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 (P_i) and organic phosphorus (P_o) forms. However, the extent to which these redox-driven processes affect P_i and P_o pools and speciation, and their contribution to phosphorus (P) availability for rice, remains poorly understood.

This study examined P_i and P_o 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 P_i and P_o 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 P_i and P_o 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 P_o and were rapidly hydrolyzed during rice growth to alleviate P limitation.

These findings highlight how TP content modulates the contribution of P_i and P_o pools to rice nutrition, emphasizing the need to account for P_o dynamics when evaluating P availability in paddy systems under fluctuating redox conditions.