Utilization of organic materials in soil aggregates by bacteria and fungi

Cultures of six fungi and two bacteria were added to samples of aggregates in which either ¹⁴C-labelled glucose or starch was thoroughly distributed in macro- and micropores or in control samples where the labelled substrates were added to the preformed aggregates and considered to be mainly in macropores. The release of ¹⁴CO₂ was monitored over a 24-day incubation.In the control samples with substrates mainly in macropores, the bacteria were as active as fungi in releasing ¹⁴CO₂ from both soils. When the substrates were distributed in macro- and micropores in aggregates made from a fine sandy loam, the fungi were more efficient than bacteria in releasing ¹⁴CO₂. This was not the case in a self-mulching clay.The initial flush of ¹⁴CO₂ released during incubation of the amended fine sandy loam was due mainly to fungi, which were followed by a secondary bacterial population. The change in populations occurred simultaneously with a step in the cumulative ¹⁴CO₂ release curve thought to be due to the utilization of all the labelled substrate added, followed by renewed respiration as the secondary population flourished. The results presented fit well with an efficiency of C assimilation by micro-organism in soil of about 60%.