Sulfur K-edge XANES spectroscopy as a tool for understanding sulfur dynamics in soil organic matter

Sulfur K-edge X-ray absorption spectroscopy (XANES) was used to identify S oxidation states and assess the impact of land use changes on the amount, form, and distribution of organic S in particle-size separates and their humic substance extracts. Soil samples (0-10 cm) were collected from natural forest, tea plantations, and cultivated fields at Wushwush and from natural forest, Cupressus plantations and cultivated fields at Munesa sites in Ethiopia. Sulfur XANES spectra measured directly from the size separates could not be quantitatively analyzed due to high background noise. However, qualitative comparison of spectra from size separates and their humic extracts were very similar and thus provides a characteristic fingerprint of S in mineral soils. X-ray absorption near-edge spectroscopy showed the presence of most reduced (sulfides, disulfides, thiols, and thiophenes), intermediate (sulfoxides and sulfonates) and highly oxidized S (ester-SO4-S) forms. Sulfur in intermediate oxidation states was dominant (39-50%; where 66-96% of it being sulfonate S) in humic extracts from clay, while highly oxidized S dominated (40-56%) the silt spectra. Concentrations of C-bonded and ester-SO4-S extracted by the HI fractionation did not correlate with those from XANES (ester-SO4-S revealed by XANES vs. HI-fractionation, r=0.23; P<0.001). A major shift following land use changes occurred in the most reduced and intermediate S species. Their proportion decreased in the order: natural forests>plantations>cultivated fields. In contrast, highly oxidized S increased in the order: natural forests<plantations<cultivated fields at both sites. Our results indicated that C-bonded S (most reduced and intermediate S) may represent the more labile forms of organic S compounds compared with ester-SO4-S. Therefore, S K-edge XANES has a significant potential to evaluate the influence of anthropogenic changes on the nature and distribution of S and to follow its dynamics in terrestrial ecosystems.