The effect of long-term mowing and ungulate exclusion on grass species composition and soil nutrient status on the Athi-Kapiti plains, central Kenya

Abstract

The long-term (>30 years) impacts of mowing (for hay-making) and ungulate exclusion, on natural, unfertilised rangelands were investigated. Changes in grass species composition and soil nutrient status were measured. The study site, situated on the Athi-Kapiti plains of central Kenya, comprised of an ungulate exclusion camp, subjected to an annual mow, and an adjacent control plot, exposed to herbivory from cattle and game. Significant changes were noted in four of the seventeen grass species. Relative percentages of Pennisetum mezianum and P. stramineum increased significantly within the mowed/exclusion plot, while those of Microchloa kunthii and Sporobolus iocladus showed a significant drop. No significant differences were noted in relative frequency of grass species designated as 'decreasers', or in basal cover index. Analysis of soils inside the mowed/exclusion plot showed significantly higher soil pH, and soil Ca levels, but significantly lower plant available soil P and inorganic N, when compared to the control plot. The long-term treatment of mowing/herbivore exclusion, has resulted in severe depletion of plant available soil P and inorganic soil N. Results suggest that differences in species composition can be primarily attributed to the lower defoliation levels, and shading effects inside the exclusion plot, while differences in soil nutrient status can be attributed primarily to the export of nutrients with bailed hay, and to the absence of the stimulating effect that large herbivores have on nutrient recycling. Results are consistent with the hypothesis that herbivory plays an important role in maintaining P recycling in plant available forms, particularly for soils where P may be limiting.

Keywords: East-African rangelands, mowing, nutrient cycling, rangeland ecology, soil phosphorus depletion.

African Journal of Range and FOrage Science 16(2&3): 62–70