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Organization and hydrobiogeochemical functioning of the lateritic covers of Amazonia


The Amazon basin is the biggest area of the world covered by rain forest and lateritic soil. There is a growing concern about extensive land clearing in the basin wich might alterete the climate and the productivity of the soils. However, are still poorly understood and only known through dispersed pedological and hydrological studies. The project is a first attempt to link soil distribution in the Amazon lateritic landscapes to weathering and pedogenetic processes as well as to water flow and water quality. Soil distribution at subcatchement scale and the scale of the whole Amazon basin is principally deduced from the broad scale soil maps of the radambrasil project. Therefore our actual knowledge on soil distribution and soil water processes were used to propose in the project a conceptual soil-water landscape model for the laterites of the Amazon basin with strong emphasis on natural land degradation. The model illustrates in seven superimposed cross sections the morphological change of the uppermost laterites due to sncient and/or recent expansion of waterlogged areas in the landscape. Soil changes under aquic conditions are linked to the successive developement of the following processes: 1) redistribution and exportation of iron; 2) exportation of si and al and 3) redistribution and exportation of organic matter and iron-clay residue. By activating the geochemical erosion, these processes modify the morphology of the landscape. They also reduce the productivity of the soil, increase the electrolyte load ground watertables and rivers. The project also tend to demonstrate that the Amazon laterites have been placed into desiquilibrium in reponse to regional environmental conditions thereby developing severe land degradation problems at specific places in the basin. It is suggested that human activities will accelerate land degradation by activating pre-existing processes. Accordingly the model and the soil maps could be used in the future by land and water supply managers to develop more efficient manegement strategies according to conditions of increasing land degradation. The objective of the project is carry out detailed pedological and hidro-geochemical investigations at four key sites representatives of the main lateritic soil landscapes of the Amazon basin in order to 1) validate or modify the global soil-water landscape model suggested in the project; 2) better chacaracterise the processes involved in the transformation of these landscapes and 3) state whereas these processes are ancient or still ocurring nowadays. (AU)

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