Monday, 10 July 2006

Hydrolyzable Carbohydrate In Tropical Soils Under Adjacent Forest And Savanna Vegetation In Lamto, Côte d'Ivoire.

Hassan Bismarck Narco1, Marie Christine Larre-Larrouy2, Christian Feller2, and Luc Abbadie3. (1) Univ Polytechnique de Bobo-Dioulasso, Institut du Développement Rural, Bobo-Dioulasso, Burkina Faso, (2) Laboratoire Matière Organique des Sols Tropicaux IRD, Montpellier, France, (3) Biogeochemistry and Ecology of continental environment laboratory UMR 7618, 46 rue d'Ulm, Paris, 75230, France

Carbohydrates represent 5-25% of the organic matter in soils. They constrain microbial activities and mineral nutrient production in soil and also reflect the whole micro-organism community dynamic. The objective of this study was to determine the contents and composition of hydrolyzable carbohydrates in soils collected in a forest-savanna mosaic landscape in the region of Lamto (Côte d'Ivoire). Capillary gas chromatography was used to identify and determine carbohydrates in soil profile under four tropical ecosystems: gallery and plateau forests, grass and shrub tree savannas. Forest soils were higher in organic matter than savanna soils (0.50-2.96% C v. 0.53-1.22% C). The carbohydrate-C content of soils, expressed as percent of total soil organic C was low, a likely consequence of the tropical climate that promotes a rapid decomposition of surface plant debris. The carbohydrate-C content was higher under savanna soils (5-7%) than under forest ones (3-4%). Glucose, ribose, mannose, xylose and galactose were the 5 most abundant extractable monosaccharides in all soils. Between them, only xylose and ribose are controlled by the vegetation type. The [(galactose + mannose):(arabinose + xylose)] and [mannose:xylose] ratios suggested that most of soil sugars derive from microbial biomass. The large abundance of microbial carbohydrates indicates intense microbial activities in the soil and then, rapid decomposition of soil organic matter favored by the long wet season, with high temperatures and soil water availability at the site study. Results suggest clearly that the climate likely controls the amount and composition of carbohydrates in Lamto soils. Keywords: monosaccharide, soil microorganisms, soil organic matter, microbial-derived compounds, carbon, nitrogen.

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