Interaction Between Nutrient-Laden Biochar and PGPR Reshapes Rhizosphere Microbiome to Reclaim Coastal Saline–Alkali Soil Fertility

Biochar and plant growth-promoting rhizobacteria (PGPR) are promising for coastal saline–alkali soil remediation, but their combined effect is often limited by nutrient scarcity. This study investigated whether nutrient-laden biochar (saturated with livestock wastewater) synergizes with a PGPR inoculant (Paenibacillus mucilaginosus PM12) to enhance maize productivity by reshaping the rhizosphere microbiome. A field experiment included five treatments: control (CK), sheep manure biochar alone (BC), nutrient-laden biochar (NBC), BC + PGPR (MBC), and NBC + PGPR (MNBC). The MNBC treatment showed the most pronounced improvements, increasing maize yield by 52.5% compared to CK, while reducing soil pH by 0.30 units and enhancing soil organic matter, total nitrogen, and available phosphorus. Metagenomic analysis revealed that MNBC uniquely enriched beneficial genera (e.g., Nocardioides) and saprotrophic Basidiomycota, while suppressing pathogenic Fusarium. This restructuring elevated the genetic potential for nitrogen transformation, phosphorus solubilization, and carbon metabolism. Structural equation modeling identified increased soil available phosphorus and total nitrogen as the primary direct drivers of yield enhancement. The integration of nutrient-laden biochar and PGPR creates a synergistic system that reclaims saline–alkali soil by alleviating stress, supplying nutrients, and directing the assembly of a functional microbiome.