SOIL PROPERTY CHANGES FOLLOWING CONVERSION OF ACACIA WOODLAND INTO GRAZING AND FARMLANDS IN THE RIFT VALLEY AREA OF ETHIOPIA

Continued conversion of woodlands into grazing and farmland is seriously undermining the natural ecosystem of the dry and fragile Rift Valley areas of Ethiopia. This study investigated the effects of land-use change on soil organic carbon (SOC), total nitrogen (N), pH, exchangeable bases, cation exchange capacity (CEC) and base saturation (per cent) in three adjacent land-use types: controlled grazing, open-grazing and farmland. A total of 81 soil samples were collected and analysed. Contents of SOC and total N decreased drastically in open-grazing and farmland (p < 0.001), and were significantly higher in the top 0.2m than in the subsurface soil layer. Compared with the controlled grazing, reductions in the contents of SOC and total N in the top 1 m soil layer were 22-30 and 19 per cent, respectively, due possibly to the decrease in plant biomass input into the soil and the fast decomposition of organic materials. Long-term cultivation had significantly increased the concentration of exchangeable K. Exchangeable Na was high in the lower layers, while Mg was higher in the top surface soil. CEC also varied with soil depth (p = 0.016); it was higher in the topsoil than in the subsurface soil, which may be, among others, due to the differences in soil organic matter distribution with depth. Although these semi-arid soils are known to have low organic carbon and CEC levels, the values from the current study area are critically low, and may indicate the further impoverishment of the soils under high agricultural and grazing pressures. Copyright (C) 2010 John Wiley & Sons, Ltd.