Influence of herbivores and trees on soil biochemical properties of a semi-arid savanna

Abstract

The study evaluates the interactive effects of mammalian herbivores and trees on soil biochemical properties of a semi-arid savanna ecosystem in South Africa. Composite soil samples were collected at three radial distances from a tree base, namely besides the tree trunk, canopy edge, and outside canopy, for two trees (Combretum apiculatum and Grewia bicolor), in the full exclosure and open access area in the Nkuhlu exclosures of Kruger National Park. We measured total nitrogen (TN), total carbon (TC), pH, available phosphorus (P), microbial activity, exchangeable cations (calcium [Ca²⁺], magnesium [Mg²⁺], sodium [Na⁺] and potassium [K⁺]) and cation exchange capacity (CEC). Result indicates that pH and Mg²⁺ in full exclosure were significantly higher than outside, whereas TN, TC, available P, microbial activity, K⁺ and CEC were lower. Under canopy samples had more pH, TN, TC, available P, K⁺ and Ca²⁺ than those collected from other sampling zones, mainly because of the effect of litter accumulation under the tree canopies. With the exceptions of CEC and microbial activity, the effects of the two tree species on soil nutrients were similar. Microbial activity was significantly high, whereas CEC was low under G. bicolor than C. apiculatum. The canopy edge of G. bicolor had the highest microbial activity, while the area outside the canopy of C. apiculatum had the lowest than all the other treatments. These results indicate that the presence of herbivores and woody species differentially affects the spatial distribution of the various nutrients, soil microbiota and other chemical properties depending on their radial distances from the tree base.

Conservation implications: It is ecologically unwise to completely eliminate trees and herbivores from savanna ecosystems, as they help to maintain soil fertility and biodiversity.