Rationalizing Microbial Strategies for Coastal Soil Restoration: Functional Complementarity and Trade‐Offs in PGPR – AMF Interactions

Coastal sandy soils pose a challenge for microbial nutrient retention due to low organic matter and high leaching. While plant growth‐promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) show promise, their functional synergies in these ecosystems—particularly beyond phosphorus limitation—remain unclear. We investigated their complementary roles in enhancing soil function and Casuarina equisetifolia growth under nutrient‐depleted conditions. A 150‐day pot experiment evaluated the synergistic effects of PGPR ( Paenibacillus kribbensis LB18/LB19, P. cellulositrophicus LB46, Brucella pseudogrignonensis LQ10) and AMF ( Funneliformis mosseae ) through soil enzymatic activity, nutrient dynamics, and plant growth metrics. Structural equation modeling (SEM) and redundancy analysis (RDA) were employed to dissect soil–plant–microbe interactions. Dual PGPR‐AMF inoculation enhanced soil enzymatic activity (dehydrogenase: 1.6‐fold; catalase: +57%) and total nitrogen (18.28 mg g −1 ). However, single inoculation with LQ10 outperformed dual treatments, increasing plant height (+102.2%) and biomass (+254.1%) via root architecture optimization. Indole‐3‐acetic acid (IAA) synthesis and cellulase activity correlated strongly with nutrient cycling (AN: r = 0.54–0.61; AP: r = 0.56–0.67). SEM identified soil nutrient dynamics ( R 2 = 0.506) and antioxidant enzyme networks as growth determinants, with potassium availability ( λ = 0.835) and superoxide dismutase activity ( λ = 0.527) dominating RDA. Structural equation modeling (SEM) revealed that dual inoculation triggered metabolic trade‐offs, suppressing host plant secondary metabolism ( β = −0.514) while concurrently enhancing plant physiological resilience, as evidenced by coordinated upregulation of antioxidant enzymes and osmoprotectant levels. Microbial functional traits (e.g., IAA production, cellulolysis) critically regulate soil–plant feedback in nutrient‐poor systems. We propose a time‐resolved framework for microbial consortia design, where PGPR–AMF synergism is constrained by host carbon allocation thresholds and potassium availability.