Effects of land use on structure and hydraulic properties of Vertisols containing a sodic horizon in northern Ethiopia

In recent decades significant clearing of native Acacia seyal and Balanites aegyptiaca savannah has preceded expansion of agricultural lands in the semiarid Sahel regions of northern Ethiopia. The main objective of this study was to determine the effects of changes in land uses on structure and saturated hydraulic conductivity (K_s) of a Vertisol under sodic conditions. Disturbed soil samples were taken from savannah-woodland landscape and from cultivated sorghum and sesame fields in the Humera region of Ethiopia, for determination of chemical properties, aggregate stability and K_s. Exchangeable sodium percentage (ESP) increased with soil depth, from ~2% in the 0-0.15m layer to 8.1-10.6% in the 0.9-1.2m layer. Swelling and dispersion was more pronounced in the subsoil (0.9-1.2m) than in the topsoil of the three land uses, due to the higher ESP values of the former. In contrast, the topsoil was more sensitive to slaking forces than the subsoil, probably due to increased particle cohesion in the subsoil. This led to lower K_s values of the topsoils under fast than slow prewetting. The steady-state K_s values under slow prewetting and leaching with deionized water were significantly higher in the savannah-woodland soil than in the cultivated soils, down to 1.2m depth. These differences in K_s values were associated with higher swelling values in the cultivated soils than in the savannah-woodland soil. The differences in the swelling values were manifested at the field scale, where the cracking in the cultivated soils was more intense than in the savannah soil. It was suggested that conversion of natural savannah vegetation to cultivated crops and tillage operations destabilized soil structure in the cultivated plots mainly by an increase of the swelling forces, which, in turn, reduced the K_s values.