The roots of living plants fuel a complex network of microbial interactions, involving both root-infecting and free-living microorganisms. About 80% of all land plants live in symbiosis with arbuscular mycorrhiza (AM) fungi that provide the host plant with mineral nutrients in exchange for photosynthates. Microfaunal interactions with mycorrhiza provide an ideal model system to study the interplay of plants with multiple microbial root associations. Bacterivorous protozoa release nutrients from consumed bacterial biomass (microbial loop), which may be translocated by AM mycelium to plant roots. Bacteria-protozoa interactions induce the development of an extensive and highly branched root system and this likely affects plant AM-symbiosis. The response of plants to these direct and indirect microbial interactions was investigated in a two-compartment microcosm system. Both soil compartments were inoculated with a diverse community of soil bacteria. The forb Plantago lanceolata colonized by the arbuscular mycorrhizal (AM) fungus Glomus intraradices was grown in one compartment, spatially separated from a 15N labelled patch of grass litter in another soil compartment. Both compartments were separated by either a 0.45 μm membrane (no AM access) or a 20 μm mesh (AM-access to the patch). The latter treatment was set up either only with mycorrhiza, or with protozoa inoculated in the patch, or with protozoa inoculated on both sides. Mycorrhiza alone was not able to compete with bacteria for mineral N in the litter patch, it rather acted as a parasite for plant nitrogen. In contrast, protozoa in the litter patch increased N-mineralization and the N-transfer via VA-mycelia to the plant host, thus rendering the plant AM symbiosis mutualistic. The most remarkable plant growth response occurred when protozoa were inoculated in the plant rhizosphere: Protozoa strongly increased plant net CO2 assimilation and root growth, leading to increased total root mycorrhization and C-transfer into the litter patch. As a consequence, N-mineralization in the patch and N-transfer to the plants increased. Remarkably, Protozoa-mycorrhiza-plant interactions resulted in a win-win situation for each of the partners, suggesting that for the full exploitation of the mutualistic potential of rhizosphere organisms more than two partners are necessary.