Factors Affecting Rates of Change in Soil Bacterial Communities

Sammanfattning: The soil bacterial community is under the constant influence of its environment. Changing any of the factors affecting the bacterial community will induce a selection pressure which, with time, will change the community. In this work the effect of changing the temperature or pH on the soil bacterial community was studied. Measurements using the [3H]-thymidine incorporation technique showed that the soil bacterial community had an optimum temperature for growth and activity between 25 and 30°C. Compared to fungi, the soil bacterial community was more adapted to higher temperatures (above 30°C) and vice versa at low temperatures (below 10°C). Below optimum temperatures for growth the temperature dependency of the bacterial community was well modelled by the square root (Ratkowsky) model. When measuring instantaneous respiration at different temperatures an uncoupling was found between respiration rate and bacterial activities at temperatures above 30°C, indicating that respiration might not be an accurate activity measurement at higher temperatures. Adaptation of the soil bacterial community to higher temperature occurred when increasing the temperature from 5 to 30°C, but later decreasing it from 30 to 5°C had a much smaller effect. Thus, the turnover rate of the bacterial community, which is lower at 5°C than at 30°C, affected the rate of adaptation. When increasing the temperature, the change in PLFA pattern was faster than the change in community temperature tolerance, indicating changes due to phenotypic plasticity. The effect of changing the pH of the soil bacterial community has also been studied. The temperature influenced the rate of adaptation to higher pH of the soil bacterial community after liming from pH 4.9 to pH 7.5. The community tolerance to high pH increased fastest at the highest temperature (30°C) and slowest at the lowest temperature (5°C). The change in phospholipid fatty acids showed a similar pattern, although the changes were slower, indicating changes in species composition. Moreover, the rate of change of the bacterial community tolerance to pH was not correlated to the effect of temperature on bacterial activity, indicating that the rate of adaptation and the turnover of the bacterial community were not directly correlated. The bacterial recolonisation after fumigation (used to kill 99% of the bacteria) was studied in limed and unlimed soil inoculated with high (6.1) or low pH (4.9) soil. The rate of recolonisation was found to be faster in the limed soil than in the unlimed one. The fastest recolonisation was found when the limed soil was inoculated with a high pH soil, indicating that the intrinsic properties of the soil bacterial community are important for the development of a community after a perturbative event. The bacterial community tolerance to high pH was also highest for this soil during the 78 days of the experiment, indicating that the effect of the inoculum was a long-lasting one. Studies of the development of community structure during recolonisation using PLFA and DGGE analysis revealed that changes in these measurements coincided with the change in function (community pH tolerance and activity) of the soil bacterial community.