Measurement of bacterial growth rates in soil

Measurements of the average growth rates of soil bacteria from a soil under three treatments; a conventional high input corn soybean rotation, a reversion to a native successional community (4 years) and a never ploughed grassland, were made by two methods. The specific rates of synthesis of bacterial DNA were determined from the specific activities of the DNA precursor, deoxythymidine triphosphate (dTTP), and purified bacterial community DNA after H-3 thymidine incorporation. These rates were compared with the specific respiration rates of the soil microbiota determined from the initial rate of C mineralization and the microbial biomass C. We show how these two measurements can be used to estimate values for microbial growth yield and maintenance coefficients in soil. Both methods ranked the soil treatments in the same order. The reversion treatment had the highest thymidine incorporation and specific respiration rates and the corn soybean rotation the lowest. The grassland soil had the highest microbial biomass but an intermediate growth rate. Based on thymidine incorporation the doubling time of the bacterial biomass at 25 degrees C was 160 days in soil from the corn soybean rotation and 107 days in the reversion treatment. Fractionation of the H-3-labelled DNA according to its %guanine + cytosine content showed growth of soil bacteria to be widespread among bacterial genera and not confined to a few active species. The soil bacterial growth rates found here are much lower than those found in some other studies using thymidine incorporation but are in agreement with annual C budgets for the soils.