Nature of carbohydrates associated with water-stable aggregates of two cultivated soils

The distribution and composition of carbohydrates were investigated in slaking-resistant aggregates and in particle size fractions of two silty soils from the Paris Basin, France. Three hydrolysis procedures, involving hot water, dilute acid and concentrated acid were used and hydrolysates were analysed on a HPLC system to further assess the origin of the carbohydrates. Results showed the predominantly plant origin of the particulate organic matter (>50 μm) and the microbial origin of the clay+silt fraction (<50 μm) carbohydrates. Total organic C content and carbohydrate content, both increased with aggregate size. The proportion of plant-derived carbohydrates also increased with aggregate size, which supported previous results on the distribution of particulate organic matter in aggregates. The clay+silt fraction (<50 μm), located within stable aggregates >50 μm, was enriched in carbohydrates produced by microorganisms. This is consistent with the hypothesis that aggregate stability is mediated partly by the extracellular polysaccharides of microorganisms developing on plant debris occluded within the aggregates.