CHANGES IN THE AMOUNT AND DISTRIBUTION OF NEUTRAL MONOSACCHARIDES OF SAVANNA SOILS AFTER PLANTATION OF PINUS AND EUCALYPTUS IN THE CONGO

In the Congo, near Pointe-Noire, Pinus and Eucalyptus were planted on the savanna for 30 years. We have characterized the effects of this ch ange on land-use on the composition of carbohydrates in whole soil and particle-size fractions of the soil. Carbohydrates represent variable proportions of the total soil organic carbon (TOC) of various particl e size fractions. The largest proportions of sugar-C were found in the savanna soil with as much as 250 mg g(-1) TOC in the coarsest plant r emains and approximately 190 mg g(-1) TOC in the finest organo-mineral fractions, whereas there was always less sugar in plantation soils. T he monosaccharide xylose and mannose have different distributions: xyl ose appears to be the marker of the vegetal inheritance, whereas the d ominance of mannose in the clay fraction bears the signature of curren t microbial sugar synthesis. The quantitative and qualitative evolutio n of the whole soil carbohydrates was studied as a function of plantat ion age. Carbohydrate-C represents 131 mg g(-1) of the soil organic ca rbon in the savanna soil, but decreases to an average value of 75 mg g (-1) in plantations more than 6 years old. This appears to be due main ly to the stimulation of the mineralization of the glucose, which repr esented 60% of the total sugars in savanna soil and only 45-48% in tre e plantations. The ratio [arabinose + galactose + fucose]/[rhamnose xylose], which is the largest in the oldest plantations, is significan t for evaluating the replacement of carbohydrates of the original gras s savanna by those of the trees.