Sulfur speciation in well-aerated and wetland soils in a forested catchment assessed by sulfur K-edge X-ray absorption near-edge spectroscopy (XANES)

In forested catchments, retention and remobilization of S in soils and wetlands regulate soil and water acidification. The prediction of long-term S budgets of forest ecosystems under changing environmental conditions requires a precise quantification of all relevant soil S pools, comprising S species with different remobilization potential. In this study, the S speciation in topsoil horizons of a soil toposequence with different groundwater influence and oxygen availability was assessed by synchrotron-based X-ray absorption near-edge spectroscopy (XANES). Our investigation was conducted on organic (0, H) and mineral topsoil (A, AE) horizons of a Cambisol-Stagnosol-Histosol catena. We studied the influence of topography (i.e., degree of groundwater influence) and oxygen availability on the S speciation. Soil sampling and pretreatment were conducted under anoxic conditions. With increasing groundwater influence and decreasing oxygen availability in the sequence Cambisol-Stagnosol-Histosol, the C : S ratio in the humic topsoil decreased, indicating an enrichment of soil organic matter in S. Moreover, the contribution of reduced S species (inorganic and organic sulfides, thiols) increased systematically at the expense of intermediate S species (sulfoxide, sulfite, sulfone, sulfonate) and oxidized S species (ester sulfate, SO42-). These results support the concept of different S-retention processes for soils with different oxygen availability. Sulfur contents and speciation in two water-logged Histosols subject to permanently anoxic and temporarily oxic conditions, respectively, were very different. In the anoxic Histosol, reduced S accounted for 57% to 67% of total S; in the temporarily oxic Histosol, reduced S was only 43% to 54% of total S. Again, the extent of S accumulation and the contribution of reduced S forms to total S closely reflected the degree of O-2 availability. Our study shows that XANES is a powerful tool to elucidate key patterns of the biogeochemical S cycling in oxic and anoxic soil environments. In contrast to traditional wet-chemical methods, it particularly allows to distinguish organic S compounds in much more detail. It can be used to elucidate microbial S-metabolism pathways in soils with different oxygen availability by combining soil inventories and repeated analyses of a sample in different stages of field or laboratory incubation experiments under controlled boundary conditions and also to study (sub)microspatial patterns of S speciation in aggregated soils.