Underassessed phosphorus fixation mechanisms in soil sand fraction

Eutrophication caused by phosphorus (P) rich leachate from sandy soils has been one of the major agro-environmental issues in the world. In predicting P bioavailability in soils, P reactivity in soil clay fractions (e.g., iron oxyhydroxides) has been a major focus of soil chemistry research in the past. However, the role of sand fraction has been rarely investigated to accurately understand the retention and release mechanisms of P in soils, and remains poorly understood. In this case study, we report here on macroscopic and spectroscopic evidence for environmentally important P partitioning mechanisms in soil sand fractions. In the high P sandy soils, total desorbable P from the sand fraction was as high as ~50% of total desorbable P from the whole soils, and these bioavailable P were associated with ammonium oxalate extractable Al and Fe. Scanning transmission electron microscopy/energy-dispersive X-ray analysis further revealed that P was associated with amorphous mixed Fe/Al/Si precipitates in the sand grain coating, and was not present with crystalline iron oxyhydroxides like goethite. Bulk- and microfocused-Fe K-edge X-ray absorption spectroscopy analysis showed that average FeO6 coordination environments in the grain coating did not exactly coincide with that in synthetic goethite, hematite, and ferrihydrite. The coordination numbers of second and third Fe shells were slightly larger/smaller than those in reference iron oxyhydroxides (hematite and goethite), suggesting the perturbation of local chemical structure of FeO6 linkages by Si/Al/P. Our research findings suggest that amorphous Al/Si/Fe grain coatings in soil sand fractions might be important in predicting the release of labile P in the surface and subsurface environment.