Effects of scattered Acacia tortilis (Forssk) hayne on soil properties in different land uses in Central Rift Valley of Ethiopia

Among several agroforestry practices in the Central Rift Valley (CRV) of Ethiopia, Acacia tortilis-dominated Parkland agroforestry systems are common. Utilizable information regarding the effects of the tree species on soil conditions in Ethiopia is very scarce to be of use for improved agricultural productivity. This study was conducted in three land use types in CRV areas in Oromia National Regional State in Ethiopia. The aim of the study was to determine the effects of A. tortilis on soil fertility variations along a gradient from the tree base to open area in different land use types. Soil samples from surface layers (0–15 cm) were taken at four concentric transects distances from tree base (0.5, 2 and 4 m), compared with soil samples from the adjacent open areas (15 m distance from the tree canopy cover), and then analyzed following the standard procedures. The results of the study indicated that except for Na the amount of soil nutrients under A. tortilis were significantly varied (P < 0.05) in the land use types. Generally, comparisons between under the canopy and outside the canopy of the tree species indicated a highly significant difference on major soil fertility parameters. The effect of the tree species on soil fertility parameters was significantly higher with the distance from the tree base to outside of the canopy. But soil texture was not affected, indicating that it is more related to parent material and land use practices than the tree influence. The study revealed that the indigenous Parkland agroforestry practices of A. tortilis tree improve soil fertility. Hence, the soil patches observed under A. tortilis trees can be important local nutrient reserves, leading to soil heterogeneity in an otherwise uniform agricultural landscape. This may be important for the agricultural landscape health and on farm biodiversity conservation in agricultural landscapes of similar agroecological conditions.