The speciation of sulfur in bulk soil differs from the S speciation in humic acid, fulvic acid, and most particle size fractions

Abstract

Synchrotron-based X-ray absorption near edge spectroscopy (XANES) at the S K-edge (2476 eV) is a powerful, non-invasive tool for the speciation of sulfur in soil. However, in many studies instead of bulk soil samples, which often yielded spectra which were too noisy to analyze, alkaline extracts or soil particle size fractions have been investigated, accepting that the S speciation of these fractions represents that of the bulk soil.

To test that assumption we compared the S speciation as revealed by S K-edge XANES of bulk soil, humic acid, fulvic acid, and several particle size fractions from A horizons of two cropland Alfisols, from an O and a Bs horizon of a forested Spodosol, and from an H horizon of a peat bog. The XANES spectra at the K-edge of S (2476 eV) were acquired at beamline ID 21 of the European Synchrotron Radiation Facility (ESRF) in Grenoble/France. For all except one (Spodosol Bs) sample, the XANES spectra of the humic and the fulvic acid fractions differed considerably from those of the respective bulk soil, with the humic and fulvic acid fractions generally being depleted in reduced S (thiols, organic disulfide) and enriched in oxidized S (sulfone, sulfonate, ester sulfate). Also the spectra of most particle size fractions were clearly different from the spectrum of the respective bulk soil, with the clay fraction being enriched in oxidized S, and the sand fractions being enriched in reduced S. Our results show that neither the sulfur K edge XANES spectra of alkaline soil extracts, including humic and fulvic acids, nor those of the clay particle size fraction can be used as proxies for the S speciation of the original soil from which the fractions had been isolated.