Abstract
Sequential chemical extraction is a common analytical approach used to separate soil P into operationally defined fractions based on solubility in increasingly strong extractants. However, there are some concerns that the relationship between these operationally defined pools and the true chemical speciation of P may not always be correct. This study was conducted to compare the speciation of P in soils amended with either biosolids or an inorganic fertilizer as using a combination of sequential chemical extraction and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy on the solid-phase residues after each extraction step. The combined results from sequential chemical extraction and XANES analysis indicate that although sequential extraction steps may remove the same broad class of P from soil, the solubility and precise chemical speciation of that P may be quite different. For example, P K-edge XANES analysis determined that calcium phosphates are removed by the HCl extraction step, but this takes the form of a poorly crystalline dicalcium phosphate in biosolid-amended plots and apatite-type calcium phosphate in the commercially fertilized plots.
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