Natural nitrogen-15 abundance and carbohydrate content and composition of organic matter particle-size fractions from a sandy soil

Sandy soil samples collected from under a woody/grass savanna in the Lamto experimental area (6degrees13N, 5degrees20W; Cote d'Ivoire, West Africa), were fractionated according to particle size with the aim of measuring the natural abundance of N-15 and determining the contents and composition of hydrolysable carbohydrates of soil organo-mineral particles for a better understanding of the contribution of each individual fraction to the soil function. The contributions of the fractions <20 mum to the total pool of organic matter were 77% for C and 84% for N. Larger amounts of carbohydrates were found in the clay and silt fractions (3,784-6,043 mug g(-1) soil). The carbohydrate composition indicated that microbe-derived carbohydrates [e.g. galactose (Gal) and mannose (Man)] accumulated preferentially in the fine fractions while plant-derived sugars [e.show $132#g. arabinose (Ara) and xylose (Xyl)] were dominant in coarse fractions. A negative relationship was observed between C:N ratio and N-15 natural abundance on the one hand, and on the other hand between C:N and (Gal+Man):(Ara+Xyl), Man:(Ara+Xyl) and Man:Xyl ratios, clearly indicating that the chemistry of the organic materials of the particle-size fractions reflects a change from soil chemistry dominated by plant materials to that dominated by microbial biomass and metabolites. The contribution of a given fraction to soil microbial activity is controlled by the quality or quantity of associated soil organic matter, its microbial biomass and also by the accumulation of microbial-derived carbohydrates which can be resynthesized or recycled.