Interet de l'etude du cycle biogeochimique du silicium pour interpreter la dynamique des sols tropicaux

Abstract

Recent studies on the biogeochemical cycle of silica put new constraints into models of genesis and evolution of silica in tropical environments. The role of plants in weathering mass balances are illustrated by phytolith studies. In Dimonika (Congo) latosols, dissolved silica originates mainly from the dissolution of phytoliths rather than from the dissolution of non-biogenic silicates. In andosols from La Reunion island (Indian Ocean), a 15 cm thick, biogenic silica accumulation formed within 4000 years from bamboo forests fires. The turn-over of silica by plants must therefore be taken into account in studies of weathering rates. Weathering rates of trachytic ash layers in La Reunion island show that all the primary minerals are destroyed and that 50% of amorphous secondary Al/Si products are transformed into halloysite. Theses rates, which are faster than the ones obtained in the Hawaii islands, may strongly influence the turn-over of carbon in soils. Besides, phytoliths preserved from dissolution may help to decipher the records of environmental changes in soils. In Salitre (Brazil), the phytolith distribution has been calibrated with charcoal and pollens. The age of phytoliths, which increases with depth, allow to trace the savanna/forest changes.