pH-dependent release of goethite-bound phosphate by biochar-derived DOM: Non-negligible role of aromatic nitrogen and highly unsaturated sulfur compounds.

Iron oxide-mediated phosphate immobilization (e.g., goethite) in acidic soils severely constrains phosphorus bioavailability through mineral-water interfacial reactions, resulting in a significant agricultural bottleneck. Enhancing phosphorus availability is therefore essential to sustain crop yields while minimizing phosphorus leaching into aquatic systems. Biochar-derived dissolved organic matter (BDOM), recognized for enhancing nutrient accessibility and mediating redox-driven transformations of pollutants through electron-donating functional groups, remains poorly understood in its interactions with goethite-bound phosphate (Ge-P). This study elucidates the pH-dependent mechanisms of Ge-P release by BDOM, emphasizing the roles of aromatic nitrogen and highly unsaturated sulfur compounds. The results demonstrate that the addition of BDOM at pH 7.5 induces 55.6 % phosphate release from Ge-P through synergistic Fe(III) reductive dissolution and ligand competition. This release efficiency is 21.5 times higher than that at pH 6.0 and 3.09 times greater than with pH adjustment alone. Conversely, at pH 4.5, BDOM-associated phosphate undergoes irreversible adsorption onto Ge-P. Fluorescence spectroscopy identifies low-emission-wavelength humic-like and tryptophan-like compounds in BDOM as dominant contributors to phosphate mobilization. Fourier-transform ion cyclotron resonance mass spectrometry reveals pH-dependent intensification of interactions between BDOM components, especially aromatic nitrogen and highly unsaturated sulfur compounds, and Ge-P. Aromatic amino (Ar-NH), carboxylic (-COOH), and thiol (-SH) moieties are further identified as dominant redox groups driving phosphate liberation via electron transfer and ligand displacement. This study proposes a biochar strategy to enhance mineral-bound phosphorus availability in acidic soils, synergizing fertilizer-efficient sustainable agriculture with water quality-protected environmental governance against eutrophication.