Species of plants and associated arbuscular mycorrhizal fungi mediate mycorrhizal responses to CO 2 enrichment NANCY COLLINS JOHNSON *, JULIE WOLF w , MELISSA A. REYES *, ASHLEY PANTER *, GEORGE W. KOCH * and A N D R E A R E D M A N * *Departments of Environmental Sciences, Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ 86011-5694, USA, wMolecular Plant Pathology Laboratory, United States Department of Agriculture, Agricultural Research Service, Beltsville, MA 20705-2350, USA Abstract It has been suggested that enrichment of atmospheric CO 2 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 CO 2 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, C 3 and C 4 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 CO 2 enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO 2 enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO 2 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 CO 2 levels further mediate AM function. In general, when CO 2 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 CO 2 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. Key words: AM fungi, arbuscular mycorrhizas, carbon dioxide enrichment, CO 2 , experimental scale, Gigasporaceae, Glomus, mutualism, mycorrhizal function, nitrogen Received 18 August 2004; revised version received 20 December 2004 and accepted 7 January 2005 Introduction Arbuscular mycorrhizal (AM) symbioses facilitate plant uptake of soil nutrients in most temperate and tropical ecosystems. The mutualistic effects of these associations are predicted to be sensitive to anthropogenic enrich- ment of atmospheric CO 2 , because elevated CO 2 should simultaneously increase plants’ photosynthetic rates and soil nutrient requirements (O’Neill, 1994). From a plant perspective, AM function is determined by the balance between photosynthate costs and nutrient benefits (Fitter, 1991). Allocation of photosynthate to AM fungi represents a major C cost to plants (Koch & Johnson, 1984), and increased photosynthetic rates at elevated CO 2 should make more C available to support AM symbioses (Lovelock et al., 1997; Jifton et al., 2002). Changing C acquisition costs through CO 2 enrichment is expected to increase the relative benefits of AM Correspondence: Nancy Collins Johnson, Environmental & Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5694, USA, tel. 1 1 928 5236473, fax 1 1 928 5237423, e-mail: Nancy.Johnson@nau.edu Global Change Biology (2005) 11, 1156–1166, doi: 10.1111/j.1365-2486.2005.00967.x 1156 r 2005 Blackwell Publishing Ltd