Growth response of the bacterial community to pH in soils differing in pH

The effect of pH on the instantaneous growth of soil bacterial communities was studied in five soils with different pH (4.5-7.8) using leucine (Leu) and thymidine (TdR) incorporation. The pH dependency of bacterial growth was modelled using three different unimodal functions, and the pH(opt) for growth and the pH range in which growth was > 50% of the optimal growth were compared. Leu and TdR incorporation yielded very similar results. The best fits were obtained using a third-degree polynomial function and the cardinal pH model. However, a simple second-degree function was adequate in most cases, yielding very similar pH(opt) values to the other two models. Bacterial growth was highly influenced by pH, showing optimum growth at a pH related to the soil pH. The lowest pH(opt) was found in the most acidic soil and the highest pH(opt) in the soil with the highest pH. The pH(opt) for bacterial growth was close to the soil pH measured in water, but higher (0.7-2.1 units) than the pH measured with 0.1 M KCl. The pH range in which bacterial growth was > 50% of that at optimum was, on average, 1.7 units below and above the optimum pH.