SummaryIn an acid soil, amounts or organic carbon resistant to peroxidation with hydrogen peroxide were strongly related to the proportions of expandable phyllosilicate (vermiculite, smectite) in the clay fraction, as measured by its K2O content and cation exchange capacity. This was taken to indicate that a substantial part of the organic matter resistant to oxidation was interlayered. The organic compounds in the clay‐organic matter complexes were identified by Pyrolysis Field Ionization Mass Spectrometry (Py‐FIMS).Alkyl‐aromatics as well as carbohydrates, phenols, lignins, lipids and N‐compounds were found in the C‐horizon sample, whereas mainly alkyl‐aromatics and N‐compounds, more resistant to chemical oxidation, were associated with the A1and Bwfine‐clay fraction. This was interpreted as being due either to the evolution of the clay‐associated compounds paralleling the mineralogical transformations, or to the selective adsorption of the more
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