Graduate Thesis Or Dissertation

Variation in communities of ammonia-oxidizing and denitrifying bacteria in Fennoscandian boreal forest soils

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  • The objective of this study was to examine the variation among ammonia-oxidizing and denitrifying communities in soils of a natural Fennoscandian boreal forest and of a forest with a long history of fertilization with different levels of nitrogen (N), and to examine whether there are any changes in the microbial communities after the termination of N fertilizer. The natural forest at Betsele, Sweden, represents the inherent variation in N supply, plant productivity, and contrasting soil microbial communities. The long-term fertilized forest, Norrliden, represents an on-going N fertilization experiment with up to 34 years of annual N loading at four levels of ammonium nitrate additions, of which the highest was terminated 14 years ago. Although many studies in the past have been done with microbial communities related to the N cycle, the effect of N and N fertilizer on the ammonia-oxidizing and denitrifying communities is still unclear. Terminal restriction fragment length polymorphism (T-RFLP), a fingerprinting method, was used to examine the variation of the ammonia-oxidizer and the denitrifier communities. Functional biomarkers, the amoA gene for ammonia-oxidizing bacteria (AOB) and the nirK, nirS and nosZ genes for denitrifying bacteria, were subjected to PCR amplification. Amplification of ammonia-oxidizers was more successful in the soils with a higher level of N. The AOB community could be assessed only within the two fertilized plots, which showed a difference in the communities with different level of N fertilization. The difference in denitrifying communities regarding the nitrite reductase (nirK and nirS) and nitrous oxide reductase (nosZ) genes were found to be associated with pH and organic matter concentration in the natural forest at Betsele, followed by nitrate and ammonium concentrations. In the case of the fertilized forest, Norrliden, the community difference was driven predominantly by nitrate and ammonium concentrations, and pH. A fertilization effect was found in the nirK communities but this was less apparent with the nirS communities. The nosZ communities showed fertilization effects. The nirK and nirS communities each represent a part of the denitrifying community whereas the nosZ communities gave the entire picture of the denitrifying communities. Nevertheless, the nirS, nirK, and nosZ communities were all significantly correlated. A shift in the nosZ community was observed towards the control and less fertilized plots after the cessation of the fertilizer at Norrliden. The study of these functional genes provided insight about the variability of the microbial communities responsible for regulating important steps in the N cycle.
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