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Nitrifier denitrification is a poorly quantified microbial process leading to emissions of N2O from soils. Nitrous oxide emissions, particularly from agricultural soils, are currently being targeted for reduction due to the contribution of this gas to anthropogenic climate change. Measurements specific to nitrifier-denitrification are hampered by poor culturability of many of the strains involved, and the inability of single isotope labelling methods to distinguish it from denitrification carried out by other organisms, dual-labelling approaches and evidence form pure cultures suggest that its contribution to nitrous oxide emissions may be large. Environmental conditions favouring the pathway are unknown, leading to difficulties in mitigation or modelling. In this thesis data from both dual-labelling isotopic techniques and isotopomer measurements of the nitrous oxide emitted are used to determine whether isotopomer techniques can quantify nitrifier-denitrification in situ, and the conditions under which nitrous oxide emissions from the pathway are increased are investigated. Data is also presented on site preference (isotopomer ratio) from ammonia oxidation in several Nitrosospira strains for which this has previously not been measured. The capacity of Nitrosospira strains in pure culture to reduce N2O to N2, an environmentally neutral product of the nitrogen cycle, are investigated. Site preference results from this research suggest that nitrifier denitrification cannot be distinguished from heterotrophic denitrification by site preference, indicating that previously published data stating proportional outputs of N2O from dentrification may overestimate heterotrophic contributions. Several Nitrosospira strains are found to be capable of a reduction step from N2O to N2 in pure culture conditions. Nitrifier denitrification is found to respond to environmental factors of soil N-level, pH and rainfall events, and changes in site preference also occur under these conditions. Site preference is linked to microbial phylogeny for the strains of Nitrosospira tested, indicating a possible effect of enzyme structure above pathway level determination. |
