Microbial Amendments for Improving Alfalfa Productivity and Soil Health in Saline–Alkali Environments: Underlying Mechanisms and Practical Challenges

Alfalfa (Medicago sativa L.) is a widely cultivated forage legume, but its productivity is severely constrained under saline–alkali conditions due to impaired nutrient uptake, ionic imbalance, and disruption of its symbiosis with Ensifer meliloti. Conventional reclamation methods rely on chemical inputs, which are often costly and environmentally unsustainable. This review synthesizes sustainable strategies for improving alfalfa performance in salt-affected soils, with particular emphasis on microbial bioinoculants, including plant growth-promoting rhizobacteria, arbuscular mycorrhizal fungi, rhizobia, and endophytes. These microorganisms can alleviate stress by improving ion homeostasis, osmotic adjustment, antioxidant defense, phytohormone regulation, and nutrient solubilization. Recent findings indicate that saline–alkali stress and plant genotype are associated with shifts in alfalfa rhizosphere and endophytic communities, including changes in potentially beneficial taxa. The review also integrates physiological, molecular, and multi-omics insights underlying stress tolerance, along with advances in breeding resilient cultivars. It also highlights microbiome dynamics, inoculant delivery systems, and agronomic practices that influence field performance. Overall, combining stress-tolerant alfalfa genotypes with tailored microbial consortia and improved soil management offers a promising strategy to improve alfalfa productivity and support saline–alkali soil management, although long-term field validation remains limited.