Impact of plant species, N fertilization and ecosystem engineers on the structure and function of soil microbial communities

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Soil microbial communities are directly and indirectly influenced by a complex system of cross-interactions between different biotic and abiotic factors influencing each other, such as plant species and their respective traits, soil nutrient content, and pH. Microorganisms shape their environment, as important drivers of the C and N cycle. Within the present thesis, several studies were conducted under controlled field and laboratory conditions as well as under natural conditions to unravel the contribution of different influencing factors. The soil prokaryotic community composition of the different soil samples was analyzed DNA-based and RNA-based using 16S rRNA genes and 16S rRNA as phylogenetic marker. The amplicon-based data were processed and diversity and richness estimates were calculated. Betadiversity analyses were conducted to assess overall differences between the different treatments. The obtained DGGE profiles were used for cluster analyses to reveal similarities or differences in the bacterial community structure. The present thesis provided insight into the impact of tree species, tree species diversity, leaf litter and sampling time on the composition and diversity of soil bacterial communities. The obtained data revealed that the leaf litter layer was the major driver of the bacterial community composition in the rhizosphere of young beech and ash trees. It was indicated that different tree species and tree species diversity levels as well as seasonal differences have a minor effect on bacterial community composition. The results revealed that the microbial community composition was not affected significantly by beech and ash saplings, possibly due to the early developmental stage of the tree saplings. Nevertheless, the obtained data revealed that beech saplings inhibited bacterial growth and promoted fungal growth by a root exudation induced soil pH shift. Tree species, differing in their morphology differentially impact soil microbial communities. The analysis of the soil bacterial and fungal communities in natural forest soils under adult beech and spruce trees revealed a significant impact of the analyzed tree species on soil bacterial and fungal community composition. It was indicated that the bacterial and fungal diversity in the analyzed spruce forest soil was driven by soil pH. The impact of high NO3- depositions on CH4 and N2O gas fluxes, and the soil-inhabiting active bacterial and archaeal communities was studied in mesocosms containing soil from a temperate broad-leaved forest. Strong impacts of NO3- fertilization on CH4 uptake rates and N2O emissions in fertilized soil columns were recorded. N fertilization inhibited the CH4 uptake, while the N2O emission increased. The soil bacterial community shifted over the course of the survey towards a denitrifying community, which was dominated by the genus Rhodanobacter. Furthermore, the bacterial diversity and CO2 emissions were reduced within the N-fertilized soil columns. Moreover, CO2 emission rates dropped in both treatments throughout the experiment. This indicated a reduced activity of soil microorganisms, which might be due to C limitation in the used forest soil. Although a shift in the relative abundance of the nitrifying archaeal genus Nitrosotalea occurred, a significant shift in the archaeal community composition was not observed. The results indicate a considerable contribution of methylotrophic, methanotrophic and nitrifying bacterial species, occurring in low abundance, to the observed CH4 uptake. The impact of ants and their activity on the activity and diversity of soil microbial communities were studied. Ant activity channeled honeydew into soils and thereby reduced the microbial biomass in the litter layer. The δ15N signature, the basal respiration and microbial biomass increased in the soil. In contrast, the cluster analysis of the derived DGGE profiles revealed no distinct differences of the microbial community structure in response to the different treatments. Ant activity affected the structure of bacterial communities in grasslands, due to nest building activity and the input of organic substances. Cluster analysis of the obtained DGGE profiles revealed differences in bacterial community composition in response to the sampling site and ant activity. In addition, bacterial community structures in ant nests differed from the surrounding soil. A secondary project of this thesis was the assessment and comparison of microbial communities present in biological soil crusts, sampled at two sites in extrazonal mountain dry steppes in northern Mongolia. The study revealed clear differences in microbial community structure of the two sampling sites differing in their disturbance history.