Microheterogeneity of element distribution and sulfur speciation in an organic surface horizon of a forested Histosol as revealed by synchrotron-based X-ray spectromicroscopy

Abstract

In recent years, the relevance of physico-chemical heterogeneity patterns in soils at the micron and submicron scale for the regulation of biogeochemical processes has become increasingly evident. For an organic surface soil horizon from a forested Histosol in Germany, microspatial patterns of element distribution (sulfur, phosphorus, aluminium, silicon) and S speciation were investigated by synchrotron-based X-ray spectromicroscopy. Microspatial patterns of S, P, Al and Si contents in the organic topsoil were assessed for a sample region of 50 μm × 30 μm by spatially resolving μ-XRF. Sulfur speciation at four microsites was investigated by focused X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge. The results show a heterogeneous distribution of the investigated elements on the (sub)micron scale, allowing the identification of diatoms, aluminosilicate mineral particles and sulfide minerals in the organic soil matrix. Evaluation of the S K-edge μ-XANES spectra acquired at four different microsites by linear combination fitting revealed a substantial microspatial heterogeneity of S speciation, characterized by the presence of distinct enrichment zones of inorganic sulfide and zones with dominant organic disulfide S within a few micrometers distance, and coexistence of different S species (e.g. reduced inorganic and organic S compounds) at a spatial scale below the resolution of the instrument (60 nm × 60 nm; X-ray penetration depth: 30 μm).

Introduction

In recent years, the relevance of physico-chemical heterogeneity patterns in soils at the micron and submicron scale for the regulation of important biogeochemical processes has become increasingly evident (Benzerara et al., 2005, Chorover et al., 2007, Herrmann et al., 2007, Totsche et al., 2010). Recent research has revealed considerable small scale heterogeneity of important biochemical reactions such as nitrification, production of CO₂, CH₄, N₂O, and other gases, which is related to corresponding heterogeneity patterns of physico-chemical boundary conditions (Heilmann and Beese, 1992, Revsbech et al., 1999, Stoyan et al., 2000, Hinsinger et al., 2006). In aggregated soils, redox potential shows significant microspatial heterogeneity (Fischer et al., 1989, Fiedler et al., 2007); both anoxic microsites in oxic soils as well as oxic microsites in anoxic soils have been reported (Revsbech et al., 1999, van der Lee et al., 1999, Küsel et al., 2002, Yang et al., 2006, Fiedler et al., 2007). The microstructural heterogeneity in soils allows the coexistence of antagonistic reactions (such as oxidation–reduction, adsorption–precipitation–desorption–dissolution) at different microsites in the same soil layer or soil horizon (Alewell et al., 2006, Paul et al., 2006, Hesterberg et al., 2011).

Synchrotron-based spectromicroscopy has been applied successfully to investigate the physico-chemical heterogeneity of mineral soils at the micron or submicron scale (e.g. Strawn et al., 2002, Prietzel et al., 2007, Prietzel et al., 2010, Frommer et al., 2011, Hesterberg et al., 2011). Applications of synchrotron-based spectromicroscopy to assess the microspatial heterogeneity of organic soils, however, up to now have not been reported, even though the small scale spatial heterogeneity of important biochemical reactions such as nitrification or microbial production of CO₂, CH₄ and N₂O suggests considerable physico-chemical heterogeneity in organic soils. In this paper we reveal for the first time microspatial heterogeneity patterns of S, P, Al, and Si contents and S speciation in an organic surface layer of a Histosol, for which the spatial coexistence of antagonistic S oxidation and reduction reactions had been reported (Alewell et al., 2006, Paul et al., 2006) and relate microspatial differences in S speciation to other soil properties.