Soil pathogenic fungi have the potential to affect the co-existence of twotallgrass prairie species

1 Negative feedback may exist between plant species and their biotic soil c ommunities, Two co-occurring native tallgrass species (Andropogon gerardii, a perennial grass, and Chamaecrista fasciculata, an annual legume) were re ciprocally transplanted into pots containing soil from areas dominated by o ne of the species. Half of the soil derived from each area was microwaved t o reduce soil fungi, resulting in four different 'soil types'. 2 Chamaecrista fasciculata plants were smaller when grown in their native s oil vs. that from under A. gerardii, but were unaffected by microwaving (i. e. fungal reduction). Andropogon gerardii plants were shorter with fewer in florescences in non-microwaved soil, with the poorest growth occurring in I ron-microwaved C. fasciculata soil. 3 Fungi were isolated from roots of A. gerardii growing in the different so il types. We tested whether the fungi found differed between the four soil types and whether any species characteristic of C. fasciculata soil were re sponsible for the poor growth of A. gerardii in this medium. 4 Fungi unique to the non-microwaved C, fasciculata soil type reduced tille ring and caused an early reduction in growth of A. gerardii. These effects were partially ameliorated when potentially mycoparasitic fungi associated with A. gerardii were also included. By the end of the experiment, both fun gal treatments increased above-ground biomass but greatly reduced below-gro und biomass of A. gerardii compared with controls, suggesting that the expo sure to potential fungal pathogens from C, fasciculata soil altered biomass allocation within plants. 5 There was no evidence of negative feedback between the prairie species an d their own soils. However, fungi associated with Chamaecrista fasciculata were detrimental to A. gerardii, one of the dominant perennials in the surr ounding area, and may facilitate the annual's long-term persistence. Arbusc ular mycorrhizae did not ameliorate the pathogenic effects of these fungi a s there was little colonization of A. gerardii roots in C. fasciculata soil .