Growth rates of bacterial communities in soils at varying pH:: A comparison of the thymidine and leucine incorporation techniques

The growth rate of bacteria in 19 soils with pH values ranging from 4 to 8 was determined using the thymidine (TdR) and leucine (Leu) incorporation techniques. The variation in isotope dilution and unspecific incorporation was also studied. The mean Leu incorporation into protein was 45% of the incorporation into total macromolecules, and was not affected by soil pH. TdR incorporation into DNA varied between 5 and 20% of that into total macromolecules, with the lowest values in the low-pH soils. Isotope dilution plots for Leu incorporation were linear. This was not the case for TdR incorporation, indicating non-Michaelis-Menten kinetics. The degree of participation (DP) of the added labeled compound in Leu incorporation varied between 0.4 (in low-pH soils) and 0.7 and was directly affected by pH. DP for TdR incorporation varied more (from 0.1 to I), with the lowest values in the low-pH soils. The variation in DP in TdR incorporation was, however, not directly affected by pH. Calculated bacterial turnover times at 20 degrees C varied between 2.3 and 33 days (mean 9.3 days) using TdR incorporation data, and between 2.1 and 13.1 days (mean 5.9 days) using Leu incorporation data. Turnover limes were longer for bacteria in low-pH soils, calculated using the Leu incroporation data, while no effect from pH was found using the TdR incorporation data. Comparing data from aquatic habitats indicated that bacterial growth rates in soil were lower.