Transformation of Calcium Phosphates in Alkaline Vertisols by Acidified Incubation

Acid-soluble soil phosphorus (P) is a potential resource in P-limited agricultural systems that may become critical as global P sources decrease in the future. The fate of P in three alkaline Vertisols, a major agricultural soil type, after acidic incubation was investigated using synchrotron-based K-edge X-ray absorption near-edge structure (XANES) spectroscopy, geochemical modeling, wet chemistry soil extraction, and a P sorption index. Increases in labile P generally coincided with decreased stability and dissolution of calcium phosphate (CaP) minerals. However, only a minor proportion of the CaP dissolved in each soil was labile. In two moderate-P soils (800 mg P kg^–1), XANES indicated that approximately 160 mg kg^–1 was repartitioned to sorbed phases at pH 5.1 of one soil and at pH 4.4 of the second; however, only 40 and 28% were labile, respectively. In a high-P soil (8900 mg P kg^–1), XANES indicated a decrease in P of 1170 mg kg^–1 from CaP minerals at pH 3.8, of which approximately only 33% was labile. Phosphorus mobilized by agricultural practices without concurrent uptake by plants may be repartitioned to sorbed forms that are not as plant-available as prior to acidification.