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Ludwig-Maximilians-Universität München (2009)

Impact of Environmental Factors on Key Functional Groups involved in Nitrogen Cycling in Agricultural Ecosystems

Hai, Brigitte

Titre : Impact of Environmental Factors on Key Functional Groups involved in Nitrogen Cycling in Agricultural Ecosystems

Auteur : Hai, Brigitte

Université de soutenance : Ludwig-Maximilians-Universität München

Grade : Doktorgrades der Naturwissenschaften 2009

Résumé
The investigation of biochemical cycles in different environmental habitats is becoming more and more vital to improve and sustain the productivity as climate change and management practices drastically change speed, intensity and balance of nutrient cycling. The conversion of nitrogenous compounds in terrestrial ecosystems, mainly mediated by microorganisms, is regarded as a central turnover process in ecology, as nitrogen is one of the limiting factors for plant growth. The different processes of the nitrogen cycle are controlled by several biotic and abiotic factors. The adaptation mechanisms of functional microbial groups in response to governing factors, however, are still poorly understood. In this thesis, nitrogen-fixing, ammonia-oxidizing and denitrifying communities were examined with regard to their responses in gene abundances to different agricultural treatments in a long-term agro-ecosystem in semi-arid Burkina Faso (Westafrica). Data were obtained from the rhizosphere of Sorghum over three plant development stages to follow developmental and seasonal effects. The different microbial groups were targeted via respective functional genes that encode the nitrogen-transforming enzymes. The observed changes in community abundance along the entire growth period lasting over the rainy and the dry season indicated that climatic factors and plant development stages interacted in diminishing nitrogen inputs and related effects. As compared to continuous straw and/or urea amendment, manure emerged to be the most important driver increasing gene abundances and shaping the ratios of nitrogen-fixing, ammonia-oxidizing and denitrifying microbes. As a result, coherences between community proportion shifts, amendments and productivity could be observed. Another long-term agricultural experimental site, located in a moderate climate region (Theix, France) and managed as grassland, was chosen to study switches in grazing and their effects on two phylogenetically different but functionally similar microbial groups : ammonia-oxidizing archaea (AOA) and their bacterial counterparts (AOB). Urine application altered abundance, activity and composition of both targeted nitrifying communities. The microbial populations adapted quickly to the new management regimens and nitrifier activity was always higher in grazed sites as compared to sites that were not exposed to urine. Nitrifier activity correlated with nitrifier community size, however, a coupling between community structure and activity was not found. Altogether, both long-term experiments provided new insights into the ecology of nitrogen-transforming microorganisms and how they are affected by different management practices in regard to sustainable agriculture, in soils that strongly depend on anthropogenic nitrogen input.

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