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The loss of organic matter (OM) from soil during long-term agricultural cropping results in a decrease in the inherent fertility of the soil as well as releasing greenhouse gases. Despite the importance of organo-mineral associations in the stabilization of OM within soils, much remains unknown about these organo-mineral associations. We used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate the incorporation and stabilization of ¹³C and ¹⁵N labelled residues of lucerne (Medicago sativa) and buffel grass (Cenchrus ciliaris) when incubated in a Vertisol from temperate Australia for up to 365 d. We show that newly-added OM forms organo-mineral associations through two mechanisms. Firstly, it was observed that the newly-added OM forms associations with the existing mineral-bound OM. However, this apparent stabilization of newly-added OM by associating with existing mineral-bound OM …