Tuesday, November 14, 2006 - 11:30 AM

Relationships between the speciation of sulfur and iron in oxic and anoxic soils and soil aggregates as revealed by synchrotron-based X-ray spectromicroscopy.

Joerg Prietzel1, Nora Tyufekchieva1, Karin Eusterhues1, Jürgen Thieme2, Diane Eichert3, and David Paterson4. (1) Chair of Soil Science, Technische Universität München, Freising-Weihenstephan, D-85350, Germany, (2) Institute of X-Ray Physics, University of Göttingen, Göttingen, D-37077, Germany, (3) ID 21, European Synchrotron Radiation Facility, Grenoble, F-38043, France, (4) Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439

Synchrotron-based X-ray spectromicroscopy is a powerful tool to assess the speciation of different bioelements, such as S, Fe, and Mn. Moreover, it can provide spatially-resolved information on the speciation of these elements in soil samples on a micron and submicron spatial scale. This is of particular interest, when spatially heterogeneous samples, such as soil aggregates are to be investigated. Here, spatially resolved X-ray spectromicroscopy allows the speciation of Fe and S in specific microenvironments, such as e.g. the surface of aggregates or root channels. We investigated bulk soil samples from an Inceptisol – Planosol – Histosol catena and aggregates from different forest soils in Germany.  X-ray transmission and µ-XRF images as well as spatially-resolved XANES spectra at the K-edges of S (2476 eV) and Fe (7112 eV) were acquired with the scanning transmission X-ray microscopes at beamlines ID 21 of the European Synchrotron Radiation Facility in Grenoble/France and ID 2B of the Advanced Photon Source at the Argonne National Laboratory (Argonne, IL). For the Inceptisol – Planosol – Histosol catena, a clear relationship between the partitioning of reduced and oxidized S and reduced and oxidized Fe was observed, and the contribution of reduced S and Fe compounds to total S and Fe increased systematically in the sequence Inceptisol – Planosol – Histosol. Dissected aggregates from soil horizons with stagnic properties showed iron enrichment in their interior relative to their surface, whereas the opposite was the case for aggregates from soils with gleyic properties. Zones of iron enrichment in most cases consisted of Fe(III) oxyhydroxides, but sporadically also of Fe(II) minerals. Depending on the parent material, the Fe(II) minerals could be identified either as sulfidic or as non-sulfidic phases.