Mobilization of Readily Exchangeable Organic P - A Potential Driver of Enhanced P Acquisition Efficiency in Upland Rice.

High phosphorus (P) fixation in soils is a major constraint on crop production worldwide. To address this challenge, we investigated plant-induced changes in soil P pools, aiming to identify superior P uptake strategies by examining whether different upland rice genotypes access various P sources in the rhizosphere. Two genotypes (DJ123 and Nerica4) with varying P acquisition efficiencies (PAEs) were grown in an Andosol under low- and high-P fertilization. Fertilizer-P was labeled with 33P, and plants were harvested 34 days after emergence. Hedley fractionation was conducted on initial soil, as well as on bulk and rhizosphere soils after harvest, to analyze changes in fertilizer/native soil and inorganic/organic P in different fractions. Fertilizer-P entered all Hedley fractions, with the largest share being found in the moderately labile (NaOH-P, + 72%) and stable (H2SO4-P, + 19.8%) P fractions under both P treatments. The plant presence resulted in a decrease in fertilizer-P in the most labile P fraction (resin-P), whereas native soil-P and organic P increased in the other labile P fraction (NaHCO3-P). Moreover, a sharp decline in organic NaOH-P fraction in the rhizosphere, along with an increase in inorganic NaOH-P under both P conditions, was observed. Genotypic differences were evident, with DJ123 exhibiting increased organic resin-P concentrations in the rhizosphere compared to Nerica4. DJ123 demonstrated superior access to readily exchangeable organic P in the rhizosphere, highlighting a potential driver for its enhanced PAE. These findings emphasize the importance of genotype-specific strategies for optimizing P mobilization and acquisition in highly P-fixing soils.