Relation between carbon and nitrogen turnover in soil organic fractions of microbial origin

Labelled ¹⁴C-acetate and ¹⁵N-(NH₄)₂SO₄ were added to a clay soil in the laboratory to follow transformations of microbial C and N, A fungal population developed initially, reaching a maximum by day 5, then rapidly declined and was replaced by a population dominated by bacteria and actinomycetes. Soil samples containing doubly-labelled microorganisms and their metabolites were extracted by Na₄P₂O₇, and the extracted material further separated with phenol.The highly labelled acid-soluble (fulvic acid) fraction of the Na₄P₂O₇ extract contained extracellular metabolites of low molecular weight which were rapidly attacked and converted to new microbial biomass, metabolites, mineral N or CO₂. Na₄P₂O₇ also removed an acid-insoluble (humic acid) fraction of which up to 70 per cent of the labelled C and N could be removed by phenol. Attack of these recently synthesized extracellular materials was indicated by a rapid decline of Na₄P₂O₇ extractable C and N during the growth of bacteria and actinomycetes.Following Na₄P₂O₇ extraction, the residue was sonicated and peptized in water and the components of the microbial biomass were partitioned into sedimentation fractions by centrifugation. The components concentrated in the > 0.2 μm fraction, which were hypothesized as being cell wall components, were more resistant to attack than materials in the < 0.04 μm fraction. The materials in the latter fraction were thought to originate from cytoplasmic constituents. The labelled materials in the < 0.04 μm sized fraction, which accumulated as the fungal population developed, were utilized less rapidly by the developing bacterial population.Decomposition of the microbial population resulted in transfer of C and N through various sediment fractions. The organic fraction (considered to be cytoplasmic material and adsorbed extracellular metabolites) which became labelled as the bacterial population developed, was utilized less rapidly by the developing bacterial population than components removable by Na₄P₂O₇. Evolution of ¹⁴CO₂, production of microbial material and immobilization of N closely paralleled the incorporation and release of these elements from the fractions. The similarity of the behavior patterns of these elements suggested they were intimately associated within the soil microbial system studied. This demonstrated that N transformations were highly dependent on C transformations.