Hill-slope agro-ecosystems and their implications on environmental systems in rural southwestern Nigeria

This study investigates the effects of continuous cultivation of hill-slopes for arable crops on the vegetation and soil components of the environment in the rural area of southwestern Nigeria. Annual rainfall in this area is 1300-1600 mm, mostly between April and October. The arable crops planted include rice, maize, vegetables, yams and cassava. The paper also discusses the probable consequences of such cultivation on the overall environmental systems of hill-slopes, including the headwaters. The results showed that all vegetation parameters considered were significantly lower on hill-slopes than in the forest. The value of 23.79 m(3) ha(-1) for the volume of wood on hill-slopes compared with 1088.01 m(3) ha(-1) in the forest demonstrates the extent of forest degradation in the area of study. Not only are the structural and nutrient properties of soils significantly degraded on hill-slopes, but the textural properties of sand, silt and clay are degraded compared with forest soil. The following values were found for forest and hill-slope soils, respectively: water holding capacity, 55.8 vs. 40.5%; organic matter content, 4.27 vs. 2.0%; available phosphorus, 15.8 vs. 12.7 ppm; total nitrogen, 0.4 vs. 0.2%; exchangeable calcium 8.9 vs. 3.2 meq per 100 g; exchangeable magnesium 2.4 vs. 1.9 meg per 100 g; exchangeable potassium, 0.6 vs. 0.3 meq per 100 g. The implications of the results and the management measures which could ameliorate soil and vegetation degradation to protect the hill-slope and surrounding environmental systems are discussed.