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Tall fescue [ Lolium arundinaceum (Schreb.) S.J. Darbyshire] is naturally infected with a fungal endophyte, Neotyphodium coenophialum, which produces toxic ergot alkaloids that negatively affect herbivores and may alter soil microbial communities. A 60-week mesocosm study with a factorial arrangement of soil type (clay loam and loamy sand) and endophyte infection (with and without) was conducted to determine changes in soil microbial community function (substrate utilization using Biolog), structure (phospholipid fatty acid profile), and glomalin concentration. Microbial utilization of carbohydrate, carboxylic acid, and miscellaneous substrate groups was lower in soil planted to endophyte-infected tall fescue than in soil planted to endophyte-free tall fescue. Gram-positive bacteria, arbuscular mycorrhizae, and glomalin in small (0.25-1.0 mm) and large (>1 mm) water-stable macro-aggregates were also negatively affected by endophyte infection. Although microbial changes due to endophyte infection were not ubiquitous and overwhelming, they were consistent with previous observations of reduced decomposition of endophyte-infected tall fescue plant litter, which may lead to greater soil C sequestration. [ABSTRACT FROM AUTHOR]