Effects of ungulate grazers on arbuscular mycorrhizal symbiosis and fungalcommunity structure in tallgrass prairie

Complex interactions among mycorrhizal fungi, plants, and herbivores occur in grasslands. Grazing of aboveground vegetation may influence plants direc tly and indirectly through the alteration of mycorrhizal symbiosis and othe r below-ground processes, and mycorrhizae in turn can influence plant respo nses to defoliation. An understanding of these interactions is important to our understanding of the dynamics of plant and fungal communities and to t he sound management and conservation of grassland ecosystems. In this study , the effects of grazing in tallgrass prairie on mycorrhizal colonization o f plant roots, fungal community composition, and extraradical mycorrhizal h yphal (EMH) development were examined. In October 1994 and 1995 rhizosphere samples were taken at different topographical positions in tallgrass prair ie sites grazed for several years by cattle at varying intensities, in ungr azed sites, and inside and outside permanent 25-m(2) exclosures at each sit e. Spores of 19 species of AM fungi were encountered in these sites, and Gl omus heterosporum was the most abundant species present. Moderate and inten se grazing increased root colonization by mycorrhizal fungi in both years o f this study. Similar root colonization levels inside and outside permanent exclosures in the ungrazed sites indicated no confounding effects of the e xclosure structure itself. In 1995, EMH development was also increased unde r intense grazing. AM fungal species diversity (based on AM fungal spores p resent) decreased with grazing under both moderate and high grazer densitie s in both years. Different fungal species varied in abundance with topograp hical position, however topography did not significantly affect AM fungal s pore species diversity, mycorrhizal root colonization or EMH development in either year. Although overall total spore density was unaffected by grazin g intensity or soil type, several individual species increased or decreased in response to these factors. These results suggest that defoliation alter s plant resources which stimulates greater development of mycorrhizal symbi osis. The shifts in fungal species composition and decrease in diversity wi th grazing also indicates that defoliation, or alteration of the soil micro environment by grazers, favors certain species of grazing-adapted AM fungi that increase under grazed conditions. The high mycorrhizal dependency of m any tallgrass prairie grasses and these results together suggest significan t interactions between plant-grazer and plant-fungal relationships in tallg rass prairie. In addition to direct effects of herbivory, our results indic ate that grazers may influence grassland plants indirectly through alterati ons in soil communities and in mycorrhizal symbiosis.