Soil-Specific Redox Effects on Phosphorus Availability and Diagnostic Approaches in Flooded Paddy Soils

Accurate evaluation of plant-available phosphorus (P) in flooded paddy soils requires consideration of redox dynamics and soil-specific properties. This study evaluated five soil P extraction methods, such as Truog, Bray 2, Mehlich 3, Olsen, and ascorbic acid-reduced Bray 2 (AR Bray 2), using soils collected from 20 paddy fields in a cold region of Japan that have received long-term fertilization. All four methods, except AR Bray 2, were conducted under air-dried and flooded incubation conditions. Additionally, we conducted pot experiments with the two rice cultivars to measure P uptake. Bray 2 extracted the highest amount of P (543.6–1045.4 mg P kg−1). Incubation increased extractable P by factors of 2.4–4.9 with the Mehlich 3 and Truog methods, indicating enhanced P solubility under reduced conditions. The Olsen method showed minimal sensitivity to redox changes (−31.4 mg P kg−1). Principal component and cluster analyses suggested three patterns of soil P behavior under changing redox conditions: (1) stable P extractability regardless of redox status; (2) increased P availability after incubation; and (3) P extractability depending on the extraction method used. These patterns were not explained by regional or taxonomic classifications. A comparison of soil extractions and P uptake indicated that no single method consistently predicted shoot P concentrations across all soils, suggesting that conventional P extraction methods may have limited ability in long-term fertilized paddy soils. Our findings demonstrate that soil-specific redox behavior and cultivar-specific P demand critically influence the effectiveness of standard P tests. Therefore, selecting diagnostic methods tailored to soil characteristics and crop requirements is essential for accurate P evaluation and sustainable fertilizer management in rice cultivation.