| Plants interact in myriad ways with microorganisms to influence ecosystem processes such as nutrient cycling, which can regulate ecosystem response to global change. One important plant-microbe symbiosis occurs between cool-season grasses and asexual fungal Epichloe endophytes, such as tall fescue (Schedonorus arundinaceus) and Epichloe coenophiala. Because the common toxic strain of the endophyte (CTE) harms grazing livestock, non-livestock toxic endophyte (NTE) strains have been developed and are increasingly deployed in pastures. Little is known about how these symbioses impact other plant-microbe interactions and microbe-mediated soil processes in grassland ecosystems. I conducted three studies to determine how E. coenophiala presence (+) or absence (-) and differences in endophyte strain affected plant-microbe-soil interactions both within tall fescue and in surrounding plants. I hypothesized that presence of CTE in tall fescue (CTE+) would suppress presence and/or activity of other microbial symbionts and related processes compared to E- tall fescue, and NTE+ tall fescue effects would be intermediate.;My first field study examined how endophyte presence and strain in tall fescue influenced symbiotic biological nitrogen fixation (BNF) in red clover, biologically-fixed N uptake in tall fescue, and non-symbiotic BNF in soils. I found that tall fescue hosting different NTE+ strains utilized different amounts of biologically-fixed N. My second field study investigated how endophyte presence and strain impacted belowground mycorrhizal colonization within the same host plant. I found no significant differences in either AMF or dark septate endophyte (DSE) colonization in tall fescue in this study. In my third field study, I investigated how these belowground symbioses were potentially altered both by tall fescue-E. coenophiala genetics and future climate change. AMF functional structures such as arbuscules in roots and extraradical hyphae in soils were significantly affected by tall fescue genotype and endophyte status. I also found that some competitive symbiont interactions were ameliorated whereas others were exacerbated by future climate change conditions such as warming and added precipitation.;Overall, the results of these studies suggest that genetically distinct E. coenophiala-tall fescue associations, through alteration of plant-microbe-soil interactions, will have divergent roles and long-term impacts on host-symbiont species interactions and nutrient cycling within pasture ecosystems.;KEYWORDS: arbuscular mycorrhizal fungi, biological nitrogen fixation, climate change, dark septate endophyte, grassland, Neotyphodium . |
