N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species
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Background and aims
Below-ground grass competition limits woody establishment in savannas. N2-fixing legumes may, however, have a nutritional advantage over broad-leaved species. We hypothesised that broad-leaved non-legume savanna thicket species would be more severely constrained by grass competition for N and consequently respond more to N-fertilization than the legume, Acacia karroo.
A. karroo and five non-legume thicket species (Maytenus senegalensis, M. heterophylla, Euclea divinorum, Ziziphus mucronata, Schotia brachypetala) were grown together in an irrigated competition experiment with clipped-, unclipped-grass and without grass with/without N-fertilizer. The biomass, foliar nutrient, δ13C and δ15N of grasses and woody species were determined.
Growth of both A. karroo and the non-legume species was equally sensitive (c. 90 % reduction) to both clipped- and unclipped-grass competition, regardless of N-fertilization. With grass competition, however, foliar [N] increased and δ15N decreased in response to N-fertilization. Grass biomass accumulation was also unchanged by fertilisation, despite increases in foliar [N] and decreases in δ15N.
The N2-fixation capacity of A. karroo provided no growth advantage over non-legumes. The lack of responsiveness of biomass accumulation by both the woody species and the grasses to N-fertilization, despite evidence that plants accessed the N-fertilizer, indicates limitation by other nutrients.
KeywordsAfrica Bush encroachment Competition Grass Legume N2 fixation
We thank Julia Wakeling, Linda Nell and the field staff of the Zululand Tree Project for field assistance and Ian Newton (Department Archeometry, University of Cape Town) for mass spectrometer analysis. Ezemvelo KZN Wildlife staff are thanked for assistance and permission to work in Hluhluwe-iMfolozi nature reserve. We are grateful for funding from the Mellon Foundation and National Research Foundation. We are also grateful to the reviewers for extensive and helpful comments.
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