Phosphorus Release from Mineral−Biochar Composites: Implications for Soil Phosphorus Speciation and Retention

Mineral modification of biochar enhances its structural stability, improving nutrient retention. This study evaluated spent mushroom substrate biochar modified with bentonite and diatomaceous earth at 350 and 700 °C, focusing on carbon stability and phosphorus release. Results indicated that higher pyrolysis temperatures (700 vs 350 °C) reduced oxygen-containing functional groups, aliphatic compounds, and dissolved organic carbon in both pristine biochar and mineral−biochar composites. At 700 °C, mineral−biochar composites slightly increased aromaticity, potentially via Si−O−C associations, and significantly reduced phosphorus release, primarily due to mineral-mediated hydroxyapatite formation. Soil incubation experiments revealed distinct phosphorus transformation patterns, depending on the soil type, when mineral−biochar composites were added. Alkaline sandy loam favored hydroxyapatite accumulation, whereas acidic sandy clay loam showed a gradual conversion of Fe(III)-phosphate to hydroxyapatite, thereby reducing phosphorus mobility. These findings provide mechanistic insight into phosphorus stabilization by mineral−biochar composites and support their potential use as controlled-release P amendments in agricultural soils.