Résumé : Selenium is an essential nutrient that is potentially toxic: one of its radio-elements is also a component of long-lived radioactive waste for which long-term deep geological storage is envisaged. The chemistry of Se in soils is complex and very sensitive to redox potential and microbial activity...Selenium is an essential nutrient that is potentially toxic: one of its radio-elements is also a component of long-lived radioactive waste for which long-term deep geological storage is envisaged. The chemistry of Se in soils is complex and very sensitive to redox potential and microbial activity which largely determine its oxidation state and chemical form. The dynamics of Se have been extensively studied in soils where it is deficient, and even more so when concentrations are potentially toxic. In contrast, relatively little information is available on the fate of Se in soils at intermediate concentrations (1-5 mg kg(-1)). Some chemical reactions and biological processes that influence Se dynamics may be strongly concentration dependent. We have followed microbial activity by monitoring soil gas composition and Se volatilization and measured changes in Se fractionation using chemical extractions in a column of aggregated soil. A small proportion of soil Se was accumulated in the leaves, stems and fruits of tomato plants. Net Se volatilization losses were small (0.12% in a two-month period). There was a considerable upward movement of freshly added Se, but not of native soil Se. This vertical mobility was greater than that predicted from solute movement driven by evaporation. Selenium was strongly immobilized at the water-saturated, anoxic base of the soil columns. Straw amendment and the growth of a tomato plant did not lead to stronger association with soil organic matter. It was not possible to correlate changes in fractionation of Se between treatments and along a soil profile with the calculated fraction of anoxia, except in the completely anoxic zone. (C) 2007 Elsevier B.V. All rights reserved.
Résumé : The evolution of clay soil porosity is currently demonstrated via the shrinkage curves in a large water content domain spreading from a shrinkage limit to a liquidity limit. In fact, the parallel between in situ profiles and the shrinkage curves in such a large water content range is difficult to...The evolution of clay soil porosity is currently demonstrated via the shrinkage curves in a large water content domain spreading from a shrinkage limit to a liquidity limit. In fact, the parallel between in situ profiles and the shrinkage curves in such a large water content range is difficult to obtain because of the lack of earth pressure in the laboratory tests and in situ limited water contents. The vertical distribution of porosity throughout a clay-rich marsh soil profile was studied in a grassland field with samples taken from the soil surface characterized by water contents near their shrinkage limit down to 2.00 m deep saturated sediments over their liquidity limit. The depth of the plasticity limit isolates a soil in a solid state characterized by a vertical prism-like structure from a plastic to pseudo-liquid state in depth. The porosity was calculated from the measurements of the density of intact samples by double weighing and image analysis of 100 cm(2) polished sections. The initial structure of clay soil was maintained by impregnation based on water-acetone-resin exchange. An ultraviolet photo luminescent pigment added to the resin allowed the capture of images from which shrinkage cracks and microporosity of the clay rnattix were easily separated. The distribution of porosity between the shrinkage crack mesoporosity and the clay matrix microporosity was evaluated after the mathematical decomposition of the grey level curves characteristic of each level. Vertical evolution of the porosity distribution from the soil surface in a solid state to the plastic and pseudo-liquid sediment in depth was presented on the shrinkage curve of the clay material. The measurements point out how the clay matrix microporosity and mesoporosity of shrinkage cracks are complementary and the role of the scale effect on the shrinkage curve. The analysis of images captured on an optical microscope under polarized and analyzed light and the SEM observation of freeze-dried samples demonstrated the isotropic arrangement of the clay particles in typical honey-comb' architecture in the in situ plastic-to-liquid saturated domain. Eventually the distribution of porosity through the profile results from the evolution of the initial honey-comb microstructure of the sediment induced by the desiccation phenomenon. It is governed by the depth of plasticity limit of the clay material and by the depth of the water table. (c) 2007 Elsevier B.V. All rights reserved.