Multidecadal persistence of organic matter in soils: multiscale investigations down to the submicron scale

Minerals, particularly clay-sized minerals, protect soil organic matter (SOM) from decomposition by microor-ganisms. Here we report the characterization of SOM andthe associated minerals over decades of biodegradation, ina French long-term bare fallow (LTBF) experiment startedin 1928. The amounts of carbon (C) and nitrogen (N) in thestudy area declined over time for six fractions (sand, coarsesilt, fine silt, coarse clays, intermediate clays, and fine clays).The C:N ratios of SOM associated with silt fractions re-mained constant, whereas the ratios significantly decreasedin clays, reaching very low values in intermediate and fineclays (C:N