TitleContrasting Effects of Grazing vs Browsing Herbivores Determine Changes in Soil Fertility in an East African Savanna
Publication TypeJournal Article
Year of Publication2022
AuthorsCoetsee C., Wigley B.J, Sankaran M., Ratnam J., Augustine D.J
Date Published03/2022

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.