Birnessite-Mediated Phosphorus Transformation and Speciation in Dissolved and Soil Organic Matter

Phosphorus (P) speciation in natural environments is shaped by mineral interactions. Manganese oxy(hydr)oxides (MnOx) are powerful oxidants in nature and may promote the oxidative transformation of organic P compounds; yet little is known about the role of MnOx in the molecular transformation and speciation of P from natural organic matter. Here, we investigated the molecular transformation of vermicompost-derived organic P under acidic and alkaline pH conditions in dissolved organic matter (DOM) and soil over a 35 d period. Phosphorus K-edge X-ray absorption near edge structure revealed the formation of birnessite-adsorbed phosphate and Mn(II/III) phosphate at pH 4 and calcium phosphate at pH 8 in the DOM system. Birnessite-adsorbed phosphate was dominant in soils, followed by Mn(III) phosphate after 35 d. Fourier transform ion cyclotron resonance mass spectrometry indicated enhanced organic P sorption and transformation into higher molecular weight aromatic compounds under acidic conditions. These findings reveal that birnessite not only mediates P speciation through surface and redox-driven interactions but also facilitates organic P stabilization via molecular transformation. The study provides new insights into how MnOx can influence both P fate in natural environments, with implications for mechanisms controlling P mobility and long-term P retention in soils.