Extractable dsDNA and product formation as measures of microbial growth in soil upon substrate addition

We combined measurements of dsDNA, using a newly developed assay for quantitative determination of dsDNA in crude soil extracts (Sandaa et al., 1998, Soil Biology & Biochemistry 30, 265-268), with measurements of biomass C according to the fumigation extraction (FE) technique and calculations of growth characteristics obtained from respiration curves to measure microbial growth in soil after glucose addition. Our results showed that the exponential increase in respiration rate after glucose addition was accompanied by an exponential increase in the amount of dsDNA extracted from the soil. Values of the specific microbial growth rate (mu) obtained from respiration rates and from dsDNA concentrations were almost identical. This suggests that changes in dsDNA quantitatively reflected microbial growth in soil after glucose addition. However, changes in chloroform labile C (CL-C) did not reflect microbial growth during the exponential phase. The increases in CL-C preceded the formation of dsDNA. This resulted in a 4-fold decrease in the ratio of dsDNA-to-CL-C 5 h after glucose addition compared to the initial value. This ratio then increased and towards the end of the incubation (216 h) had reached that of the non-amended soil. With an increase in the rate of glucose addition the proportion of glucose C respired increased while the proportion of glucose C recovered in the chloroform labile fraction decreased. The lowest rate of glucose application (100 mu g C g(-1)) resulted in microbial uptake of glucose C, without there having been an increase in the amount of dsDNA nor evidence of growth from the respiration data. Such uptake without growth confirms earlier suggestions that at low rates of glucose addition the C assimilated is stored or incorporated in the microbial cytoplasm. We conclude that measurements of dsDNA and respiration rates can be used to measure specific microbial growth rates after substrate addition to soil, and that dsDNA is an alternative to quantifying microbial biomass under conditions where the FE technique is not wholly reliable. (C) 1999 Elsevier Science Ltd. All rights reserved.