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N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species

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Abstract

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.

Methods

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.

Results

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.

Conclusions

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.

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Acknowledgments

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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Authors and Affiliations

  1. Department of Botany, University of Cape Town, Private Bag X1, Rondebosch, 7701, South Africa

    Michael D. Cramer & William J. Bond

  2. School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia

    Michael D. Cramer

Authors
  1. Michael D. Cramer
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  2. William J. Bond
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Correspondence to Michael D. Cramer.

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Responsible Editor: Hans Lambers.

Electronic supplementary material

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Online resource 1

Initial and post-harvest soil characteristics (mean ± SE) averaged across all plots after determining that there were no significant differences between soils of various treatments. Soils were dried at 70 °C for 48 h prior to analysis. Different letters indicate significant differences (P < 0.05) as determined by two-way ANOVA. (DOCX 28 kb)

Online resource 2

Clipped (left) and unclipped (right) grass foliar a, b) δ13C values, c, d) δ15N values, e, f) [P] and g, h) [N] of foliar samples collected regularly throughout the growing period. Arrows indicate the addition of N fertilizer at three times during the growing period. Symbols represent the mean ± SE (n = 3 plots) (DOCX 446 kb)

Online resource 3

Contribution of fertilizer N to the N-budgets of both clipped (left) and unclipped (right) grass estimated on the basis of δ15N values of foliar samples collected regularly throughout the growing period. Arrows indicate the addition of N fertilizer at three times during the growing period. Symbols represent the mean ± SE (DOCX 65 kb)

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Cramer, M.D., Bond, W.J. N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species. Plant Soil 366, 563–574 (2013). https://doi.org/10.1007/s11104-012-1456-4

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