Background and aims <p>In multi-symbiotic plant communities, interactions between below- and aboveground plant symbionts modify nutrient acquisition for host and co-occurring plants, yet the relative importance of distinct pathways remains poorly resolved.&#xa0;We aimed to determine how common mycorrhizal networks (CMNs) modulate the interactive effects of foliar endophytes and arbuscular mycorrhizal fungi (AMF) on nitrogen (N) dynamics and plant interactions in a multi-symbiotic grass-legume system.</p> Methods <p>We conducted a factorial greenhouse experiment using three-compartment microcosms. Central compartment contained <i>Trifolium repens</i>. One outer compartment held non-endophytic grass <i>Bromus catharticus</i>, and <i>Lolium multiflorum</i> (with high/low <i>Epichloë occultans</i> status) on the other. Forty-micrometre mesh allowed CMN connections while preventing root contact. We evaluated how endophyte status and CMN connection in <i>L. multiflorum</i> modified fixed-N transfer and soil N uptake within neighbouring plants using ¹⁵N natural abundance.</p> Results <p>Fixed-N transfer to grasses depended on the interaction between endophyte status of <i>L. multiflorum</i> and its CMN connection. The endophyte enhanced N transfer to its host but restricted transfer to the neighbour. <i>B. catharticus</i> received fixed-N transfer only when its low-endophyte neighbour was disconnected from the CMNs. Low-endophyte <i>L. multiflorum</i> depended more on the CMNs for soil-N uptake. Growth resembled these dynamics: CMN connection enhanced biomass in both grasses under low-endophyte status, whereas high-endophyte status favoured <i>L. multiflorum</i> biomass over that of <i>B. catharticus.</i></p> Conclusions <p>Foliar endophytes and CMNs interactively shape N dynamics and competitive balance among co-occurring grasses. Endophytes regulate N partitioning, shifting plant interactions from facilitation to competition, particularly when plants remain connected through the CMN.</p>