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Notes: Organic matter can be removed from soil to varying extents using many chemical methods, but little information may be obtained on the nature of the organic matter thus removed and its chemical interactions with soil mineral components. We selected two surface soils, with similar organic carbon contents. Chemical analysis showed the presence of a range of amino acids and amino sugars. Quantitative mineralogical analysis revealed some differences between the soils, particularly in the poorly crystallized components. The clay-sized fractions were submitted to various chemical treatments, then the organic carbon remaining was investigated using infrared spectroscopy. We also studied the adsorption of purified soil polysaccharide to some of these residues. The nature and proportion of organic matter removed by each of the treatments was broadly similar for both soils. Residual organic matter remaining was enriched in amide groups. The infrared spectrum after maximum removal of organic matter was very similar to that of the clay-sized fraction of soil from the equivalent C horizon. Differences in the ability of the clay residues from the two surface horizons to adsorb polysaccharide correlated with their original organic matter content and is postulated to be related to poorly ordered amorphous mineral matter. Oxidation of added polysaccharide showed no enhancement of nitrogen in the residue, indicating that it was not bonded to the clay fraction of soil by absorption through the amino groups, in addition to the conventionally postulated hydrogen bonding involving hydroxyl groups.〈section xml:id="abs1-2"〉〈title type="main"〉L'interaction entre la matière organique et les minéraux argileux du sol par l'extraction sélective et l'addition contrôlée de la matière organique〈section xml:id="abs1-3"〉〈title type="main"〉RésuméLa matière organique du sol peut être détruite avec une efficiacité variable par diverses techniques chimiques, par contre peu d'information est ainsi obtenue sur la nature de la matière organique et ses interactions avec les composés minéraux du sol. Nous avons choisi deux sols de surface, ayant des teneurs en matière organique similaires. L'analyse chimique a mis en évidence une gamme importante d'acides aminés et de sucres aminés. Une analyse minéralogique quantitative a montré quelques différences entre les sols, notamment pour les composés faiblement cristallisés. Les fractions argileuses ont subi divers traitements chimiques, puis le carbone organique restant a été analysé par spectroscopie infrarouge. Nous avons également étudié l'adsorption de polysaccharide de sol purifié sur certains de ces sols traités. La nature et la proportion de matière organique détruite par chacun de ces traitements étaientassez similaires pour les deux sols. La matière organique restant était enrichie en groupements amide. Les spectres infrarouges après destruction maximale de la matière organique étaient très similaires à celui de la fraction argileuse du sol de l'horizon C correspondant. Les différences dans la capacité des deux sols de surface après traitement à adsorber un polysaccharide était corrélé avec la teneur en matière organique initiale et nous postulons qu'elles sont liées à la phase minérale amorphe. L'oxydation du polysaccharide ajouté n'était pas accompagnée d'un enrichissement d'azote dans le solide restant, ce qui indique que la liaison avec la fraction minérale du sol n'implique pas les groupements aminés, en plus des liaisons hydrogène des groupements hydroxyles, postulées classiquement.

Notes: Radiocaesium is more available to biological systems in organic soils than in mineral soils. Part of the reason is that the adsorption of caesium on minerals, particularly illites, is very specific, whereas its adsorption on organic matter is non-specific. However, increasing evidence shows that even organic upland soils contain enough illitic material to immobilize caesium effectively. The presence of organic matter may reduce the affinity of soil minerals for caesium.We have studied adsorption of radiocaesium on the clay fractions of a mineral soil obtained from a surface, and the corresponding C, horizon. The soil clay fractions were treated to remove organic matter and Fe oxides progressively.Adsorption isotherms of caesium were obtained in dilute suspension using trace amounts of 137Cs and stable caesium. In general, adsorption was greater when the organic matter content was less, namely on the soil from the C horizon rather than the surface horizon, or when organic matter had been removed. However, adsorption is not a simple function of organic matter content: Cs adsorption was greater on the topsoil fraction after organic matter removal than on the subsoil fraction. X-ray diffraction analysis indicated that the surface soil clay contained more weathered minerals and less illite than the subsoil clay. The effect of organic matter removal was more marked at large Cs concentrations than when only radiocaesium was added, suggesting that very selective Cs adsorption sites are less sensitive to the presence of organic macromolecules. Iron oxides do not play a role in the affinity of soil clays for Cs. The form of the Freundlich isotherm was unchanged by any of the chemical treatments.