Solid-Phase Speciation and Solubility of Phosphorus in an Acid Sulfate Paddy Soil during Soil Reduction and Reoxidation as Affected by Oil Palm Ash and Biochar.

Understanding phosphorus (P) speciation and how redox conditions control P solubility in acid sulfate paddy soils with limited P availability is crucial for improving soil P availability. We examined P speciation and extractability in an acid sulfate paddy soil incorporated with oil palm ash (OPA) and biochar (OPB) during soil reduction and subsequent oxidation. Phosphorus K-edge X-ray absorption near edge structure (XANES) spectra of the soil samples revealed that P in the soil mainly occurred as P adsorbed to ferrihydrite and P adsorbed to gibbsite. During soil reduction, gibbsite-bound P was transformed into variscite, which was back-transformed to gibbsite-bound P during soil reoxidation. Sequential extraction results confirmed the dominance of Fe/Al (hydr)oxides-bound P (average 72%) in the soils. The OPA incorporation increased the exchangeable P pool concurring with the decrease in gibbsite-bound P. The OPB incorporation enhanced the dissolved P from the residual pool presumably due to electron shuttling of biochar with Fe(III) minerals during soil reduction. Our results highlight P dynamics in paddy soils, which are of immense importance for effective P-management strategies in rice cultivation.