasporaceae (i.e. Gigaspora and Scutellospora spp.). Our results do not support the hypothesis that CO2 enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO2 enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO2 enrichment and decreases plant growth responses to AM fungi.
Our results show that the strength of AM mutualisms vary significantly among fungal and plant taxa, and that CO2 levels further mediate AM function. In general, when CO2 enrichment interacted with AM fungal taxa to affect host plant dry weight, it increased the beneficial effects of Gigasporaceae and reduced the benefits of Glomus spp. Future studies are necessary to assess the importance of temperature, irradiance, and ambient soil fertility in this response. We conclude that the affects of CO2 enrichment on AM function varies with plant and fungal taxa, and when making predictions about mycorrhizal function, it is unwise to generalize findings based on a narrow range of plant hosts, AM fungi, and environmental conditions.