Decoupling Phosphorus Pools and Plant Uptake: Chemical Speciation and Phytoavailability of Legacy P in Taiwanese Rice Paddies

Long-term phosphorus (P) fertilization has resulted in substantial P accumulation in Taiwanese rice paddy soils, with concentrations reaching several thousand mg kg⁻¹. To assess the phytoavailability of this legacy P to rice, soil P speciation was characterized using X-ray absorption near-edge structure (XANES) spectroscopy and a sequential extraction procedure, and quantified P accumulation in rice as soil-to-plant translocation. Despite lower total and extractable P, acidic soils showed greater soil-to-root P translocation, whereas alkaline soils contained larger soil P pools but exhibited more constrained P translocation. Sequential extraction and XANES consistently indicated the coexistence of Ca-bound P (Ca–P) and Fe-bound P (Fe–P) across the pH range, including species not predicted to dominate from thermodynamic considerations. In acidic soils, the persistence of Ca–P suggests a potentially available pool that may supply P through gradual dissolution. In alkaline soils, abundant Fe–P implies retention within mineral phases that could remain chemically labile over long timescales. Overall, these findings highlight the need to account for soil P speciation when evaluating legacy P use and guiding fertilizer management, and the information is essential for developing strategies to sustain rice growth while reducing or eliminating P inputs.