Influence of Altitude on the Petrological Features of a Soil Climosequence in the Humid Tropical Zone of Cameroon
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.
Weathering and soil formation in rhyolitic tephra along a moisture gradient on Alcedo Volcano, Galápagos
Abstract The Galapagos archipelago has extensively been studied because of its geological and climatic setting and especially for its peculiar flora and fauna. However, there is still little information about its soils. The objective of this study was to investigate weathering and soil formation on Alcedo Volcano, which is the only volcano of Galapagos that has erupted abundant rhyolitic materials. Soils were sampled from three sites along an elevation/moisture gradient, site 1 at 872 m above sea level (a.s.l.) (humid zone), site 2 at 621 m a.s.l. (dry zone) and site 3 at 377 m a.s.l. (very dry zone). Soil pH (in H2O) ranged from 6.0 to 6.6 but did not vary significantly among the three sites. Water retention (at 1500 kPa) and phosphate retention increased with elevation/moisture. Similarly, soil organic carbon (SOC) stocks increased from approx. 1.1 kg m−2 in the very dry zone to approx. 7.5 kg m−2 in the humid zone. Allophane and ferrihydrite were the dominant colloidal constituents in the humid zone (site 1), whereas halloysite was found in the dry zone (site 2) and only traces of halloysite in the very dry zone (site 3). At site 1, the soil was borderline between andic and vitric properties and was classified as Silandic Tephric Andosol or Vitric Tephric Andosol for andic and vitric character, respectively; at site 2, the soil had vitric properties and was classified as Vitric Tephric Skeletic Andosol; and at site 3, the soil was borderline with respect to the expression of vitric properties and was classified as Vitric Tephric Skeletic Andosol in the presence and as Tephric Skeletic Regosol in the absence of vitric properties. Our results show that different moisture availability strongly affected SOC accumulation and favored the formation of different types of colloidal constituents (amorphous vs. crystalline) in the rhyolitic tephra of Alcedo Volcano. This has important bearings on the expression of andic and vitric soil properties and, hence, on the distribution of Andosols in the studied environment.
THE CATEGORIES OF ALUMINIUM‐ AND IRON‐HUMUS COMPLEXES IN ANDO SOILS DETERMINED BY SELECTIVE DISSOLUTION
Summary 
Thirty-three samples of Ando soils were extracted by sodium pyrophosphate (O.IM; pH 10) and by dithionite-citrate. The Al, Fe, and C contents of these two extracts offer a means of differentiating the status of humus in relation to Al and Fe in the different horizons. The humus that forms at first in the Al horizon has a very low complexing ability for Al and Fe and little is dissolved by pyrophosphate. The humus evolves with time or pedogenesis into forms that complex Al and Fe released from volcanic ash by weathering, and which are dissolved by pyrophosphate. In the old horizons, the humus further reacts with additional Al and Fe, some of which may be present as hydrous oxides or allophane-like constituents, allophane and imogolite. This reaction probably makes the humus less soluble in pyrophosphate.