Grass-Fungal Endophyte Symbiosis: Effects on Nitrogen Fixation and Dynamics in a Kentucky Pasture.

Tall fescue, an important forage species in the eastern U.S., can associate with an endophytic fungus, Epichloë coenophiala. However, endophyte presence can negatively impact grazing animals through production of toxic alkaloids; therefore, fescue infected with non-toxic ‘novel’ strains of the endophyte is being increasingly utilized by forage producers, but the environmental consequences of this symbiotic change are largely unknown. In this study, the impact of grass-endophyte symbiosis on nitrogen fixation and dynamics was investigated by measuring ¹⁵N natural abundance in plant and soil samples. The study site consisted of tall fescue that was either infected with the common toxic strain of the endophyte, infected with one of two non-toxic strains, endophyte-free, or contained an equal mixture of endophyte treatments.  To assess the effect of endophyte presence and strain on the amount of nitrogen derived from biological fixation via legume symbiosis and nitrogen use in co-occurring tall fescue, δ¹⁵N natural abundance was measured in red clover (RC), tall fescue associated with red clover (TF+RC), and tall fescue not associated with clover (TF-RC) collected from each of the tall fescue – endophyte treatment plots. In addition, δ¹⁵N natural abundance and potential activity of free-living nitrogen-fixing organisms were measured in bulk soil samples from each plot over a 4 year period in order to gain insight regarding long-term changes in N-cycling. While endophyte presence and strain had no significant effect on red clover or soil δ¹⁵N, both impacted the δ¹⁵N of tall fescue.  For TF(-RC), endophyte-free samples were more ¹⁵N-depleted than endophyte-infected, but for TF(+RC) samples, fescue infected with one of the novel strains was more enriched with ¹⁵N than common toxic endophyte-infected stands. These results indicate that endophyte infection and strain can significantly affect nitrogen dynamics in both tall fescue and neighboring red clover.