Intraspecific AM fungal variation contributes to plant-fungal feedback in a serpentine grassland

<p>Feedback between plants and arbuscular mycorrhizal (AM) fungi can affect species diversity in plant and fungal communities. Feedback depends on (1) some specificity between plants and fungi and (2) fungi exhibiting specificity either improving (positive feedback) or decreasing (negative feedback) host performance relative to other fungi. Associations between AM fungi and plant species in a serpentine grassland dominated by <em>Andropogon gerardii</em>, <em>Schizachyrium scoparium</em>, <em>Sorghastrum nutans</em>, and <em>Sporobolus heterolepis</em> were examined, and their performance consequences were evaluated. Specificity was determined from AM fungal spore abundance under plants in the field and from trap cultures established by inoculating greenhouse plants with field-collected roots containing fungal material. Seven AM fungal species were unevenly distributed among plant species, with differences in total spore numbers and evenness. In the field, <em>Gigaspora gigantea</em> exhibited specificity to <em>Sporobolus</em> compared to <em>Andropogon</em> and <em>Sorghastrum</em>, and <em>Glomus microcarpum</em> exhibited specificity to <em>Schizachyrium</em> compared to <em>Andropogon</em> and <em>Sporobolus</em>. In trap cultures, <em>Glomus etunicatum</em> exhibited specificity to <em>Andropogon</em> compared to <em>Sorghastrum</em>, and an unidentified species of <em>Glomus</em> exhibited specificity to <em>Sorghastrum</em> compared to <em>Sporobolus</em>. The AM fungal community associated with <em>Smilax rotundifolia</em>, a grassland invader, was not qualitatively different from those of grasses. In the greenhouse, plant and fungal performance was examined as a function of three host plant species (excluding <em>Sporobolus</em>), four AM fungal species, and the same three plant species on which the fungi had been collected in the field. Fungal species affected host plant biomass and fungal performance, measured by AM root length and percentage of colonization. Compared to other fungal species, <em>Gigaspora gigantea</em> increased plant biomass and had a higher percentage of colonization and AM root length on both <em>Andropogon</em> and <em>Schizachyrium</em>. <em>Glomus etunicatum</em>, which exhibits specificity on <em>Andropogon</em>, depressed growth of <em>Andropogon</em> relative to other fungi, implicating negative feedback. <em>Schizachyrium</em> had reduced biomass when inoculated with <em>Glomus microcarpum</em>, its specific fungus. Additionally, fungi originating on a conspecific host depressed plant growth compared to fungi collected from other host species, an effect strongest for <em>Andropogon</em>. Percentage of AM colonization on <em>Andropogon</em> was also greater for fungi originating on <em>Andropogon</em>. The latter two effects serve as another example of negative plant-fungal feedback and could represent intraspecific fungal variation within this community.</p>