The mineral matrix, particularly clay-sized minerals, protects soil organic matter (SOM) from decomposition by microorganisms. Here we report the characterization of SOM and associated minerals over decades of biodegradation, in a French long-term bare fallow (LTBF) experiment started in 1928. The amounts of carbon (C) and nitrogen (N) declined with time for six fractions (sand, coarse silt, fine silt, coarse clays, intermediate clays and fine clays). The C : N ratios of SOM associated to silt fractions remained constant whereas they significantly decreased in clays, reaching very low values in intermediate and fine clays (C : N < 5) after 8 decades of LTBF. X-ray absorption spectroscopy revealed that (i) bulk-scale SOM chemical speciation remained almost constant, (ii) submicrometric particulate OM was present in coarse clays, even after 79 years of LTBF, (iii) illite particles became progressively SOM-free with time whereas mixed layered illite/smectite and smectites were always associated to OM throughout the bare fallow. Altogether, these results suggest that clay-sized minerals preferentially protect N-rich SOM and that smectites and mixed layered illite/smectite protect SOM more efficiently than illites.