『Abstract
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 oxyhydrates (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 if labile P in the surface and subsurface
environment.
Keywords: Phosphorus; Fixation; Sand fraction; Coatings; XAS;
STEM』
1. Introduction
2. Materials and methods
2.1. Material characterization
2.2. Inorganic phosphorus fractionation
2.3. Phosphorus desorption experiments
2.4. Synchrotron based X-ray microprobe analyses
2.5. Scanning electron microscopy analyses
2.6. Transmission electron microprobe analysis
3. Results and discussion
3.1. Characterization of soils and sand fractions
3.2. Phosphate desorption
3.3. Synchrotron based X-ray fluorescence and STEM analysis
3.4. Fe K-edge microfocused- and bulk-XAS analysis
4. Conclusion
Acknowledgments
References