Soil symbiotic microorganisms promote recruitment of legume forage species in grassland ecosystems by enhancing provision for nutrients. We studied the effects of arbuscular mycorrhizal fungi (AMF) on plant growth, mineral nutrition and biological nitrogen fixation of Melilotus alba Med., a forage legume with potential use in subtropical grasslands. Responses to AMF inoculation (a mix of Funneliformis mosseae, Rhizophagus irregularis and Simiglomus hoi) and phosphate fertilization were evaluated in plants growing for 12 weeks in a subtropical soil. All plants were previously inoculated with a suspension of Rhizobium meliloti. Plant responses were assessed in terms of mycorrhizal colonization, biomass production, mineral nutrition (P and N) and proportion of biological nitrogen fixation by 15N natural abundance method. The results showed that when soil phosphorus availability was very low, regardless of the degree of AMF colonization, the generation of mycorrhizae did not reach a significant impact on plant nutrition and biomass production. But under a relatively higher phosphorus provision, AMF symbiosis showed positive effects on phosphorus accumulation, nitrogen nutrition and biomass production, linked to an enhancement of biological nitrogen fixation. The results strengthen our understanding of how synergistic effects between belowground symbionts (in this case AMF+Rhizobium) could promote the recruitment of forage legumes in subtropical grassland soils.