We demonstrate and explain differential patterns in SOM dynamics in C3/C4 mixed ecosystems at various spatial scales across contrasting climate and soils. This study shows that the interdependence between biotic and abiotic factors ultimately determines whether SOM dynamics of C3- and C4-derived vegetation are at variance in ecosystems where both vegetation types coexist. The results also highlight the far-reaching implications that vegetation thickening may have for the stability of deep SOM.