Optimal stimulation of rhizosphere nutrient mobilization and grassland plant growth by intermediate but not by high diversity of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are prevalent in grasslands and could aid in their restoration. It's still unclear whether increased AMF species diversity is universally beneficial for grassland plants. This study examined the impact of AMF diversity on plant growth using Lolium perenne L., Dactylis glomerata L., and Trifolium repens L in degraded grassland soil with the addition of five AMF species both individually and in various combinations. We found that a mix of three AMF species (3AMF) maximized plant growth, increasing shoot biomass by up to 58% compared to the non-mycorrhizal control, while a five-species mix (5AMF) showed a lesser effect. The 3AMF treatment enhanced soil nitrogen and phosphorus cycling, increased activities of key enzymes (e.g., acid phosphatase by ∼40%), and favored three key bacterial genera ( Citrifermentans, Rhodococcus, Flavisolibacter ) positively correlated with nutrient cycles. In contrast, 5AMF intensified competition for plant-derived carbon, likely reducing the abundance of these functional bacteria. Synthesis: the study provides evidence that the benefits of AMF diversity follow a non-linear pattern, with an optimum at intermediate diversity, mediated by shifts in cooperation and competition with rhizosphere bacteria. This insight emphasizes the importance of optimizing rather than maximizing AMF diversity in grassland restoration strategies. • Intermediate AMF diversity optimizes grassland plant growth. • Three-AMF mix boosts nutrient cycling via key bacterial genera. • AMF-bacteria shift from cooperation to competition with diversity. • Carbon competition drives reduced benefits at high AMF diversity. • Study informs microbial management for grassland restoration.