Foliar fungal endophytes and common mycorrhizal networks shape nitrogen dynamics in multi-symbiotic grass–legume systems

Abstract

Background and aims In multi-symbiotic plantcommunities, interactions between below- andaboveground plant symbionts modify nutrient acquisitionfor host and co-occurring plants, yet the relativeimportance of distinct pathways remains poorlyresolved. We aimed to determine how common mycorrhizalnetworks (CMNs) modulate the interactiveeffects of foliar endophytes and arbuscular mycorrhizalfungi (AMF) on nitrogen (N) dynamics andplant interactions in a multi-symbiotic grass-legumesystem.Methods We conducted a factorial greenhouseexperiment using three-compartment microcosms.Central compartment contained Trifolium repens.One outer compartment held non-endophytic grassBromus catharticus, and Lolium multiflorum (withhigh/low Epichloë occultans status) on the other.Forty-micrometre mesh allowed CMN connectionswhile preventing root contact. We evaluatedhow endophyte status and CMN connection in L.multiflorum modified fixed-N transfer and soil Nuptake within neighbouring plants using 15N naturalabundance.Results Fixed-N transfer to grasses depended onthe interaction between endophyte status of L. multiflorumand its CMN connection. The endophyteenhanced N transfer to its host but restricted transferto the neighbour. B. catharticus received fixed-Ntransfer only when its low-endophyte neighbour wasdisconnected from the CMNs. Low-endophyte L.multiflorum depended more on the CMNs for soil-N uptake. Growth resembled these dynamics: CMNconnection enhanced biomass in both grasses underlow-endophyte status, whereas high-endophyte statusfavoured L. multiflorum biomass over that of B.catharticus.Conclusions Foliar endophytes and CMNs interactivelyshape N dynamics and competitive balanceamong co-occurring grasses. Endophytes regulate Npartitioning, shifting plant interactions from facilitationto competition, particularly when plants remainconnected through the CMN.