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

We combined measurements of dsDNA, using a newly developed assay for quanti tative determination of dsDNA in crude soil extracts (Sandaa et al., 1998, Soil Biology & Biochemistry 30, 265-268), with measurements of biomass C ac cording to the fumigation extraction (FE) technique and calculations of gro wth characteristics obtained from respiration curves to measure microbial g rowth in soil after glucose addition. Our results showed that the exponenti al increase in respiration rate after glucose addition was accompanied by a n exponential increase in the amount of dsDNA extracted from the soil. Valu es of the specific microbial growth rate (mu) obtained from respiration rat es and from dsDNA concentrations were almost identical. This suggests that changes in dsDNA quantitatively reflected microbial growth in soil after gl ucose addition. However, changes in chloroform labile C (CL-C) did not refl ect microbial growth during the exponential phase. The increases in CL-C pr eceded the formation of dsDNA. This resulted in a 4-fold decrease in the ra tio of dsDNA-to-CL-C 5 h after glucose addition compared to the initial val ue. 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 p roportion of glucose C recovered in the chloroform labile fraction decrease d. The lowest rate of glucose application (100 mu g C g(-1)) resulted in mi crobial uptake of glucose C, without there having been an increase in the a mount of dsDNA nor evidence of growth from the respiration data. Such uptak e without growth confirms earlier suggestions that at low rates of glucose addition the C assimilated is stored or incorporated in the microbial cytop lasm. We conclude that measurements of dsDNA and respiration rates can be u sed to measure specific microbial growth rates after substrate addition to soil, and that dsDNA is an alternative to quantifying microbial biomass und er conditions where the FE technique is not wholly reliable. (C) 1999 Elsev ier Science Ltd. All rights reserved.