X-ray Microspectroscopy Analysis of Phosphorus Speciation in an Acidic Organic Soil.

Because organic soils are often located near surface water bodies, they have a vital role in protecting water quality. To determine how phosphate mobility is affected by reduction and oxidation processes in poorly-drained organic soils, our goal is to quantify proportions of phosphate bound with redox-active soil Fe(III) vs redox-inactive Al(III). We studied an acid soil (pH 4.8) containing total P, Fe, and Al concentrations of 23, 43, and 380 mmol/kg. Micro-XANES analyses at both the phosphorus and aluminum K-edges were performed on the sample to try to identify Fe(III)-bonded PO₄ based on a pre-white-line peak, and PO₄ bonded with Al by comparison with mineral standards.  Micro-XANES helps overcome limitations of bulk XANES analysis for soil samples with low P concentrations.  Based on micro-x-ray fluorescence analysis using an ~50 x 50 nm synchrotron x-ray beam; P, Al, and Si were heterogeneously distributed within the sample as expected. Phosphorus K-edge micro-XANES spectra collected at various locations on the sample showed little or no evidence for Fe(III)-bonded PO₄. Aluminum K-edge micro-XANES spectra were usually characteristic of Al-hydroxide mineral, likely because of the 16-fold greater concentration of Al vs P.  Detailed and quantitative speciation of soil P remains a challenge, but will be important for determining mechanisms and predicting the extent of reductive P dissolution in a given soil.