Soil total phosphorus content is a driver of P forms in continuously flooded paddy soils

Redox fluctuations in submerged paddy soils strongly influence the transformation and availability of inorganic (Pi) and organic phosphorus (Po) forms. However, the extent to which these redox-driven processes affect Pi and Po pools and speciation, and their contribution to phosphorus (P) availability for rice, remains poorly understood.This study examined Pi and Po dynamics in twelve paddy soils with different total P (TP) content, classified as high-P (>800 mg P kg−1), medium-P (500–800 mg P kg−1), and low-P (<500 mg P kg−1). Soils were analysed before and after 60 days of rice growth using sequential P fractionation, liquid-state 31P nuclear magnetic resonance (31P NMR) spectroscopy, and phosphomonoesterase activity assays to assess P pools (soluble, exchangeable, redox-sensitive, and residual), organic P composition, and enzymatic hydrolysis potential.Redox-sensitive Pi and Po were the dominant pools across all soils, accounting for ∼50 % and ∼18 % of total P, respectively. Soluble and exchangeable P pools remained minor. Concentrations of Pi and Po were highest in high-P soils and lowest in low-P soils. In high-P soils, orthophosphate monoesters dominated and remained quite stable during plant growth, likely due to selective accumulation of inositol phosphates under repeated Fe redox cycles. In contrast, orthophosphate diesters in medium- and low-P soils represented the most labile component of Po and were rapidly hydrolyzed during rice growth to alleviate P limitation.These findings highlight how TP content modulates the contribution of Pi and Po pools to rice nutrition, emphasizing the need to account for Po dynamics when evaluating P availability in paddy systems under fluctuating redox conditions.