Density fractions of soil macroorganic matter and microbial biomass as predictors of C and N mineralization

Macroorganic matter of arable soils which had received different inputs of organic residues for 25 y and grassland soils that had been under grass for at least 8 y was fractionated into light, intermediate and heavy fractions using a stable silica suspension as heavy liquid. For all residue treatments, the C-to-N ratios of organic matter decreased in the order light, intermediate, heavy macroorganic matter (fraction > 150 μm) and non-macroorganic matter (fraction < 150 μm). Residue application had a stronger effect on the amount and C-to-N ratio of macroorganic matter fractions than on the amount and C-to-N ratio non-macroorganic matter. Textural effects were apparent with the proportions of soil N in the light and intermediate fractions being higher in coarse-textured grassland soils than in fine-textured grassland soils. C and N mineralization were positively correlated with the amount of C and N in the light fraction and the active microbial biomass. The correlation with mineralization decreased with increasing stability of the organic matter fractions. C and N mineralization per unit of total microbial biomass were lower in fine-textured soils than in coarse-textured soils. This is ascribed to a greater physical protection of the organic matter in fine-textured soils than in coarse-textured soils.