Epsilon Open Archive

Abstract

Quantification of changes in soil properties (particularly organic carbon and total nitrogen) due to natural and anthropogenic influences is essential in understanding carbon fluxes between land and atmosphere. This thesis examines the effects of topographic aspect, vegetation community and land use on physical and chemical properties of soils in south-eastern Ethiopia. Soil samples were collected under three vegetation communities, Schefflera abyssinica/Hagenia abyssinica (SHaD), Hypericum revoltum/Erica arborea/Schefflera volkensii (HESD) and shrub-sized Erica arborea (EAD), at four topographic aspects (north/south/east/west-facing). Soil samples were also collected from three land use types (native forest, cropland, grazing) between 3000-3150 m altitude. The soil properties examined generally exhibited significant variations with respect to vegetation and aspect. Sand, silt and clay content was high under EAD, HESD and SHaD respectively. Soil bulk density was lower in A- than B-horizons for all vegetation types and aspects. Available P was high under all south-facing communities and in east-facing A-horizon soils under SHaD. Soil pH was high in both horizons under SHaD. Base cation adsorption in soil followed the trend Ca2+>Mg2+>K+>Na+ for all communities and aspects. CEC was high under south- and east-facing SHaD and EAD. Overall, percentage base saturation was high under SHaD across all aspects. Soil organic carbon (SOC) and total nitrogen (N) stocks to 1.0 m depth were highest under EAD (46.03 kg C m-2, 3.61 kg N m-2) and southern aspect (44.97 kg C m-2, 3.75 kg N m-2). Mean annual temperature was important for variations in SOC and total N stocks along vegetation gradients across all aspects. About 45% of SOC was held in the upper 0.3 m, indicating that large amounts of CO2 can be released to the atmosphere if the vegetation communities are cleared for arable/grazing land. Conversion of native forest into cropland significantly increased soil bulk density and pH while reducing SOC, total N and CEC concentrations by 31, 32 and 38%, respectively (1.0 m layer). Protecting remnant afroalpine/afromontane vegetation communities or improving existing cropping systems could mitigate nutrient losses while enhancing organic carbon sequestration for sustainable agriculture, ecosystem functioning and climate change mitigation.