An exotic Australian Acacia fixes more N than a coexisting indigenous Acacia in a South African riparian zone

Abstract

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 N 2 -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 15 N natural abundance field survey suggested that both mature alien and native acacias fix N under current conditions in the riparian zone. Significantly depleted d 15 N was observed in both acacias relative to reference species, although variation in d 15 N was not correlated with N concentrations. Calculated contributions of N 2 -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 d 15 N and %Ndfa in the native acacia, suggesting relatively high plasticity in its N 2 -fixation contributions. This plasticity was interpreted as a facultative N 2 -fixation strategy for the native acacia, while the N 2 -fixation strategy of the alien acacia remained unclear. Our results emphasize the importance of potentially elevated N inputs through N 2 -fixation by invasive legumes in invaded landscapes. Furthermore, they suggest that N 2 -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.

Biomass and nutrient mass of Acacia dealbata and Eucalyptus globulus bioenergy plantations

“…It is not surprising that A. dealbata would have higher nitrogen concentrations and ultimately nitrogen masses compared with E. globulus given that it is a nitrogen fixing plant [33]. A. dealbata can produce up to 80% (reported range from 42 to 80%) of its nitrogen mass through nitrogen fixing [34,35]. If we use the middle of the range (60%) to estimate the amount of nitrogen mass fixed by A. dealbata in our study, the amount of nitrogen accumulated in A. dealbata that was not fixed by the plant is approximately If B1 and or B0 are the same for multiple species and stocking combinations for a given dependent variable then these variable did not significantly affect the relationship (e.g.…”

Effects of land use change on streamflow and stream water quality of a coastal catchment

“…The land use change from natural to alien vegetation probably resulted in changes in the nutrient concentrations observed. Leguminous invasive species such as Acacias are efficient nitrogen fixers (Tye and Drake, 2012) and are known to contribute nitrogen via soil and groundwater due to leaching, adding to nutrient loads in the rivers, especially if these species are cleared. Jovanovic et al (2009) found that NO x was quickly released into groundwater after clearing Acacia saligna from plots, due to high residual nitrogen reserves in the rooting zone, decreased evapotranspiration due to plant removal and increased groundwater recharge.…”

Invasion, alien control and restoration: Legacy effects linked to folivorous insects and phylopathogenic fungi

“…Apart from the influences of host abundance and native plant species richness, the observed higher levels of folivory and fungal diseases may be related to changes in leaf chemical components due to high rates of nutrient inputs to the ecosystems through the presence of nitrogen-fixing invasive alien plants (e.g. A. mearnsii is a putative nitrogen fixing legume (Tye & Drake 2012), that are known to elevate soil nutrient levels (van der Waal, 2009). A study by Naudé (2012), at some of the same riparian sites used in this study, showed that a legacy effect remained after clearing invasive alien plants in that levels of available nitrogen in soil remained high.…”