Browsing by Person "Ditterich, Franziska"

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Microbial community structure and function is shaped by microhabitat characteristics in soil
(2016) Ditterich, Franziska; Kandeler, Ellen
Soil microorganisms play a key role in degradation processes in soil, such as organic matter decomposition and degradation of xenobiotics. Microbial growth and activity and therefore degradation processes are influenced by different ecological factors, such as substrate availability, pH and temperature. During soil development different microhabitats are formed which differ in their physiochemical properties. There is some evidence that mineral composition is a driver for specific microbial colonization. Thereby, the heterogeneity of soils with differences in mineral composition and substrate availability can lead to a spatial distribution of soil microorganisms. At the soil-litter interface, a biogeochemical hot spot in soil, the abundance and activity of soil microorganisms increases due to high substrate availability, and degradation processes such as pesticide degradation are enhanced. This thesis aimed to clarify the influence of habitat properties on the structure and function of the microbial community in soil. In particular, focus was on mineral-microbe interactions that result from the mineral composition and substrate availability in an artificial soils system. Furthermore this thesis was designed to increase our understanding of the bacterial and fungal roles in pesticide degradation at the soil-litter interface using 4-chloro-2-methylphenoxyacetic acid (MCPA) as a model xenobiotic. These two aspects of the thesis were examined in three studies. The first study focused on the succession of microbial communities and enzyme activities in an artificial soils system with varying mineral composition and substrate availability over a period of 18 months. In the second study a microcosm experiment was used to study the bacterial pathway of MCPA degradation at the soil-litter interface. Over a period of 27 days the succession of bacterial degraders was followed. The third study focused on the degradation of MCPA in soil by nonspecific fungal enzymes, through the addition of fungal laccases as well as litter during 42 days of incubation. Both studies indicated the involvement of fungi in MCPA degradation and the importance of the ecological behavior of different degraders as a function of substrate availability. Results of the first study indicated that the microbial community was affected by mineral properties under high substrate availability and by the availability of beneficial nutrients at the end of incubation when substrate had become limited. The measured enzyme activities provided clear evidence that microbial community structure was driven by nutrient limitation during incubation. In the presence of easily available organic substrates at the beginning of the experiment, the soil microbial community was dominated by copiotrophic bacteria (e.g. Betaproteobacteria), whereas under substrate limitation at the end of incubation, more recalcitrant compounds became important to oligotrophic bacteria (e.g. Acidobacteria), which then became dominant. The results of the second study indicated that the contribution of the potential degraders to degradation of MCPA differed, and this was also seen in the succession of specific bacterial MCPA degraders. Added litter stimulated MCPA degradation due to the availability of litter-derived carbon and induced a two-phase response of fungi. This was seen in the development of pioneer and late stage fungal communities. Both fungal communities were probably involved in MCPA degradation. Therefore, the third study focused on the fungal pathway. These results indicated that the fungal laccases used had no direct influence on degradation and were as efficient as litter in providing additional nutrient sources, increasing MCPA degradation by bacteria and fungi. The observed differences between litter and enzyme addition underscored the observation that the enzyme effect was short-lived and that substrate quality is an important factor in degradation processes. In conclusion, this thesis demonstrated that soil microbial communities and therefore degradation processes are driven by mineral composition as well as substrate availability and quality. In addition, this thesis extends our understanding of degradation processes such as the degradation of xenobiotics, with MCPA as model compound, in soil. The combined insights from all three studies suggest that the use of a simple system such as the artificial soil system can increase our understanding of complex mechanisms such as degradation of pesticides.