Phosphoric acid-triggered phosphorus transformation: Structural regulation and long-term phosphorus release mechanism...

Addressing the low phosphorus-use efficiency in soils requires sustainable strategies for phosphorus replenishment. Herein, biochar-based slow-release phosphorus fertilizers (BSRPFs) were fabricated through co-pyrolysis of waste pomegranate branches with Ca₃(PO₄)₂, while the addition of H₃PO₄ was precisely regulated to manipulate internal phosphorus transformation and release behavior. Results demonstrate that phosphate particles were successfully loaded onto the BSRPFs surface. Increasing H₃PO₄ dosage raised the total phosphorus (25.26-30.20 mg g^-1) and inorganic phosphorus (24.23-28.47 mg g^-1) contents and promoted the conversion of organic phosphorus to inorganic phosphorus during pyrolysis. BSRPFs obtained at higher temperatures exhibited enhanced10 aromatization and structural stability, accompanied by the formation of Ca₈H₂(PO₄)₃(OH)) and a dominant HCl-P fraction, which collectively contributed to superior slow-release performance. Phosphorus release was positively correlated with H₃PO₄ addition and followed kinetics best described by the Pseudo-second-order (R² > 0.988) and Elovich models (R² > 0.935). Soil incubation experiments confirmed that BSRPFs could sustainably supply available phosphorus without significantly altering soil pH. Systematic evaluation of H₃PO₄ addition ratios revealed that a moderate addition (30%) provides an optimal balance, yielding a high total phosphorus content (27.29 mg g^-1) while achieving a desirable slow-release profile (41.04%) and maximizing soil available phosphorus increase. This work highlights a feasible route to convert agricultural waste into efficient slow-release phosphorus fertilizers via H₃PO-modulated co-pyrolysis, offering a promising strategy for improving phosphorus utilization in sustainable agriculture.