Impacts of Molybdate Unreactive Phosphorus on Phosphate Release from Intensively Fertilized Soil

Abstract

Total phosphorus (P) in soil is classified as molybdate-reactive P (MRP) and molybdate-unreactive P (MUP) based on bioavailability, and P tests are generally focused on MRP fraction of soil, despite the fact that MUP can contribute significantly to total extracted P. This survey study indicated that water-soluble P (WP) contributed as much as 9% to total P (TP) in two intensively fertilized (rice–onion cropping upland and greenhouse) soils, and most (ca. 80%) of WP was present in MUP form. The total P lost through runoff and leaching was high (>5 ppm), due to rotating greenhouse into submerged paddy soil, especially at the initial stage of rice cultivation, and MUP contributed 43% and 77% to total P lost through runoff and leaching, respectively. Therefore, it is necessary to monitor MUP to develop suitable soil-management strategies to reduce MUP release and P loss from high-P-containing soils.