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Acacia mearnsii is an introduced Australian acacia in South Africa and has invaded more than 2.5 million ha, primarily establishing in rangeland and riparian areas. Because acacias have the capability to fix N, A. mearnsii invasions may fundamentally change N dynamics in invaded systems. This study compares biological N2-fixation in the alien invasive A. mearnsii and the native A. caffra growing in a grassland riparian zone in the Komati Gorge Reserve, Mpumalanga, South Africa. A 15N natural abundance field survey suggested that both mature alien and native acacias fix N under current conditions in the riparian zone. Significantly depleted ?15N was observed in both acacias relative to reference species, although variation in ?15N was not correlated with N concentrations. Calculated contributions of N2-fixation (%Ndfa) suggest that alien acacias fix significantly more of their N than native acacias (~75 ± 5% SE and 53 ± 9% SE, respectively). There was a larger variation in ?15N and %Ndfa in the native acacia, suggesting relatively high plasticity in its N2-fixation contributions. This plasticity was interpreted as a facultative N2-fixation strategy for the native acacia, while the N2-fixation strategy of the alien acacia remained unclear. Our results emphasize the importance of potentially elevated N inputs through N2-fixation by invasive legumes in invaded landscapes. Furthermore, they suggest that N2-fixation by invasive acacias may not respond to fine-scale patchiness in soil N in the same manner as native acacias, making them potential contributors to N excess in Southern Africa.