Impact of Land Use Changes on Soil Quality of a Biosphere Reserve in Southwestern Nigeria

Many primary forests in the tropical regions of the world have been converted into degraded secondary or intensively used agricultural areas. Using the Spatial Analogue technique of studying ecosystem dynamics, soil impact of deforestation and conversion was evaluated by comparing soil properties under the natural forest, secondary regrowth of a logged-over forest, monoculture plantation of Cedrela odorata (Linn), and maize/cassava farm at 0 10, 10 20 and 20 30cm soil depths. % sand decreased with soil depth in all the land use types and was significantly different between the natural forest and each of the logged-over forest regrowth, monoculture plantation and farmland except at 0 10cm depth. % Clay increased with soil depth but did not vary significantly among land use types except at 20 30cm depth. Available phosphorus decreased gradually with soil depth in all land use types with the natural forest and farmland having the highest and lowest mean values respectively. Organic matter varied significantly between each of the natural forest, logged-over forest regrowth and monoculture plantation, with the farmland at the three soil depths. Total nitrogen was generally low in all land use types, though, the natural forest compared better than the others. Soil pH, calcium, magnesium, effective cation exchange capacity, and percentage base saturation decreased with soil depth in all land use types with the natural forest having higher mean values than the others. Indices of change computed for organic matter, total nitrogen, available phosphorus, calcium, magnesium, potassium, effective cation exchange capacity and percentage base saturation, showed that soil management in the introduced land use types, is tending away from sustainability. Inter-cropping with nutrient-regenerating agroforestry trees on the farmland and the use of low-impact logging techniques are suggested for soil conservation.