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 change 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 particle 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 remains and approximately 190 mg g^?1 TOC in the finest organo-mineral fractions, whereas there was always less sugar in plantation soils. The monosaccharide xylose and mannose have different distributions: xylose appears to be the marker of the vegetal inheritance, whereas the dominance of mannose in the clay fraction bears the signature of current microbial sugar synthesis. The quantitative and qualitative evolution of the whole soil carbohydrates was studied as a function of plantation age. Carbohydrate-C represents 131 mg g^?1 of the soil organic carbon 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 mainly to the stimulation of the mineralization of the glucose, which represented 60% of the total sugars in savanna soil and only 45–48% in tree plantations. The ratio arabinose + galactose + fucose]/rhamnose + xylose], which is the largest in the oldest plantations, is significant for evaluating the replacement of carbohydrates of the original grass savanna by those of the trees.