Contrasting effects of grazing vs browsing herbivores determine changes in soil fertility in an East African savanna

Authors: COETSEE, C - Kruger National Park; WIGLEY, B - Nelson Mandela University; SANKARAN, M - National Centre For Biological Sciences; RATNAM, J - National Centre For Biological Sciences; Augustine, David

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2022
Publication Date: 3/11/2022
Citation: Coetsee, C., Wigley, B.J., Sankaran, M., Ratnam, J., Augustine, D.J. 2022. Contrasting effects of grazing vs browsing herbivores determine changes in soil fertility in an East African savanna. Ecosystems. 1-11. https://doi.org/10.1007/s10021-022-00748-7.
DOI: https://doi.org/10.1007/s10021-022-00748-7

Interpretive Summary: African savannahs often have a patchy distribution of vegetation and soil nutrients, some of which arises from the effects of grazing and browsing herbivores. We used a long-term herbivore removal experiment where paired exclosures – open treatments were established at the Mpala Research Centre, Laikipia, Kenya in 1999 to examine changes in the distribution of soil nitrogen (N) and phosphorus (P). Specifically, we studied nutrient in glades, which are treeless patches created by high inputs of dung and urine from abandoned cattle corrals, and in the surrounding nutrient-poor matrix of grassland and woodland. We examined soil nutrients in these different patch types after almost two decades of herbivore removal. In woodland patches, soil N increased slightly over time in the presence of herbivores (+10%) but increased much more (+30%) with herbivore removal, mainly because removal of browsers allowed the woody plants to increase N fixation (conversion from N in the atmosphere to N in roots). In the glades, we found that herbivore removal again allowed soil N to increase over time, whereas soil N in grazed glades declined over time. Here, herbivores affected soil N in a different way, primarily by creating a more “open” N cycle, with increased potential for N loss in gaseous forms. In other words, large herbivore effects on glade soil N stocks are not only influenced by spatial movements of nutrients in and out of glades, but also by the degree to which herbivores accelerate gaseous losses of N. In conclusion, grazers and browsers provide unique mechanisms in creating and reinforcing variation in soil nutrient stocks in African savannas.

Technical Abstract: Savanna rangelands are characterized by substantial spatial heterogeneity in plant communities and soil nutrients arising from edaphic and topographic variability as well as from feedback effects generated by herbivores. 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) and phosphorus (P) at glade and adjacent bushland sites. Glades are treeless, high nutrient patches created by high inputs of dung and urine from previous long-term corralling of cattle in an otherwise nutrient-poor matrix of woodland (bushland). We examined nutrient dynamics after almost two decades of herbivore removal and we expected long-term herbivore removal to result in 1) a net gain of soil nutrients at bushland sites (i.e. inputs of nutrients > losses) and 2) a net loss of soil nutrients at glade sites (i.e. inputs of nutrients < losses). Soil N increased slightly over time in the presence of herbivores at bushland sites (10%) but increased substantially (by 30% after 17 years) with herbivore removal. We suggest that increased N-fixation by Acacia spp. combined with increased canopy cover and increased rates of leaf litter deposition with herbivore removal resulted in elevated soil N at bushland sites implicating that browser species (primarily elephant, dik-dik, and impala) strongly affect N balance at the bushland sites. Contrary to our expectations, we found that soil total N declined in grazed glades (-11%) but increased with herbivore removal (+22%). Here, herbivore-induced increases in the rate of N transformations between organic and mineral forms result in a more “open” N cycle, with increased potential for N loss in gaseous forms. In other words, large herbivore effects on glade soil N stocks are not only influenced by spatial movements of nutrients in and out of glades, but also by the degree to which herbivores accelerate gaseous losses of N. In conclusion, grazers and browsers provide unique mechanisms in creating and reinforcing heterogeneity in African savannas.