Influence of Altitude on the Petrological Features of a Soil Climosequencein the Humid Tropical Zone of Cameroon

摘要

Although lateritic soils are well documented in the humid tropical zone, work on adiabatic effect, whichimposes local climatic variations with increasing altitude, has received very little attention in this zone. The aim of thiswork was to study and specify a soil climosequence in the humid tropical zone of Cameroon and to show the influence ofaltitude on the geochemical functioning of tropical soils. The work was done in the field and completed by a battery oflaboratory analyses. The results enabled the definition of a soil climosequence for the humid tropical zone in Cameroon.This climosequence is marked by soils whose evolution is conditioned by the geochemistry of iron, silicon andaluminium. In the plateau ecosystems (500-800 m altitude), iron is expressed either as hardened materials (e.g.ferruginous nodules or duricrust) or light patches on the soils, while silicon and aluminium are combined as kaolinite. Inthe high plateaus (800-2000 m altitude), soil evolution is dominated by aluminium geochemistry, where aluminium formsgibbsite (either indurated or loose) often associated with small amounts of goethite and hematite. Finally, in themountainous massifs (> 2000 m altitude), soil evolution is controlled by silica geochemistry, where silica is combinedwith amorphous aluminium to form allophane. In this soil, the presence of a dark and thick humiferous andosolic horizonin the upper part of the profile is very remarkable. Overall, this soil climosequence reflects geochemical functioningprocesses which grade with altitude in the humid tropical zone of Cameroon, similar to that commonly observed under thetemperate climate. This general scheme could be extrapolated to other humid tropical regions with similar climatic andrelief conditions as those observed in Cameroon. However, this general tendency often includes site-specificmodifications caused by human activity and local variations of environmental conditions.