Direct, nondestructive analysis using phosphorus K-XANES spectroscopy with minimal sample pretreatment provided unique insights, in addition to chemical fractionation results, on the chemical speciation of soil P in five samples from soils having highly different chemical properties. The distinct shoulder on the high-energy side of the white line peak characterizing XANES spectra for Ca-phosphate minerals and the intense white line peak typical of adsorbed forms of PO4 made it possible to identify these P species in the soil samples. In spite of a pre-edge feature present in adsorbed PO4 species on Fe-oxide compared with Al-oxide surfaces, this subtle characteristic could not be reliably used in the fitting of soil data to distinguish between these species. Spectral fitting indicated that PO4 adsorbed to Fe- or Al-oxides occurred in all of the five soil samples studied. A poorly crystalline form of Fe-phosphate was additionally found in an acidic A horizon sample. All samples studied, regardless of pH, contained Ca-phosphates. Hydroxyapatite appeared in all soils while octacalcium phosphate was present only in the two slightly alkaline samples and in one acidic soil for which calcium phosphate was a dominant P species. Chemical fractionation gave additional insights on P forms, such as organic P, that were not accounted for by the XANES analysis.
The authors are grateful to Sylvie Côté and Kimberly Hutchison for excellent laboratory work associated with this research. We are grateful to Dr. Young-Mi Oh for supplying oxide minerals and to Dr. April Leytem for supplying organic phosphorus standards. Thanks are extended to Dr. Henri Dinel, Dr. Thi Sen Tran, and Dr. Eugene J. Kamprath for reviewing an earlier version of the paper. This research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. The authors appreciate technical support of Dr. Lisa Miller, Syed Khalid, and staff at Beamline X-19A. Funding was provided in part by Agriculture and Agri-Food Canada (Ste-Foy, Québec) and by the North Carolina Agricultural Research Service. We deeply regret that Dr. Régis Simard (1956–2002) could not see the final product of this study; his valuable support is fully appreciated.