Preliminary validation of a sequential fractionation method to study phosphorus chemistry in a calcareous soil

A sequential fractionation method proposed by Jiang and Gu (1989) distinguished three types of calcium phosphates (Ca-P) according to their different plant availabilities. Three extractants, NaHCO3, NH4Ac, and H2SO4 were used to extract Ca-2-P, C-8-P, and Ca-10-P types, respectively, from soil. This sequential fractionation method was tested and modified for analyzing the P chemistry of a calcareous soil. The solubility test and the model diagrams of the stability of the major Ca-P minerals showed that NaHCO3 was able to extract brushite (Ca-2-P type), and NH4Ac extracted brushite and beta-tricalcium P (C-8-P type) as well as hydroxyapatite (Ca-10-P type). Therefore the P forms targeted by extraction with NH4Ac should include both Ca-8-and Ca-10-P types. The sum of the P extracted by all extractants in the sequential fractionation method in the calcareous soil was in agreement with the total P measured by the perchloric acid digestion method. A proportion of organic P measured by the sequential fractionation method was in agreement with the result from solution P-31 NMR spectroscopy. This study showed that the modified sequential fractionation method and its target P forms would be useful for quantifying and characterizing inorganic and organic P in a calcareous soil, even though it should be used in combination with other techniques, such as solution P-31 NMR spectroscopy. (C) 2016 Elsevier Ltd. All rights reserved.