DNA determinations during growth of soil microbial biomasses

We attempted to quantify microbial growth in soil by means of DNA determination after glucose amendment. An FDNA conversion factor of 5.0 was used to convert μg DNA g−1 soil to μg Cmic g−1 soil during the growth phase. The conversion factor acquired rested on a regression analysis between soil microbial biomass-C (Cmic) estimated by the substrate-induced respiration technique (SIR) and dsDNA using a modified, miniaturized dsDNA extraction procedure which included 44 field and forest soils with a coefficient of determination of r2 = 0.95. Verification of this conversion factor was tested on eight arable soils where Cmic was determined by substrate-induced respiration (SIR)-, chloroform fumigation-incubation (CFI)-, chloroform fumigation-extraction (CFE)-, and application of the FDNA conversion factor. The congruency between the Cmic values obtained through these different techniques was satisfactory since five of eight soils gave similar Cmic values which were not statistically significantly different. The soils were thereafter amended with glucose and microbial growth followed by Cmic determinations with CFI, CFE, and DNA conversion over a period of up to 264 h at 22 °C. Concomitant CO2 analyses gave clues to two kinds of growth processes with respect to speed. Based on DNA conversion the calculated traditional growth parameters such as the specific growth rate (μ) lay in the range between 0.0046 and 0.022 h−1 which is several fold slower than μ values based on CO2 conversion but are in accordance with data in the earlier literature on growth rates for bacteria and fungi in soil done with traditional plate counts. These results suggest that DNA determinations can be applied as an alternative index for growth studies in situ.