Metabolic efficiency and turnover of soil microbial communities in biodegradation tests

Biodegradability screening tests of soil commonly measure (CO2)-C-14 evolution from radiolabeled test compounds, and glucose has often served as a positive control, When constant amounts of radiolabel were added to soil in combination with increasing amounts of unlabeled substrates, glucose and some related hexoses behaved in an anomalous manner, In contrast to that of formate, benzoate, n-hexadecane, or bis(2-ethylhexyl) phthalate, dilution of glucose radiocarbon with unlabeled glucose increased rather than decreased the rate and extent of (CO2)-C-14 evolution, [C-14]glucose incorporation into biomass and V-max values were consistent with the interpretation that application of relatively high concentrations of glucose to soil shifts the balance of the soil microbial community from the autochthonous (humus-degrading) to the zymogeneous (opportunistic) segment. The higher growth and turnover rates that define zymogeneous microorganisms, combined with a lower level of carbon incorporation into their biomass, result in the evolution of disproportionate percentages of (CO2)-C-14. When used as positive controls, glucose and related hexoses may raise the expectations for percent (CO2)-C-14 evolution to levels that are not realistic for other biodegradable compounds.