Effects of the properties of the bacterial community on pH adaptation during recolonisation of a humus soil

Abstract:
A humus soil with a pH of 4.9 was fumigated with chloroform for 5 h, after which only 1% of the initial bacterial activity remained. Half of the fumigated soil was limed to a pH of 7.5. Both the unlimed soil (UL) and the experimentally limed soil (EL) were inoculated with unfumigated (UL) soil or a field-limed (FL) soil with a pH of 6.1. The pH tolerance and activity of each developing bacterial community were monitored over time in the different treatments (ULUL, ULFL, ELUL and ELFL, subscripts indicate inoculum). The activity and pH tolerance of the resulting bacterial communities were measured using thymidine (TdR) incorporation. To study pH tolerance the pH of the bacterial suspensions (bacteria directly extracted from soil) was altered to pH 8.3 and 3.6 using different pH buffers before TdR incorporation. The logarithmic ratio of TdR incorporation at pH 8.3 and 3.6 was used as a measure of pH tolerance. The source of inoculum had a clear effect in the experimentally limed soil. The tolerance of the bacterial community to high pH increased very rapidly and was always higher in the ELFL soil than in the ELUL soil for up to 78 days of incubation. The bacterial activity was also highest in the ELFL soil for the major duration of the experiment. The source of inoculum had little effect on the unlimed soils, since both the ULUL and ULFL soils had the same low pH tolerance and bacterial activity. The tolerance to high pH was initially higher in the ELUL Soil than that observed in a comparable non-fumigated, limed soil by Pettersson and Baath (2003), indicating that the non-colonised environment following fumigation facilitated a more rapid development of bacterial community tolerance to pH changes.