Fungal endophyte and tall fescue cultivar interact to differentially effect bulk and rhizosphere soil processes governing C and N cycling

Tall fescue (Lolium arundinaceum (Schreb.)) is a cool-season perennial grass within which can live a fungal endophyte (Epichloë coenophiala) thought to provide enhanced edaphic and climactic stress tolerance to the host compared to non-infected individuals. Our prior research demonstrated that a variety of root exudate compounds released from tall fescue were differentially affected by tall fescue cultivar, endophyte genotype and their interaction. Changes in root exudates and associated microbial communities could influence soil processes, including carbon and nitrogen cycling, but these effects may differ depending on fescue and endophyte genetics. To test this, we collected rhizosphere and bulk soil samples from six year old field plots located in Lexington, KY planted with two different tall fescue cultivars (PDF and 97TF1), each containing four endophyte treatments [endophyte-free (E−) or infected with one of three strains of E. coenophiala (common toxic, novel AR542E+ and novel AR584E+)]. The influence of fescue cultivar, endophyte strain, and soil sample location (rhizosphere vs. bulk) were assessed for soil organic carbon, soil organic nitrogen, particulate and non-particulate organic matter (POM and n-POM, respectively) - C and – N, and dissolved organic carbon and nitrogen (DOC, DON) pools. Soil functional aspects were evaluated by measuring soil respiration and the activity of seven different soil enzymes related to C, N and P cycling. We found that rhizosphere soils had greater microbial biomass, potential enzyme activity, and oxygen utilization, but lesser POM-N concentrations than bulk soils. In rhizosphere soils, tall fescue cultivar had the greatest influence on soil microbial community structure, while endophyte genotype had a stronger influence on soil C fractions (notably POM-C). Changes in root system architecture, biomass, and tissue composition, together with root exudate chemistry, which we have shown in previous studies to be affected by endophyte infection and fescue cultivar, likely explain these findings. There was greater POM-N in bulk soils which was influenced by the interaction of endophyte and cultivar potentially due to differences in the chemical composition of the tissues brought about by this interaction. Our results support current observations that tall fescue cultivar and fungal endophyte strain influence soil C and N cycling and, by analyzing bulk and rhizosphere soils separately, go further to show the level of influence cultivar and endophyte have within each of these compartments.