An increase in soil N availability by Acacia mangium in pure or mixed-species stands may reduce soil organic matter (SOM) mineralization compared to eucalypt stands. We measured the isotope composition (d13C) of coarse particulate organic matter (cPOM, 4–0.25 mm), of fine POM (fPOM, 0.25–0.05 mm) and of the remaining organo-mineral fraction (OMF, <0.05 mm), and of the forest floor in plantations established on a tropical C4-grass savannah. In contrast to our hypothesis, the presence of acacia either alone or in mixture did not impact d13C of the most stable SOM fractions derived from the previous savannah vegetation, which suggests similar rates of native SOM mineralization under acacia and eucalypt trees.