AM fungi reduce grass–legume competition by increasing nutrient access.

Aims: Mixed communities of plant species 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 soil depths, 15N tracer was injected into soil at 3- and 25-cm depths of microcosms planted with 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 plant species on mixed communities. Niche differentiation 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 enhanced nutrient uptake and facilitated N2 fixation in legumes at 3 and 25 cm soil depths. Conclusion: Our results suggest AM fungi strongly influence 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. [ABSTRACT FROM AUTHOR]