Biochar promotes the dissolution of inorganic inactive phosphorus by strengthening phoD-harboring bacterial communities during rice stover and sheep manure co-composting

Inorganic phosphorus (P) predominantly exists as insoluble fractions in natural soil environments, particularly in calcareous soils where calcium-bound stable P and occluded P represent major non-labile P pools. This study investigated the regulatory effect of biochar amendment on the transformation and fractionation of inorganic P during rice stover and sheep manure co-composting. Via high-throughput sequencing of the alkaline phosphatase gene (phoD), cultivable microbial isolation and bacterial ecological network analysis, the ecological function of phoD-harboring bacterial communities in mediating the solubilization of inorganic non-labile P was systematically elucidated. The results demonstrated that biochar addition effectively facilitated the dissolution of occluded P (O-P) and calcium-bound stable P (Ca₁₀-P), while simultaneously elevating the richness, diversity and metabolic activity of phoD-harboring bacterial assemblages. Biochar amendment also optimized the topological structure of phoD-harboring bacterial networks, manifested as enhanced network complexity, interspecies connectivity and community stability. Five culturable phosphate-solubilizing bacterial strains affiliated to Escherichia, Pseudomonas, Brucella, Cellulosimicrobium and Bacillus were screened and identified, all possessing the capacity to decompose recalcitrant inorganic P fractions. Redundancy analysis (RDA) indicated that pH, total organic carbon (TOC) and alkaline phosphatase (ALP) activity constituted the dominant abiotic drivers shaping phoD-harboring bacterial community assembly and P fraction evolution. Structural equation modeling (SEM) further verified that the synergistic regulation of pH, TOC and phoD-harboring microbiota collectively accelerated the solubilization of inorganic non-labile P in biochar-amended compost. This work clarifies the microbial mechanism underlying biochar-modulated P activation, providing a theoretical basis for efficient P recycling and availability improvement in calcareous agroecosystems.