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Gewählte Doctoral Thesis:

Katrin Ripka (2010): Bacterial Nitrogen cycles in agricultural settings.
Doctoral Thesis - Studienabteilung, BOKU-Universität für Bodenkultur, pp 145. UB BOKU obvsg FullText

Data Source: ZID Abstracts
Even though soil microbial communities play an essential role in nitrogen (N) conversion processes in arable soils, parameters and drivers influencing the diversity, activity and quantity of these communities are still a ´biotic-black-box´. Thus, in a first approach, from five different arable soils an extensive biochemical dataset was correlated to abundance and structure of functional bacterial and archaeal marker genes in order to get a deeper insight in microbial N cycling. In two follow up greenhouse pot experiments more specifically the effects on ammonia oxidizing and denitrifying communities upon application of different forms of mineral fertilizer, growth of different barley genotypes combined with inoculation with a plant growth promoting bacterium (PGPB) were tested on bulk and rhizospheric soils. Community structures and abundance of ammonium oxidizing bacteria (AOB) and archaea (AOA) as well as denitrifying bacteria were determined using a culture-independent approach targeting molecular marker genes. Soil texture and differences in C and N pool sizes mainly account for differences in denitrifier and nitrifier community structure and abundance. Nitrifier and denitrifier diversity and abundance in agricultural soils are differentially influenced by specific soil characteristics. Ammonium amendment induced changes in ammonia oxidizing bacterial (AOB) community composition and an increase in abundance in bulk soil and rhizosphere, with changes in AOB numbers lagging behind relative to changes in soil ammonium. More pronounced increases in AOB abundance were found in bulk soil but AOB numbers were also enhanced in the rhizosphere upon plant inoculation with a PGPB. AOA numbers varied in abundance in both ammonium and nitrate amended soils, also each treatment shifted the AOA composition. Alltogether AOA underwent shifts in correspondence with soil nitrate rather than ammonium. AOB were thus considered as the main agents responsible for fertilizer ammonium oxidation, while the roles of AOA in soil N cycling still need to be clarified. Community structures of denitrifiers were found to differ in soils planted with barley relative to unfertilized and unplanted control soils depending on soil type. Plant age rather than N amendment showed to be the decisive factor introducing an increase in denitrifying communities.

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