Amelioration of Sodic Soil and Enhancement of Alfalfa (Medicago sativa L.) Growth by a Consortium of Lactiplantibacillus pentosus and PGPR

Sodic stress and soil-borne phytopathogens impose severe dual constraints on alfalfa (Medicago sativa L.) production in arid agro-ecosystems. To address these constraints, an alkali-tolerant bacterial strain, Lactiplantibacillus pentosus QC, was isolated and formulated into a quadruple-strain consortium with functional plant growth-promoting rhizobacteria (Bacillus velezensis Y, Bacillus pumilus N, and Pseudomonas monteilii M). L. pentosus QC exhibited broad alkali tolerance (pH 9.0), homofermentative acidification (medium pH dropping to 4.1 within 24 h), and in vitro mycelial inhibition (58.71%) against Fusarium oxysporum. A 9-treatment pot experiment demonstrated that the quadruple inoculant (T8) provided superior multifaceted amelioration: reducing rhizosphere pH by 0.31 units, soil sodium ion content (Na+) by 38.0%, and sodium adsorption ratio and exchangeable sodium percentage by 45.2% and 44.1%. The concurrent enhancement in soil physical structure (total porosity reaching 45.93%) was accompanied by marked increases in soil urease, sucrase, and alkaline phosphatase activities. These rhizosphere modifications coincided with root system architecture adjustments (expanding total root length and volume). Concurrently, leaf antioxidant enzyme activities (superoxide dismutase, peroxidase, and catalase) were higher in T8 than in CK, malondialdehyde (MDA) content was lower by 59.4%, and total plant dry biomass was greater by 113.3%. This biological acidification–root remodeling–systemic defense framework presents a promising bio-management strategy for sodic soil reclamation, although in vivo colonization dynamics and true synergy relative to a PGPR-only control require further validation.