Coupled biotic-abiotic mechanisms of nitrous oxide production in soils during nitrification involving the reactive intermediates hydroxylamineand nitrite

Nitrous oxide (N$_{2}$O) is an important greenhouse gas that can deplete the ozone layer. Microbial nitrification and denitrification have been long considered as the major contributors of soil N$_{2}$O production. However, the mechanisms responsible for N$_{2}$O production from nitrification are still not fully understood. The current understanding is that there are mainly two routes responsible for the N$_{2}$O production from nitrification: biological ammonia (NH$_{3}$) oxidation and nitrifier denitrification of nitrite (NO$_{2}^{-}$). However, so far it has been neglected that abiotic processes could also play an important role in the N$_{2}$O production during nitrification, involving the two reactive N intermediates hydroxylamine (NH$_{2}$OH) and NO$_{2}$-via coupled biotic-abiotic mechanisms of N$_{2}$O production. While the abiotic N$_{2}$O production from NO$_{2}^{-}$ has been studied in the last decades, the abiotic N$_{2}$O production involving NH$_{2}$OH has long been ignored. One possible reason could be that NH$_{2}$OH was not detected in soils in previous research. In addition, the release of NH$_{2}$OH during NH$_{3}$ oxidation in pure cultures of ammonia oxidizers has not been studied previously, which would be the prerequisite of abiotic N$_{2}$O production involving NH$_{2}$OH. Therefore, the aim of the present thesis was to study the relevance and mechanisms of coupled biotic-abiotic N$_{2}$O formation from NH$_{2}$OH and NO$_{2}^{-}$ during nitrification in different soils. By studying different types of ammonia oxidizers (ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and complete ammonia oxidizers (comammox)), this thesis demonstrates NH$_{2}$OH release during NH$_{3}$ oxidation of various ammonia oxidizers. However, the NH$_{2}$OH:final product release ratios were different between the different microbial strains studied, ranging from 0.24% to 1.92%, and were also dependent on initial NH$_{3}$ concentrations in the medium. The presence of NO$_{2}$ - decreased the abiotic NH$_{2}$OH decay rate in the medium but increased abiotic N$_{2}$O production involving NH$_{2}$OH. The calculated fraction of NH$_{4}^{+}$ converted to N$_{2}$O via NH$_{2}$OH release during incubations ranged from 0.05% to 0.14%, which was consistent with published NH$_{4}^{+}$-to-N$_{2}$O conversion ratios for certain ammonia oxidizers. [...