Temperature-Dependent Phosphorus Speciation and Release from Magnesium-Rich Biochars

Thermal conversion of biomasses into biochar-based slow-release fertilizers presents an attractive approach to recover and reuse phosphorus (P) in agriculture, and the presence of magnesium (Mg) can stabilize the P into poorly leachable yet plant-available forms. Nevertheless, the effect of pyrolysis temperature on P speciation and release from Mg-rich biochars is poorly understood. To understand this, spent green tea leaves were treated with magnesium oxide (MgO) and phosphoric acid (H₃PO₄) and pyrolyzed at 300–700 °C. By using X-ray diffraction (XRD) and FTIR spectroscopy, three types of Mg phosphate forms were identified in the Mg/P-modified biochars: (i) crystalline Mg pyrophosphate (Mg₂P₂O₇) at 400–700 °C, (ii) crystalline Mg orthophosphate (Mg₃(PO₄)₂) at 700 °C, and (iii) an amorphous ortho-/condensed phosphate phase. Sequential (Hedley) extraction confirmed that increasing the pyrolysis temperature from 300 to 500 °C decreased the orthophosphate (P_i) extractability and increased that of polyphosphate (P_x), and the readily available P (in H₂O and 0.5 M NaHCO₃) was high at 300–500 °C. Biochars produced at 600 and especially 700 °C contained primarily long-term available P species (extractable in 1 M HCl). This temperature-dependent extractability was ascribed to phosphate phase transformations and increased crystallinity at higher temperatures. Similar trends in P release with pyrolysis temperature were found in kinetic release in water, and post-kinetics characterization confirmed that struvite or struvite-K and Mg₂P₂O₇ were rate-limiting phases for phosphate dissolution. These results provide mechanistic insights into the P transformations for the design of more efficient Mg-rich biochar-based slow-release fertilizers with minimal P risk losses.