An exotic Australian <Emphasis Type="Italic">Acacia</Emphasis> fixes more N than a coexisting indigenous <Emphasis Type="Italic">Acacia</Emphasis> in a South African riparian zone

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 δ¹⁵N was observed in both acacias relative to reference species, although variation in δ¹⁵N 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 δ¹⁵N 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.