Synergistic Effects of Arbuscular Mycorrhizal Fungi and Rhizobia on Nutrient Acquisition, Growth Promotion, and Protein Profile Optimization in Alfalfa

Arbuscular mycorrhizal fungi (AMF) and rhizobia establish mutualistic symbioses with plant roots. This study focused on their effects on alfalfa (Medicago sativa L.) growth, root morphology, nutritional quality, photosynthetic characteristics, and protein fractions; eight treatments were established: single inoculation with three AMF strains (Funneliformis mosseae, Claroideoglomus etunicatum, Glomus versiforme) or Sinorhizobium meliloti (Sm), dual co-inoculation of each AMF with Sm, and a non-inoculated control (CK). Results showed that all AMF successfully colonized alfalfa roots, with co-inoculation increasing both mycorrhizal colonization rate and nodule number. The F. mosseae × Sm treatment achieved the highest colonization (83.3%) and nodule count (76 per plant). Across two years, this treatment significantly increased aboveground biomass, plant height, and stem diameter (p < 0.05). C. etunicatum × Sm significantly reduced acid detergent fiber content, while dual inoculation markedly improved net photosynthetic rate and light-use efficiency. All inoculations increased rapidly (PB1) and intermediate-degradable protein (PB2) but decreased non-protein nitrogen (PA) and bound protein (PC). In conclusion, AMF and rhizobia exhibit significant synergistic effects. Co-inoculation (F. mosseae × Sm and C. etunicatum × Sm) enhances alfalfa productivity by optimizing root structure, improving photosynthesis, and regulating nitrogen metabolism.