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Aims Plant species mixtures with different functional traits or microbial associations can more fully utilize soil nutrient pools. However, there is a gap in our understanding of the synergistic influences of arbuscular mycorrhizal (AM) fungi in nutrient capture and resource partitioning across different soil depths in mixed plant communities, as well as their possible effects on plant coexistence. To address these knowledge gaps, we assessed the effect of AM fungi on nutrient uptake and competition between shallow- and deep-rooted pairs of grass and legume species. Methods To quantify the mycorrhizal mediation of N uptake at different rooting depths, 15N tracer was injected into soil at 3- and 25-cm depths of monoculture or bi-species mixtures, with or without AM fungi. Results AM symbioses reduced plant competition by equalizing access to N and subsequently increasing shoot N and P concentrations of co-cultivated plant species. Niche differentiation, based on rooting depth, had minimal effects on grass and legume competitive interactions. Both deep-rooted plant species, i.e., Medicago sativa and Dactylis glomerata, and shallow-rooted species, i.e., Lolium perenne and Trifolium repens, primarily acquired nutrients from shallow soil layers. However, AM fungi significantly increased the rooting depth of both shallow- and deep-rooted plant species, enhancing nutrient uptake and N2 fixation from depths of both 3 and 25 cm. Conclusion Our results suggest a strong influence of AM fungi on host-plant competitiveness and vertical nutrient capture in mixed plant communities, underscoring that microbial mutualist-mediated nutrient acquisition is a key driver of gains in productivity of grass–legume mixtures.