Pédogenèse et potentialités forestières des sols sulfatés acides salés des tannes du Sine Saloum, Sénégal

Soils of the "Tannes region" of the Sine Saloum bassin, Senegal, are characterized by great heterogenity in morphology and physical and chemical properties. Their caracteristics are linked to topographic position, material and hydrology. Their recent pedogenetical evolution is a consequence of the severe drought period which occurred all over Senegal since 1971. The drought has had dramatic social consequences but also consequences for the ecosystem. The rainfall deficit started and amplified supersalinization and acidification of the soils throughout the region. The surpersalinity, not widespread before 1971, has rapidly reached all soils, from the low terraces up to "glacis de raccordement". Three main processes enhanced the supersalinization: a. Vertical migration of salts by capillary rise from soil solution and shallow groundwaters, caused by intensive evaporation due to high tempratures (25 - 40°C), which leads to an upward salt flux from 8 to 9 months of the year. b. Inundations by supersaline water from the creeks and rivers, having salt concentration 2 to 3 times that of seawater (46 mS/cm). C. Aeolian accumulations: Saline silt blown from bare areas is deposited in places covered with a vegetation. Because of these processes, soils remain supersaline throughout the year. Nevertheless one can observe a decreasing of the salinity during the rainy season in sandy soils of the "glacis de raccordement" and of sandy levees, and also in loam soils of the upper terrace with a smooth topography, because of saturation by rain water. Soil acidification, unknown before 1971, has rapidly spread throughout the entire Sine Saloum basin. It is due to oxidation of pyritic sediments, provoked by the long drought period, which resulted in drainage, lowering the groundwater tables and evaporation. Pyrite oxidation seems to be the most important pedogenetic process. Its importance depends on the pyrite content of the sediment, local chemical conditions (exchangeable cations, salts) and topographic position. The importance of the topographic position can be illustrated as follows. Acid sulfate soils of the low terraces, which are subject to inundation, have recently formed jarosite of a pale yellow (2.5Y 8/4) colour, while soils in the mid-terraces and in the "glacis de raccordement", which are better drained, show a complete oxidation of pyrite to jarosite, followed by hydrolysis of jarosite to goethite (yellowish colours) and subsequently to haematite (10R 5/8). This evolution, which can be regarded as "chemical ripening", seems to be an extremely rapid phenomenon. In soils having oyster shells in their sediments pyrite oxidation doesn't lead to acidification, because of neutralization of the acidity by carbonates. The chemical ripening processes occur simultaneously with physical ripening, the importance of which depends on the topographic position, decreasing progressively from soils of "glacis de raccordement" to soils of the low inundated terraces. Supersalinity and acidifications have caused severe chemical soil degradation and have annihilated all possibilities of land reclamation. Research pointed out, that in view of the actual arid conditions and lack of scientific knowledge on soil development, the only possible form of land use remaining, is forestry and pasture by introduction of salt and acid tolerant species. In the large area of the Sine Saloum basin, land reclamation for other types of land use is only possible on long term periods, after having taken the necessary land and water management measures, focussing on desalinization and de-acidification of the soils.