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Contrasting Effects of Grazing vs Browsing Herbivores Determine Changes in Soil Fertility in an East African Savanna

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Abstract

We used a long-term herbivore removal experiment where paired exclosure–open treatments were established at the Mpala Research Centre, Laikipia, Kenya, in 1999 to examine changes in soil nitrogen (N) at nutrient-rich glades and adjacent nutrient-poor bushland sites after almost two decades of herbivore removal. Glades in this landscape are created by large inputs of dung and urine from previous long-term corralling of cattle in an otherwise nutrient-poor matrix of woodland (bushland). We predicted (1) a net gain of soil nutrients at bushland sites (that is, inputs of nutrients > losses) and (2) a net loss of soil nutrients at glade sites (that is, inputs of nutrients < losses) following herbivore exclusion. As expected, soil N increased (by 28% after 17 years) with herbivore removal, but remained largely unchanged in the presence of herbivores at low-nutrient bushland sites. However, contrary to our expectations, soil total N in nutrient-rich glades also increased (+ 18%) when herbivores were removed, but declined when grazed (− 11%). Although the underlying mechanisms are unclear, we suggest that increased N fixation by Acacia spp., combined with increased canopy cover and associated tree leaf litter, resulted in elevated soil N following browser removal in low-nutrient bushland sites, while grazer-induced increases in the rate of N transformations between organic and mineral forms resulted in a more “open” N cycle (as evidenced by higher N mineralization rates and foliar N), with increased potential for N loss in gaseous forms, in grazed nutrient-rich glade sites. Grazers and browsers thus appear to affect the N cycle and create and reinforce heterogeneity in unique ways.

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Acknowledgements

This work was funded by the National Centre for Biological Sciences, TIFR, India, National Geographic (grant 982815) and NERC (grant NE-E017436-1). We are grateful to MRC for logistical support and the Office of the President of the Republic of Kenya for permission to conduct the research. We thank Junior Parsare for his assistance in the field and Chengappa S.K. and Balavignesh B. for their assistance with laboratory analyses.

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Author notes

  1. MS, DA and JR initially envisioned the experiment (that is, exclosure set-up). DA, MS, BW and JR gathered data and performed research. CC conceived the particular research questions and developed it further with the help of DA and BW. CC analysed data and wrote the paper with the help and inputs of the other authors.

Authors and Affiliations

  1. Scientific Services, South African National Parks, Skukuza, South Africa

    C. Coetsee

  2. School of Natural Resource Management, Nelson Mandela University, Madiba Rd, George, 6529, South Africa

    C. Coetsee & B. J. Wigley

  3. Department of Environmental Biology, Sapienza University of Rome, P. le A. Moro 5, Rome, Italy

    B. J. Wigley

  4. National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bellary Rd, Bangalore, 560065, India

    B. J. Wigley, M. Sankaran & J. Ratnam

  5. Faculty of Biological Sciences, School of Biology, University of Leeds, Leeds, UK

    M. Sankaran

  6. U.S. Department of Agriculture, Rangeland Resources Research Unit, Agricultural Research Service, Fort Collins, Colorado, USA

    D. J. Augustine

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  1. C. Coetsee
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Correspondence to C. Coetsee.

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Coetsee, C., Wigley, B.J., Sankaran, M. et al. Contrasting Effects of Grazing vs Browsing Herbivores Determine Changes in Soil Fertility in an East African Savanna. Ecosystems 26, 161–173 (2023). https://doi.org/10.1007/s10021-022-00748-7

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