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Grass competition induces N 2 fixation in some species of African Acacia
Author(s) -
CRAMER M. D.,
CHIMPHANGO S. B. M.,
VAN CAUTER A.,
WALDRAM M. S.,
BOND W. J.
Publication year - 2007
Publication title -
journal of ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.452
H-Index - 181
eISSN - 1365-2745
pISSN - 0022-0477
DOI - 10.1111/j.1365-2745.2007.01285.x
Subject(s) - acacia , biology , legume , nitrogen fixation , botany , competition (biology) , biomass (ecology) , interspecific competition , agronomy , ecology , genetics , bacteria
Summary1 Indigenous species of Acacia are common in African savannas that have N‐rich soils. This raises doubt regarding the extent of plant dependence on N 2 ‐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 δ 15 N values were determined. We also sampled a range of legume and non‐legume saplings from the Hluhluwe‐iMfolozi Park for δ 15 N values. 3 In the pot experiment all species, except A. ataxacantha , which is a forest margin species, nodulated and consequently had lower δ 15 N isotope values than A. ataxacantha . In the field experiment the δ 15 N values of the plants grown with grass were significantly lower (δ 15 N = 0.77 ± 0.08‰) than those grown without grass (δ 15 N = 5.0 ± 0.16‰) for all species, except A. ataxacantha . The δ 15 N isotope abundances of field‐collected leaves of legume saplings were found to be significantly lower than those of non‐legume species (legume δ 15 N = 0.98 ± 0.32‰, non‐legume δ 15 N = 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 δ 15 N in plants grown with grass indicates that N 2 fixation was strongly enhanced by competition with grass for N. 5 N 2 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.