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

ABSTRACT

Although lateritic soils are well documented in the humid tropical zone, work on adiabatic effect, which imposes local climatic variations with increasing altitude, has received very little attention in this zone. The aim of this work was to study and specify a soil climosequence in the humid tropical zone of Cameroon and to show the influence of altitude on the geochemical functioning of tropical soils. The work was done in the field and completed by a battery of laboratory 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 and aluminium. 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. In the high plateaus (800-2000 m altitude), soil evolution is dominated by aluminium geochemistry, where aluminium forms gibbsite (either indurated or loose) often associated with small amounts of goethite and hematite. Finally, in the mountainous massifs (> 2000 m altitude), soil evolution is controlled by silica geochemistry, where silica is combined with amorphous aluminium to form allophane. In this soil, the presence of a dark and thick humiferous andosolic horizon in the upper part of the profile is very remarkable. Overall, this soil climosequence reflects geochemical functioning processes which grade with altitude in the humid tropical zone of Cameroon, similar to that commonly observed under the temperate climate. This general scheme could be extrapolated to other humid tropical regions with similar climatic and relief conditions as those observed in Cameroon. However, this general tendency often includes site-specific modifications caused by human activity and local variations of environmental conditions.

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