S. Christensen

The diversity and function of soil microbial communities exposed to different disturbances

To improve understanding of the relationship between the diversity and function of the soil ecosystem, we investigated the effect of two different disturbances on soil bacterial communities—long-term exposure to the heavy metal mercury and transient exposure to the antibiotic tylosin. In die mercury-contaminated soil the diversity (Shannon index) was reduced as assessed from denaturing gradient gel electrophoresis (DGGE) of amplified 16S rDNA sequences from the soil community DNA and from colony morphology typing of the culturable bacterial population. However, analysis of the substrate utilization profiles did not reveal any differences in diversity. In the tylosin-treated soil, DGGE revealed a small difference in the diversity of 16S rDNA compared to the control soil, whereas analysis of the colony morphology typing or substrate utilization results did not reveal any differences in diversity. Soil function was also affected by mercury contamination. The lag time before soil respiration increased following addition of glucose or alfalfa substrate was longer in the mercury-contaminated soil than in the control soil. Moreover, it was markedly prolonged in mercury-contaminated soil subjected to heat treatment prior to substrate addition, thus indicating reduced resistance to a new disturbance in the mercury-contaminated soil as compared to the control soil. Tylosin treatment did not have any significant effect on any of the respiration parameters measured, either with or without prior heat treatment of the soil.

Effects of tylosin as a disturbance on the soil microbial community

Abstract The effect of a strong temporary disturbance on the soil microbial community was investigated and the ability of the community to show resilience with respect to bacterial diversity and structure was examined. Soil was treated with the antibiotic tylosin and incubated for 2 months. After 3 weeks, the added tylosin and its degradation products had disappeared. During incubation, the populations of bacteria, fungi and protozoa in the soil responded to the tylosin treatment; the changes in the population sizes being strongest the first 2 weeks after treatment, after which it diminished. The diversity (number and abundance) of colony morphotypes decreased temporarily following the disturbance whereas a more permanent change in diversity was revealed investigating amplified 16S rDNA sequences from total community DNA by DGGE. The community structure (PCA) based on both colony morphology, DGGE and sole carbon source utilisation obtained by Ecoplates® was altered due to the tylosin treatment throughout the experiment. The DGGE was the most sensitive method. Differences in diversity and community structure found by this method were maintained for 2 months. However, the results were highly dependent on the DNA-extraction procedure. We have shown that diversity as a composite community parameter can attain its original value following the disturbance, whereas changes in community structure were permanent. It is therefore important to focus on community structure and not only on diversity, when evaluating the effect of disturbances on soil populations in relation to system functioning.

Soil respiration profiles and protozoan enumeration agree as microbial growth indicators

Abstract Carbon, nitrogen or phosphorus limitation of microbial growth in a forest and a field soil was evaluated from measurements of respiration and protozoan counts after nutrient addition. In both soils simultaneous addition of C (glucose) and N (NH 4 NO 3 ) resulted in microbial growth as indicated by a gradual increase in respiration rate whereas a single addition of C did not induce microbial growth. Addition of P (KH 2 PO 4 + Na 2 HPO 4 ) to C-amended soil stimulated activity slightly in forest soil but not in field soil whereas addition of P to soil amended with C and N increased growth markedly in both soils. The stimulation of microbial growth indicated by respiration profiles following C and N addition was supported by enumeration of bacterivorous protozoa. The numbers of soil protozoa increased above the control only when C and N were added simultaneously. These observations lend support to the proposal that respiration patterns may be valuable in an analysis of nutrient limitation of microbial growth in soils.