Browsing by Subject "Prokaryoten"

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Abundance and diversity of total and nitrifying prokaryotes as influenced by biochemical quality of organic inputs, mineral nitrogen fertilizer and soil texture in tropical agro-ecosystems
(2016) Muema, Esther Kathini; Cadisch, Georg
Tropical agro-ecosystems are limited in nutrient resources as a consequence of i) being composed of highly weathered soils, ii) low native soil organic matter (SOM) content due to conversion of natural forests to arable lands and iii) continuous cropping without replenishing soil nutrients. Recovery of SOM by use of organic residues is faced with other competing uses like animal fodder. Moreover, existing SOM is further reduced by increased turnover rates due to favorable climatic conditions in the tropics. Incorporation of residues is therefore a justified means to restore SOM and to provide crop nutrients through microbial mediated activities like nitrification. Nitrification is a central step of the nitrogen (N) cycle, whereby ammonia is converted into nitrite and then to nitrate by bacteria and archaea through production of the amoA gene encoding the alpha-subunit of the enzyme ammonia monooxygenase. In order to better understand the impact of organic residues of contrasting biochemical quality (i.e., high quality Tithonia diversifolia (TD; C/N ratio: 13, lignin: 8.9 %, polyphenols: 1.7 %), intermediate quality Calliandra calothyrsus (CC; 13, 13, 9.4) and low quality Zea mays (ZM; 59, 5.4, 1.2)) on nutrient provision, effects of residue quality on dynamics of relevant decomposer microbial communities were studied. In addition, mineral N fertilizer was used to compensate for mineral N limitations especially in case of low and intermediate quality residues. Since N is one of the most limiting crop nutrients in the tropics, this study therefore focused on ammonia-oxidizing prokaryotes, using DNA-based quantitative PCR (qPCR) and terminal restriction fragment length polymorphism (TRFLP) techniques. In addition, soil physicochemical properties were measured and linked to the dynamics of microbial communities. The study hypothesized that soil type due to differences in structure and nutrient background, as well as seasonality, which influences soil moisture, would shape the response of the studied communities to biochemical quality of residues. Overall, the results of this PhD research revealed specific responses of dynamics of AOB and AOA to quality of organic residues and their combinations with mineral N fertilizer. They also revealed effects of interrelations between quality of residues and soil texture as well as seasonality particularly precipitation on dynamics of microbial communities. Future investigation of active microbial communities with the use of RNA-based approaches need to be considered to further improve our understanding of quality of SOM on soil nutrient dynamics.
Impacts of the fungal bio-control agent Fusarium oxysporum f.sp. strigae on plant beneficial microbial communities in the maize rhizosphere
(2016) Musyoki, Mary Kamaa; Cadisch, Georg
Striga hermonthica causes severe yield reduction in cereal crop production in Sub-Saharan Africa. Intergrated Striga management has been proposed as one of the best options to control striga. Along this line, the use of biocontrol agent (BCA) Fusarium oxysporum f.sp. strigae (Foxy-2) has been proven as an effective and environmental friendly management strategy. It is well established that a prerequisite for a successful BCA is sufficient risk assessment analysis. Towards this direction, Foxy-2 was assessed for its potential non-target impacts on the abundance, community structure of bacterial and archaeal nitrifying prokaryotes as well as enzymatic activities of proteolytic bacteria. Maize rhizosphere soils treated with or without Foxy-2, Striga and high quality organic residues (i.e., Tithonia diversifolia) as N source were evaluated by quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (TRFLP). It was observed that Foxy-2 had a promoting effect on archaeal abundance under controlled conditions in sandy soils. Furthermore, crop growth stage, seasonality and soil type had a strong effect on abundance and community structure of nitrifying prokaryotes over Foxy-2 inoculation. In addition proteolytic enzymatic activities analysis showed that Foxy-2 did not affect their activities. Correlation analysis also showed that abundance and community structure on nitrifying communities positively correlated with extractable organic carbon, extractable organic nitrogen and soil pH, while proteolytic enzymatic activities correlated with extractable organic nitrogen and soil ammonium. It was concluded that Foxy-2 is compatible with nitrifying prokaryotes and proteolytic enzymatic activities.