Sulfur K-edge XANES Spectroscopy as a Tool for Understanding Sulfur Dynamics in Soil Organic Matter

in oxidation states. Sulfur occurs in soil both in organic and inorganic forms. However, most of the soil S Sulfur K-edge X-ray absorption spectroscopy (XANES) was used ( 95%) in temperate and tropical ecosystems is present 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 in organic forms (Janzen and Ellert, 1998; Saggar et al., separates and their humic substance extracts. Soil samples (0–10 cm) 1998; Solomon et al., 2001a). Knowledge of the speciawere collected from natural forest, tea plantations, and cultivated tion of S in organic materials could, therefore, provide fields at Wushwush and from natural forest, Cupressus plantations a clearer understanding of the amount, form, and bioand cultivated fields at Munesa sites in Ethiopia. Sulfur XANES geochemical transformations of S in the ecosystem. Our spectra measured directly from the size separates could not be quantiknowledge of the biogeochemical S cycling is, however, tatively analyzed due to high background noise. However, qualitative severely limited by relatively crude analytical techcomparison of spectra from size separates and their humic extracts niques used in S measurements and speciation. Most of were very similar and thus provides a characteristic fingerprint of S the present studies on soil organic S are based on a in mineral soils. X-ray absorption near-edge spectroscopy showed the fractionation technique using HI reduction of organic presence of most reduced (sulfides, disulfides, thiols, and thiophenes), intermediate (sulfoxides and sulfonates) and highly oxidized S (esterS compounds in soil (Tabatabai, 1982; Kowalenko, SO4–S) forms. Sulfur in intermediate oxidation states was dominant 1993a,b). The biochemical characterization of organic (39–50%; where 66–96% of it being sulfonate S) in humic extracts S as ester-SO4–S (HI-reducible S) and C-bonded S has from clay, while highly oxidized S dominated (40–56%) the silt spectra. advanced the understanding of the organic S cycle in Concentrations of C-bonded and ester-SO4–S extracted by the HI soil (e.g., McGill and Cole, 1981; Janzen and Ellert, fractionation did not correlate with those from XANES (ester-SO4–S 1998; Lehmann et al., 2001; Solomon et al., 2001a). Howrevealed by XANES vs. HI-fractionation, r 0.23; P 0.001). A ever, this technique is only an indirect method involving major shift following land use changes occurred in the most reduced differential reduction of organic S compounds to H2S and intermediate S species. Their proportion decreased in the order: and is, therefore, not an entirely satisfactory procedure natural forests plantations cultivated fields. In contrast, highly to directly speciate S within complex organic functional oxidized S increased in the order: natural forests plantations cultivated fields at both sites. Our results indicated that C-bonded S groups. Moreover, this chemical reduction method can(most reduced and intermediate S) may represent the more labile not identify intermediate oxidation states of S in organic forms of organic S compounds compared with ester-SO4–S. Therefore, matter and consequently almost no information is availS K-edge XANES has a significant potential to evaluate the influence able about their turnover rates. Other studies have atof anthropogenic changes on the nature and distribution of S and to tempted to speciate S into its functional groups using follow its dynamics in terrestrial ecosystems. pyrolysis gas chromatography mass spectrometry in aquatic humic substances (Van Loon et al., 1993) and kerogens (Carmo et al., 1997) for characterization of S is a highly reactive element, existing in several S containing moieties, but this approach is limited to oxidation states and moving freely among the lithospeciation of thermally stable forms of S (Hundal et sphere, hydrosphere, and atmosphere. The accumulaal., 2000). tion and cycling of S are central to many biological Synchrotron-based S K-edge XANES spectroscopy processes and provide a key to understanding changes in has been successfully used to speciate and quantify all the biosphere. Sulfur cycling also significantly influences oxidation states of S in a variety of geochemical samples atmospheric and oceanic chemistry and the global enranging from petroleum (Waldo et al., 1991a,b), coal ergy balance. Therefore, the chemistry of S compounds (Huffman et al., 1995), marine sediments (Vairavamurin the environment has taken on a new significance in thy et al., 1993, 1994), biosolids (Hundal et al., 2000) to recent years because human activities have dramatically soil humic and fulvic acids (Morra et al., 1997; Xia et altered the biogeochemical S cycling in terrestrial ecoal., 1998; Szulczewski et al., 2001). X-ray absorption systems. near-edge structure spectroscopy has also proven to be The soil environment is the primary component of a valuable tool in the identification and quantification the global biogeochemical S cycle, acting as a source of S oxidation states in organic soils (Martı́nez et al., and sink for various S species and mediating changes 2002). Sulfur XANES spectroscopy is a solid-state technique that circumvents the limitations of chemical reD. Solomon and J. Lehmann, Dep. of Crop and Soil Sciences, Cornell duction methods and provides a means to directly deterUniv., Bradfield and Emerson Hall, Ithaca, NY 14853; C.E. Martı́nez, mine the various S species based on the energy required Dep. of Crop and Soil Sciences, 418 ASI Building, The Pennsylvania State Univ., University Park, PA 16802. Received 18 Nov. 2002. *CorAbbreviations: CEC, cation-exchange capacity; eV, electron volt; responding author ([email protected]). PIPS, passivated implanted planar silicon detector; R-S/O-S, ratios of most reduced S to highly oxidized S; I-S/O-S, ratios of intermediate Published in Soil Sci. Soc. Am. J. 67:1721–1731 (2003).  Soil Science Society of America S to highly oxidized S; SOC, soil organic C; SOM, soil organic matter; XANES, X-ray absorption near-edge structure spectroscopy. 677 S. Segoe Rd., Madison, WI 53711 USA