Reciprocal effects between arbuscular mycorrhizal fungi (AMF) and plant communities are essential to study the complexity of interactions in a grassland ecosystem. Here, we investigated the effects of plant community density and composition on AMF colonization, plant growth, and reproduction investment. We developed an experimental system with three compartments, each containing either three or six Medicago truncatula Gaertn. plants, or three M. truncatula plants associated with three Silene vulgaris (Moench) Garcke plants. All three compartments shared the same common mycorrhizal network built either by Rhizophagus irregularis MUCL 43194, by Rhizophagus clarus MUCL 46238, or by both AMF in association grown in a central compartment on Plantago lanceolata L. Our results demonstrate an absence of effect of plant density but a positive influence of mixed cultures on AMF root colonization compared with monocultures. This higher AMF development resulted in a positive feedback on shoot biomass and number of flowers and fruits produced by M. truncatula. Although both fungal strains were present in root systems, co-inoculation did not generate a synergistic effect on plant development. These results highlight the importance of plant associations on AMF dynamics, which requires further investigation at the community scale to improve our understanding of the intricate AMF - host plant relationships.