Phosphorus K-edge XANES spectroscopy has probably often underestimated iron oxyhydroxide-bound P in soils

Phosphorus (P) K-edge X-ray absorption near-edge structure (XANES) spectra of orthophosphate (oPO₄) bound to soil Fe^III minerals (e.g. ferrihydrite, goethite) show a pre-edge signal at 2148-2152 eV. It is unknown whether organic P bound to Fe^III oxyhydroxides also show this feature. Otherwise, Fe-bound soil P may be underestimated by P K-edge XANES spectroscopy, because a large portion of Fe oxyhydroxide-bound P in soils is organic P. K-edge XANES spectra were obtained for different organic P compounds present in soils [inositol hexaphosphate (IHP), glucose-6-phosphate (G6P), adenosine triphosphate (ATP)] after sorption to ferrihydrite or goethite and compared with spectra of oPO₄ adsorbed to these minerals. P sorption to ferrihydrite increased in the sequence IHP ≪ G6P < oPO₄ < ATP. P sorption to goethite increased in the sequence G6P < oPO₄ ≪ ATP = IHP. Pre-edge signals in P K-edge XANES spectra of organic P adsorbed to Fe oxyhydroxides were markedly smaller compared with those of oPO₄ adsorbed to these minerals and absent for Fe^III oxyhydroxide-bound ATP as well as goethite-bound IHP. Linear combination fitting (LCF) performed on spectra of IHP, G6P or ATP adsorbed to ferrihydrite or goethite, using only spectra of Fe^III oxyhydroxide-bound oPO₄ as reference compounds for Fe-bound P, erroneously assigned >93% (ferrihydrite) or >41% (goethite) of Fe-bound P to non-Fe-bound P species. Inclusion of Fe^III oxyhydroxide-bound IHP as reference compounds markedly increased the recovery of oxyhydroxide-bound organic P. Thus, Fe-bound soil P has probably often been underestimated by LCF in soil XANES studies where IHP adsorbed to ferrihydrite and to goethite were not included as reference compounds.