Background/Question/Methods    Plants can influence soil microbial communities in ways that feed back to affect the relative performance of co-existing plant species.  The specific role of arbuscular mycorrhizal (AM) fungi in this plant-soil feedback, separate from the action of other soil microbes, has not been fully illuminated.  We explore intraspecific variation in the AM fungus Gigaspora gigantea in an Eastern serpentine grassland as one possible agent of plant-soil feedback previously documented in this system.
    In the greenhouse, we examined the ability of different isolates from the same grassland to promote host plant growth.  Isolates consisted of spores collected under six well-defined clumps of three grasses: Andropogon gerardii, Sorghastrum nutans, and Schizachyrium scoparium.  We thoroughly washed apparently healthy spores to remove debris and other microbes and surface sterilized them in antibiotics.  We used them to inoculate two host plant species, Sorghastrum and Schizachyrium grown in sterilized field soil and sand to which we added other native soil microbes.  Spores from a single grass clump were applied to six replicate seedlings.  We analyzed shoot biomass at harvest as a function of (1) host species of fungal origin and (2) clump nested within host species of fungal origin.

Results/Conclusions    The results show phenotypic variation among isolates of G. gigantea collected within a single grassland community.  Shoot biomass of Schizachyrium varied with the host species of fungal origin.  Shoot biomass of Sorghastrum varied among grass clumps but not with host species of fungal origin.  Both effects could contribute to spatial variation in the relative performance of Sorghastrum and Schizachyrium in the field.  However, neither result was consistent with previously documented negative feedback in this system, whereby plants grew less well in their own soils than in soils cultured by other host plant species.  Possible reasons for this difference will be discussed.