Variations de la distribution de la matière organique (carbone total et [exposant 13] C) dans les sols ferrallitiques du Brésil. Modifications consécutives à la déforestation et à la mise en culture en amazonie orientale

This study focuses on organic matter eycling in tropical soils of Brazil.The first part was concerned with the relationships, in natural ecosystems, between soilorganic matter, its [exponent 13] C isotopic composition, the climate and the vegetation. For this purpose, the distributions of total carbon and [exponent 13] C isotope in "ferralitic" soils (mainly oxisols and ultisols) fromdifferent regions of Brazil were deterrnined. These demonstrated the existence of three main patterns of the two carbon variables in these soils. Total carbon distributions were mainly influenced by temperature differences. Under identical vegetation, no correlation was found between the distribution patterns of total carbon content and ol3e values. However lower [delta exponent 13] C values werealways associated with high carbon content.The [exponent 13] isotopic composition of these soils was related to both the climate and vegetation history of each site. Soils located under tropical and subtropical forests exhibited 2 distribution patterns. A first group showed a slight variation of the isotopic composition with depth, probably related to a few changes in cover vegetation over a long period of time. A second group of profileswas characterized by a sharp increase in the ol3e values at a given depth, revealing a more complex history of the overlying vegetation which may have changed several times over the years.Finally, for a third group of profiles, under a graminaceous dominant vegetation, and located in subtropical and temperate conditions, the ol3e values showed a slight increase in the upper layers, followed by a gradual decrease with depth. Past cover changes may have been responsible in a large part for these variations.The second part of this work dealt with the modification in humus composition ofamazonian "ferrallitic" soils, after deforestation and installation of a Pennisetum purpureum pasture. Organic matter transfers, inputs and losses were studied, using fractionation and characterization of the organie matter and natural abundance isotopie measurements.These soils, called "Podzolico vermelho-amarelo" soils in the brazilian classification,seemed to undergo at times a degradation in the surface horizons. This was associated with the presence of ferruginous blocks in the underlying horizons. This degradation was revealed by (i) a clay impoverishment in the surface layers, without visible illuviation in the B horizon, (ii) a compaction in the upperlayer of this B horizon, and (iii) an evidence of mottling and aluminium and iron amorphization. The presence of the pasture seemed to increase slightly the above phenomena.Under forest vegetation, the total carbon contents did not exceed 81 T ha[exponent -1] in the first 100 cm of the soil, eventhough quantities higher than 100 T ha[exponent -1] are common in amazonian soils. After tenyears under pasture, they remained almost unchanged. ln the 0-20 cm upper layer nearly half of the carbon of forest origin was lost, but was replaced by organie inputs from the grass pasture.Cette étude est centrée sur le cycle des matières organiques dans les écosystèmes tropicaux du Brésil. Un inventaire a été réalisé qui met en évidence l'existence de trois types principaux de distributions du carbone total et du [exposant 13] C dans les sols ferrallitiques. La distribution du carbone total s'explique en partie par l'action de la température. La composition isotopique [exposant 13]C de ces sols est reliée à l'histoire du climat et de la végétation. Sous forêts tropicale et subtropicale, sont rencontrés des profils montrant une faible évolution avec la profondeur, en présence d'un même type de végétation depuis une période ancienne. Certains profils organiques, également sous forêts tropicale et subtropicale, mettent en évidence une histoire plus complexe, impliquant des successions végétales. La seconde partie de l'ouvrage plus complexe, impliquant des successions végétales. La seconde partie de l'ouvrage a trait aux modifications de la composition de la réserve organique d'un sol ferrallitique d'Amazonie, après installation d'un pâturage de pennisetum purpureum. Les quantités de carbone sous forêt ne dépassent pas 81 T ha [exposant -1] dans le premier mètre ; après dix ans de mise en culture, elles sont restées presque inchangées. Sous pennisetum, les techniques isotopiques [exposant 13] C mettent en évidence le renouvellement de 38% du carbone de la couche 0-20 cm par les restitutions organiques des cultures. L'incorporation du carbone de la graminée s'effectue principalement sous forme des débris grossiers. Une fraction du carbone d'origine forestière associe aux fractions fines s'est minéralisé au cours des dix années de culture et ce départ n'est pas compense par des entrées équivalentes de carbone d'origine graminéenne dans ces mêmes fractions