Phosphate-solubilizing bacteria drive transformation and release of phosphorus in phosphorus-enriched sludge biochar: Efficacy validation and multifaceted mechanisms

Phosphorus (P) is an essential non-renewable nutrient; however, its inefficient recovery from sewage sludge leads to considerable losses. Although phosphate-solubilizing bacteria (PSB) can convert insoluble P into bioavailable forms, their application is limited by poor colonization and stress tolerance. In this study, a biochar-microbe composite (PSB1@PBC) was prepared by immobilizing PSB Raoultella ornithinolytica (PSB1) on P-enriched sludge biochar (PBC) to enhance P transformation and release. PSB1@PBC improved P release and bacterial survival, with cell density 1.51 times that of free-living cells. Hedley sequential extraction showed a decrease in stable HCl-P from 13.20% to 8.90%, whereas bioavailable NaHCO 3 -P increased by 1.96%. P release followed a biphasic pattern, characterized by an initial rapid phase (0–4 days) followed by a slow and stable phase, peaking at 26.80 mg/g (17.5 times that of unmodified PBC) after 4 days. This improvement is attributed to three synergistic mechanisms: (i) microbial acidification via malonic acid secretion, which promotes mineral-bound P dissolution; (ii) enzymatic mineralization, which increases phosphatase activity to hydrolyze organic P; and (iii) chelation-assisted dissolution through siderophore production. The release of P shifted from slow chemical dissolution in PBC to rapid microbial solubilization in PSB1@PBC, with biochar transport becoming the rate-limiting step. This study demonstrated a sustainable strategy for P recovery, highlighting the potential of PSB1@PBC to enhance nutrient cycling and agricultural sustainability.