ABSTRACTThe effects of tallgrass prairie management practices, burning, mowing (simulated grazing), and fertilization, on mycorrhizal symbiosis were studied in a field experiment established in 1986. In 1987 and 1989, there were no significant effects of these management practices on mycorrhizal fungus species composition. While 14 and 11 species were observed in 1987 and 1989, respectively, the dominant species in both samplings wasGlomus ambisporum.Spore numbers were generally not affected by these management practices. However, in 1987 there were significant effects on spore number due to nitrogen addition and a burn×mow interaction, but these were not apparent in 1989. In 1989 there was a significant burn×nitrogen interaction, with nitrogen fertilization of unburned plots significantly increasing the number of mycorrhizal fungal spores. In winter months total % root colonization, active % root colonization and inoculum potential were low whether or not plants were fertilized. In contrast, in late spring and early summer when plants were actively growing, fertilization reduced total % root colonization, active % root colonization, and inoculum potential in soil. However, nitrogen fertilization was not as inhibitory to the symbiosis as phosphorus fertilization or phosphorus + nitrogen fertilization. The negative effects of nitrogen fertilization on mycorrhizae are probably offset by the pronounced benefit of nitrogen fertilization to plant biomass production.