Distribution of microbial activities and organic matter according to particle size in humid savanna soill (Lamto, Cote d'Ivoire)

The contributions of 6 fractions of soil organo-mineral particles of varying sizes to soil microbial activity were studied using samples of a West African savanna soil. Fractions were obtained by a physical fractionation. Microbial activity was defined by CO2 production and ammonium and nitrate + nitrite accumulations, when samples were incubated at 28 degrees C and 80% of water holding capacity for 3 weeks. The contribution of each fraction was indirectly estimated by comparing the activities of incomplete soils (lacking one fraction) to those of a completely recombined soil (all six fractions). Carbon mineralization was additive, the sum of the CO2 produced by the six fractions alone was similar to the quantity of CO2 produced from a complete recombined soil. Mineral nitrogen accumulation was not additive, the sum of the quantities of total mineral N accumulated in the six fractions was greater than that of the complete recombined soil. The dynamics of net nitrogen mineralization in individual fractions depended on the nitrogen mineralization in other fractions.