Fungal endophyte symbiosis alters nitrogen source of tall fescue host, but not nitrogen fixation in co-occurring red clover

Background and aims: Infection of tall fescue with the common toxic fungal endophyte Epichloë coenophiala harms livestock via toxic alkaloid production; therefore, non-toxic ‘novel’ strains of the endophyte have been developed and released. How different endophyte strains impact biological nitrogen fixation (BNF) in mixed species pastures is unknown. We asked whether novel endophyte or common toxic endophyte-infected (NE+; CTE+) tall fescue affects symbiotic and non-symbiotic BNF, and utilization of biologically-fixed nitrogen in tall fescue. Methods: Tall fescue was planted either endophyte-free (E-), infected with CTE, two non-toxic strains AR542 NE, AR584 NE, or a blend of endophyte treatments. We measured natural abundance of ¹⁵ N in plant and soil samples, and conducted soil acetylene reduction assays. Results: Endophyte presence and strain significantly affected the δ¹⁵N of tall fescue. Near red clover, CTE+ and AR584 NE+ tall fescue were most ¹⁵ N-depleted; but away, E- tall fescue was most ¹⁵ N-depleted. Endophyte strain significantly influenced N concentration in red clover, but not symbiotic or non-symbiotic BNF. Conclusions: Endophyte strains produce different effects on tall fescue’s competitive ability and nitrogen utilization. In mixed pastures, deployment of NE strains for decreased alkaloid toxicity will differentially impact use of biologically fixed nitrogen in tall fescue and nitrogen concentration in red clover.