Lettres à la rédaction

Nous proposons une méthode chimique permettant d’obtenir, dans une solution solide hétérogène, alliage réduit en poudre par exemple, la répartition des titres d’un constituant. La méthode s’applique si le constituant peut être engagé dans un équilibre chimique alors que les autres ne réagissent pas. Cette méthode a été appliquée à un alliage or-cuivre en poudre. Principe

N° 51. — Conductivité ionique du protoxyde de cuivre en fonction de sa composition vers 400 °C

Nous avons déterminé la partie ionique de la conductivité électrique du protoxyde de cuivre en fonction de la température entre 336 et 456 °C et de la composition entre les deux limites d’existence de l’oxyde vers le cuivre d’une part et l’oxyde cuivrique d’autre part.Cette détermination a été effectuée par analyse des caractéristiques courant-tension d’une cellule constituée par la chaîne de conducteurs : cuivre anode, protoxyde de cuivre, bromure cuivreux, cuivre cathode.Le nombre de transport ionique est de quelques dix-millièmes. La conductivité ionique varie comme la puissance — 0,92 environ de l’activité du cuivre dans l’oxyde, soit comme la puissance 0,23 de la pression d’oxygène en équilibre avec l’oxyde; ces valeurs montrent que les lacunes d’ions cuivreux et les lacunes électroniques du modèle de WAGNER doivent être très fortement associées

Sur les relations entre paramètres cinétiques et grandeurs thermodynamiques

On rappelle les formules reliant les paramètres cinétiques (énergie d’activation et facteur pré-exponentiel) de processus élémentaires en phase gazeuse aux grandeurs thermodynamiques correspondantes (variations d’enthalpie et d’entropie dans l’état standard).On montre que l’usage fait par certains auteurs de formules simplifiées peut conduire à des erreurs sensibles

Evolution des complexes organo-minéraux par la mise en culture et la modification des conditions physico-chimiques du sol

National audienc

Organo-Mineral Interactions Are More Important for Organic Matter Retention in Subsoil Than Topsoil

International audienceDecomposing crop residues contribute to soil organic matter (SOM) accrual; however, the factors driving the fate of carbon (C) and nitrogen (N) in soil fractions is still largely unknown, especially the influence of soil mineralogy and autochthonous organic matter concentration. The objectives of this work were (1) to evaluate the retention of C and N from crop residue in the form of occluded and mineral-associated SOM in topsoil (0-20 cm) and subsoil (30-70 cm) previously incubated for 51 days with 13 C-15 N-labelled corn residues, and (2) to explore if specific minerals preferentially control the retention of residue-derived C and N in topsoil and subsoil. We used topsoil and subsoil having similar texture and mineralogy as proxies for soils being rich (i.e., topsoil) and poor (i.e., subsoil) in autochthonous organic matter. We performed a sequential density fractionation procedure and measured residue-derived C and N in occluded and mineral-associated SOM fractions, and used X-ray diffraction analysis of soil density fractions to investigate their mineralogy. In accordance with our hypothesis, the retention of C and N from crop residue through organo-mineral interactions was greater in subsoil than topsoil. The same minerals were involved in the retention of residue-derived organic matter in topsoil and subsoil, but the residue-derived organic matter was associated with a denser fraction in the subsoil (i.e., 2.5-2.6. g cm −3) than in the topsoil (i.e., 2.3-2.5 g cm −3). In soils and soil horizons with high clay content and reactive minerals, we find that a low SOM concentration leads to the rapid stabilization of C and N from newly added crop residues

Effect of landuse on organic matter stabilized in organomineral complexes: A study combining density fractionation, mineralogy and delta C-13

Landuse changes for the purposes of cultivation often destabilise a substantial part of the initially stabilized organic matter (OM) in surface soils. However, the mechanisms of OM destabilisation are poorly documented, particularly with respect to organomineral complexes. The aim of this study was to characterize and quantify the effects of landuse on various OM pools, while focusing particularly on mineral-bound OM in a ferralsol. Four different parameters were assessed: the proportion of mineral-bound OM, the nature of the minerals (poorly crystalline aluminosilicates, gibbsite, halloysite and iron oxides) contributing to organomineral complexes, landuse, and soil depth (surface horizons versus non-tilled deeper horizons). The study site had a field with C4 vegetation that had been cultivated for 186 years and compared to an uncultivated reference plot with C3 vegetation. Organomineral complexes were separated by densimetric fractionation and characterized by mineralogical and carbon isotopic methods. Depending on the considered horizon, 58 to 80% of the OM was stabilized through organomineral complexation. Chemosorption of organic compounds at the surface of mineral phases was thus found to be a major stabilisation process in the ferralsol. Although cultivation significantly affected OM pools that were not bound to minerals (particulate non-occluded and particulate occluded OM), these pools represented a low proportion (similar to 5%) of the carbon budget variations in the profile. Most variations in carbon stocks within the profile were attributed to organic matter located in organomineral complexes. Several potential processes were highlighted on the organomineral complex scale: (i) cultivation may have modified the physicochemical stability of mineral phases (involving dissolution and/or formation); (ii) cultivation could have led to vertical migration of organomineral complexes and their accumulation deeper in the profile; (iii) cultivation systematically modified the amount of organic compounds linked to minerals, with a decrease in topsoil and an increase in deep soil. In further studies to determine the impact of cultivation on soil carbon stocks, it would thus be interesting to continue assessing mechanisms that control the dynamics of mineral-bound OM while also investigating soil layers to a depth of at least 1 m. (C) 2009 Elsevier B.V. All rights reserved