Grass competition induces N2 fixation in some species of African Acacia

By M. Cramer, S. Chimphango, A. van Cauter, M. Waldram, W. Bond. 2007

108 citations recorded by the source corpus.

1 Indigenous species of Acacia are common in African savannas that have N‐rich soils. This raises doubt regarding the extent of plant dependence on N2 ‐fixation. Why would Acacia spp. enjoy an advantage over other tree species on N‐replete soils? 2 We tested the hypothesis that competition by grass for nutrients would induce increased nodulation of Acacia karroo, A. nilotica, A. tortilis and A. nigrescens seedlings that would enable them to survive better than a non‐nodulating congeneric species (A. ataxacantha). A glasshouse pot experiment was conducted to determine the capacity of the Acacia spp. to nodulate. The Acacia spp. were also grown in a randomized field plot experiment in Hluhluwe‐iMfolozi Park with and without grass coexistence, and biomass accumulation and δ15N values were determined. We also sampled a range of legume and non‐legume saplings from the Hluhluwe‐iMfolozi Park for δ15N values. 3 In the pot experiment all species, except A. ataxacantha, which is a forest margin species, nodulated and consequently had lower δ15N isotope values than A. ataxacantha. In the field experiment the δ15N values of the plants grown with grass were significantly lower (δ15N = 0.77 ± 0.08‰) than those grown without grass (δ15N = 5.0 ± 0.16‰) for all species, except A. ataxacantha. The δ15N isotope abundances of field‐collected leaves of legume saplings were found to be significantly lower than those of non‐legume species (legume δ15N = 0.98 ± 0.32‰, non‐legume δ15N = 2.15 ± 0.32‰). 4 These data confirm that A. karroo, A. nilotica, A. tortilis and A. nigrescens seedlings are capable of nodulating, and do so in their native habitat. The decreased δ15N in plants grown with grass indicates that N2 fixation was strongly enhanced by competition with grass for N. 5 N2 fixation may thus be an important attribute allowing legume tree seedlings to survive competition with grass through a critical period when tree roots and grass roots must compete for nutrients.

Subfields: Ecology, Nature and Landscape Conservation, Soil Science

View Paper: https://www.semanticscholar.org/paper/2eae49bdf3dc664fb7f9f926d407b17f0ec92a75

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Citations in the corpus

What are the effects of substrate and grass removal on recruitment of Acacia mellifera seedlings in a semi-arid environment? (2011, background): “Cramer et al. (2007) ascribe the differences to a change in nitrogen fixation; that is, when competing with grasses, these Acacia species fix nitrogen (there was a decrease in d15N in plants grown with grass).”
Nitrogen fixation in legumes and actinorhizal plants in natural ecosystems: values obtained using 15N natural abundance (2011, background): “Secondly, Cramer et al. (2007) reported that δ15N values were significantly lower in Acacia spp. than C4 grasses in savanna in Kwazulu-Natal, South Africa, and it was concluded that Acacia N2 fixation was important in this ecosystem.”
Emerging patterns in the comparative analysis of phylogenetic community structure (2009, background): “…presumes a taxonomic scale making competitive interactions likely — and despite many specific exceptions (e.g. Brown & Davidson 1977; Schluter 1986; Cramer et al. 2007), the strength of competition is typically expected to be strongest between members of closely related groups (e.g. Darwin 1859;…”
Growth of N2‐fixing African savanna Acacia species is constrained by below‐ground competition with grass (2010, background): “8c) indicated little N2 fixation in these two species, which occur in arid savannas, consistent with the extent of nodulation previously observed in the field (Cramer et al. 2007) and the small amount of apparentN2 fixation ofA. nigrescens in the seedling experiment at the Hluhluwe site.”
Water and nutrients alter herbaceous competitive effects on tree seedlings in a semi‐arid savanna (2009, background): “mopane seedlings, other studies also found suppressing effects of herbaceous vegetation on the growth and survival of various Acacia species (Knoop & Walker 1985; Cramer et al. 2007) and Euclea divinorum (Sharam et al.”