Growth rates of bacterial communities in soils at varying pH: A comparisonof 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 te chniques. The variation in isotope dilution and unspecific incorporation wa s also studied. The mean Leu incorporation into protein was 45% of the inco rporation into total macromolecules, and was not affected by soil pH. TdR i ncorporation into DNA varied between 5 and 20% of that into total macromole cules, with the lowest values in the low-pH soils. Isotope dilution plots f or Leu incorporation were linear. This was not the case for TdR incorporati on, 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 incorp oration varied more (from 0.1 to I), with the lowest values in the low-pH s oils. The variation in DP in TdR incorporation was, however, not directly a ffected by pH. Calculated bacterial turnover times at 20 degrees C varied b etween 2.3 and 33 days (mean 9.3 days) using TdR incorporation data, and be tween 2.1 and 13.1 days (mean 5.9 days) using Leu incorporation data. Turno ver limes were longer for bacteria in low-pH soils, calculated using the Le u incroporation data, while no effect from pH was found using the TdR incor poration data. Comparing data from aquatic habitats indicated that bacteria l growth rates in soil were lower.