Characterization of Sulphur Species in Oxic and Anoxic Forest Soils with Synchrotron-based X-Ray Spectromicroscopy.
Nora A. Tyufekchieva1, Jörg Prietzel1, Jürgen Thieme2, and Ingrid Kögel-Knabner1. (1) Technische Universität München, Am Hochanger 2, Freising-Weiehenstephan, D-85354, Germany, (2) Institut für Röntgenphysik, Georg-August-Universität, Göttingen, Germany
Current routine methods of chemical analysis for sulphur (S) speciation in soil are highly unsatisfactory, in face of the enormous ecological relevance of S in soils. Wet chemical methods can only distinguish total soil S into operationally-defined fractions rather than destined S species. X-ray spectromicroscopy at the K-absorption edge reflects the chemical bonding state and particularly the electronic oxidation of elements and can be used to study individual species of S. Synchrotron-based X-ray absorption near edge fine structure spectroscopy (XANES) at the K-absorption edge (2465 eV to 2495 eV) of sulfur (S) was used for the speciation of different S forms in oxic and anoxic topsoil horizons. We analyzed forest soils with total S contents ranging from 840 to 6520 mg S kg-1. The analyses were conducted directly on freeze-dried and ground soil samples. To investigate the effect of aeration during the soil sampling and sample preterit on the S speciation sub sample of each soil were proceed either with or without strict maintenance of anoxic conditions. The measurements were carried out at the Advanced Photon Source (Argonne National Laboratory, USA) with an intermediate energy scanning X-ray microscope (SXM). The incident X-ray energy was decremented in steps of 0.25 eV, using a 10 s dwell time per energy step, resulting in an acquisition time for a complete spectrum of 20 minutes. Each spectrum was recorded from two different locations within the sample to improve the statistical significance of the data. Our study was conducted on four different soil types, Cambisol, Planoslol, Histosol C and Histosol D, in the Lehstenbach catchment in the Fichtelgebirge (Germany). The soils differ in the degree of groundwater influence. With increasing soils depth and concomitantly increasing groundwater influence, the allotment of reduced S-species (inorganic and organic sulphides) to the total sulphur of the respective horizon increase from 53% to 68%; simultaneously the portion of oxidesed S-species (sulphite, sulphone, sulphonate and sulphate) decries from 41% to 23% Without maintenance of anoxic conditions during soil sampling, sample storage, and sample pretreatment considerable changes of the portions of reduced, intermediate and oxic S-species in the sample can be observed. These changes are more pronounced in soils with high groundwater influence than in soil with moderate groundwater influence.Synchrotron-based X-ray spectromicroscopy (S K edge XANES) allows a quick, direct quantification of different sulphur species in oxic and anoxic soils. In the studied soils, an increasing groundwater influence is correlated with an increased contribution of reduced S species to the total soil sulphur pool. For an accurate S speciation in anoxic soils, anoxic conditions have to be maintained during the entire procedure of soil sampling, sample storage, and sample pretreatment to avoid artificial changes of S speciation, particularly in anoxic soils. Acknowledgements This work was funded by the Deutsche Forschungsgemeinschaft (DFG)PR 534/4. We would like to thank the Advanced Photon Source in the Argonne National Laboratory, USA.Keywords: X-ray spectromicroscopy, sulphur, forest soils, groundwater influence