Estimation of Available Phosphorus Under Phosphorus Fertilization in Paddy Fields of a Cold Region Using Several Extraction Methods: A Case Study from Yamagata, Japan

Assessing available phosphorus (P) in paddy fields is challenging due to waterlogging-induced reducing conditions. This study tested the applicability of the Truog, Bray 2, and Mehlich 3 extraction methods in both air-dried and incubated soils, as well as the ascorbic-acid-reduced Bray 2 (AR Bray 2), which simulates reducing conditions, for evaluating rice growth under P fertilization. In addition, to investigate the chemical characteristics of the extraction methods, active Al and Fe and P sequential extractions were measured. Soil samples from four representative regions in Yamagata Prefecture were used. Pot cultivation tests using ‘Haenuki’ and ‘Tsuyahime’ cultivars were conducted with varying P fertilizer levels. Variations in P availability across soil types were influenced by levels of active Al and Fe. Sequential extractions identified NaHCO3-P and NaOH-P fractions as important for P availability. Bray 2 in both soils and AR Bray 2 were the most effective methods, showing a strong saturating exponential correlation with rice growth and P uptake, whereas Mehlich 3 and Truog showed weaker correlations. Bray 2 and AR Bray 2 show potential but require further evaluation for practical application due to the small number of soils. Future efforts should prioritize developing methods that account for P dynamics under reducing conditions, thereby improving P management strategies and supporting sustainable rice production.