Summary. The dynamics of the forest to the north of Manaus is tightly linked to that of the soil. The latosol that covers the plateau, which supports a dense forest, consists from top to bottom of: (a) a brown, clayey organic horizon (0.3 m), (b) a yellow horizon, very rich in clay but permeable (from 0.3 to 4 m), (c) a nodular horizon rich in A1 and Fe oxides (from 4 to 9 m), and (d) a horizon which still preserves the sedimentary structures of the parent sandstone, where quartz is intensely dissolved and kaolinite crystallizes in pores. In perfectly flat areas, the clay of the organic horizon is destroyed by aeidocomplexolysis, and the dissolved A1 is transported vertically by the drainage water. A part of this A1 is used to make the gibbsite nodules of horizon (c), and the rest is used to make kaolinite in horizon (d). Because aluminum is thus conserved within any vertical prism, the rate of destruction of horizon (a) is equivalent to the rate of advance of the kaolinization zone into the sediment: the latosol is said to be in equilibrium, the surface remains perfectly flat as it slowly sinks, the quantity of kaolinite increases with time, and the silica released by quartz dissolution in the whole profile is exported by drainage water to the water table. In contrast, near drainage axes, however small initially, the drainage becomes inclined toward the axis. Part of the A1 released by acidocomplexolysis of horizon (a) is now exported to rivers, and A1 is no longer conserved within any given prism. The rate of advance of the kaolinization zone (d) into the sediment now becomes less than the rate of destruction of horizon (a) and the surface sinks faster than that of the surrounding plateau. After this differential 'podzolization' has gone on long enough, it creates a network of 'geochemical valleys' characterized by convex slopes and bounded by sandy soils (campinas). The vegetation becomes sparser and sparser. At the end, only some bushes and lichens survive on the white sand. Key words. Rain forest; sediment; latosol; podzol; geochemical land morphogenesis; dynamic equilibrium and disequilibrium; aluminium; silica.The region to the north of the Amazon, between the rivers Rio Negro and Trombetas ( fig. 1) is covered by a dense, humid, evergreen rain forest that has developed on yellow, clayey latosols. The soils cover plateaux of the tertiary continental 'Barreiras' sediments. These plateaux are more or less strongly dissected by the hydrographic system (Planalto dissecado, Rio Trombetas, Rio Negro) ~. The experimental station for forestry of the Instituto Nacional de Pesquisas da Amaz6nia, where this study was carried out, is about 60 km north of Manaus (see fig. 1) and is representative of the region. These soils have as parental material the products of the weathering of the sediments (Barreiras formations), which involves intense dissolution of quartz, concentration of aluminum hydroxide, and crystallization of kaolinite in dissolution voids 9. It turns out that these sediments, and even more their alterati...

In most soils of the humid tropics, kaolinitic topsoil horizons overlie more gibbsitic horizons. This arrangement cannot be produced simply by leaching. Quantitative measurement of the turnover of chemical elements in the litterfall in an Amazonian ecosystem indicates that the forest cycles a significant amount of elements, particularly silicon. As a result, fluids that percolate through topsoil horizons already contain dissolved silicon. This effect keeps silicon from being leached down and may account for the stability of kaolinite in the soil upper horizons. The soil mineral composition is thus maintained by biological activity.

▪ Abstract  The recycling of elements by plants and plant-induced biological activity cause the rates and products of weathering to be markedly different from what would result in abiotic processes. Plants directly control water dynamics, weathering, and the chemistry of weathering solutions, which is clearly exhibited in equatorial areas where old weathering mantles are greatly influenced by biological activity. Depending on the dynamics of plant-induced organometallic compounds, this weathering results in either clayey soils, which are in a dynamic equilibrium sustained by the forest's cycling of elements, or sandy soils. In most places (tropical as well temperate areas), the weathering mantle can be regarded as being in a dynamic equilibrium sustained by plants.

Abstract. Silicon (Si) released as H 4 SiO 4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0

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