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Background Aims and Methods Results Conclusions Fe and Al oxyhydroxides are well‐known phosphorus (P)‐retaining minerals in soils. Little information is available regarding the relevance of clay minerals for the sorption of P in mixed oxyhydroxide–clay mineral systems and pH effects on P sorption partitioning.We wanted to investigate pH effects on P sorption partitioning between Fe oxyhydroxides and high‐activity clay minerals in mixed‐mineral systems. We quantified the relative contribution of ferrihydrite or goethite versus Al‐saturated montmorillonite (Al‐MT) to the retention of orthophosphate (oPO4) and inositol hexakisphosphate (IHP) at different pH values (3–6). We combined the analysis of P solution concentration changes with the quantification of P bound to Fe(III) versus Al in the Fe oxyhydroxide/Al‐MT mixtures by X‐ray absorption near‐edge structure (XANES) spectroscopy.Orthophosphate was preferentially retained by ferrihydrite, compared to Al‐MT at pH 3–6. The opposite was observed for goethite at low P solution concentrations. The contribution of Al‐MT versus Fe oxyhydroxides to oPO4 retention increased with pH. This is attributed to a speciation change of clay‐adsorbed Al from monomeric Al3+ to polynuclear Al hydroxy species. IHP was predominantly retained by Al‐MT rather than ferrihydrite at pH 3–6, probably by the formation of adsorbed and surface‐precipitated Al phytate complexes.Synchrotron‐based XANES spectroscopy allows for quantifying P adsorbed to co‐existing pedogenic Fe(III) oxyhydroxides versus Al‐MT in mixed‐mineral systems. Soil P retention partitioning to these minerals depends on (1) dominating P form (oPO4, IHP), (2) relative abundance of high‐activity clay minerals, short‐range ordered and crystalline Fe oxyhydroxides, and (3) solution pH. [ABSTRACT FROM AUTHOR]

Direct speciation of soil phosphorus (P) by linear combination fitting (LCF) of P K-edge XANES spectra requires a standard set of spectra representing all major P species supposed to be present in the investigated soil. Here, available spectra of free- and cation-bound inositol hexakisphosphate (IHP), representing organic P, and of Fe, Al and Ca phosphate minerals are supplemented with spectra of adsorbed P binding forms. First, various soil constituents assumed to be potentially relevant for P sorption were compared with respect to their retention efficiency for orthophosphate and IHP at P levels typical for soils. Then, P K-edge XANES spectra for orthophosphate and IHP retained by the most relevant constituents were acquired. The spectra were compared with each other as well as with spectra of Ca, Al or Fe orthophosphate and IHP precipitates. Orthophosphate and IHP were retained particularly efficiently by ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated soil organic matter (SOM), but far less efficiently by hematite, Ca-saturated montmorillonite and Ca-saturated SOM. P retention by dolomite was negligible. Calcite retained a large portion of the applied IHP, but no orthophosphate. The respective P K-edge XANES spectra of orthophosphate and IHP adsorbed to ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated SOM differ from each other. They also are different from the spectra of amorphous FePO4, amorphous or crystalline AlPO4, Ca phosphates and free IHP. Inclusion of reference spectra of orthophosphate as well as IHP adsorbed to P-retaining soil minerals in addition to spectra of free or cation-bound IHP, AlPO4, FePO4 and Ca phosphate minerals in linear combination fitting exercises results in improved fit quality and a more realistic soil P speciation. A standard set of P K-edge XANES spectra of the most relevant adsorbed P binding forms in soils is presented. [ABSTRACT FROM AUTHOR]