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

Abstract

Nitrous oxide (N₂O) 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 N₂O production. However, the mechanisms responsible for N₂O production from nitrification are still not fully understood. The current understanding is that there are mainly two routes responsible for the N₂O production from nitrification: biological ammonia (NH3) 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 N₂O production during nitrification, involving the two reactive N intermediates hydroxylamine (NH₂OH) and NO₂^- via coupled biotic-abiotic mechanisms of N₂O production. While the abiotic N₂O production from NO_2^- has been studied in the last decades, the abiotic N₂O production involving NH₂OH has long been ignored. One possible reason could be that NH₂OH was not detected in soils in previous research. In addition, the release of NH₂OH during NH₃ oxidation in pure cultures of ammonia oxidizers has not been studied previously, which would be the prerequisite of abiotic N₂ production involving NH₂OH. Therefore, the aim of the present thesis was to study the relevance and mechanisms of coupled biotic-abiotic N₂O formation from NH₂OH and NO_2^- during nitrification in different soils. <br /> By studying different types of ammonia oxidizers (ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and complete ammonia oxidizers (comammox)), this thesis demonstrates NH₂OH release during NH₃ oxidation of various ammonia oxidizers. However, the NH₂OH: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 NH₃ concentrations in the medium. The presence of NO_2^- decreased the abiotic NH₂OH decay rate in the medium but increased abiotic N₂O production involving NH₂OH. The calculated fraction of NH₄⁺ converted to N₂O via NH₂OH release during incubations ranged from 0.05% to 0.14%, which was consistent with published NH₄⁺-to-N₂O conversion ratios for certain ammonia oxidizers. <br /> Hydroxylamine could not only be detected in pure cultures, but also be determined in natural soils by developing and applying a highly sensitive method using extraction under acidic conditions and oxidation of NH₂OH to N₂O with Fe₃3⁺. The determined NH₂OH content in spruce forest soil samples ranged between 0.3 and 34.8 μg N kg^-1 dry soil, which was consistent with the magnitude of NO₂^- contents reported for forest soils. This thesis further shows a positive spatial correlation between NH₂OH concentrations and aerobic N₂O production in Norway spruce forest soil, although aerobic N₂O production was also correlated with other soil basic properties, such as soil pH, NO_3^-, Mn, and soil organic carbon (SOC) content. Similar hotspots were identified for aerobic N₂O production itself as well as for the contribution of NH₂OH to aerobic N₂O production. The incorporation of the NH₂OH information largely improved the estimation of aerobic N₂O production in the study area. <br /> In a systematic experiment with artificial soil mixtures with the aim to test the relevance of the control parameters identified in the forest soil study, the abiotic conversion of NH₂OH to N₂O was strongly dependent on soil organic matter (SOM) content, pH, and MnO₂ content. More NH₂OH was chemically converted to N₂O at low SOM content, low pH, and high MnO₂ content. Based on these results, the thesis presents a model to estimate abiotic NH₂OH-to-N₂O conversion in soils by considering the SOM and MnO₂ content as well as pH. It should be noted that not only the quantity, but also the quality of SOM, e.g. certain functional groups, such as carbonyl groups, can affect the abiotic conversion of NH₂OH to N₂O.<br /> The thesis further explored the contribution of the two reactive N species NO₂^- and NH₂OH to abiotic N₂O production in different soils after oxic and anoxic pre-incubation. NO^- played the most important role in N₂O production in grassland soil, followed by the soils of upland forest, a riparian area, and cropland. Abiotic processes contributed about 10-40% to the conversion of NO_2^- to N₂O, but no significant factors responsible for the N₂O production from NO₂^- could be identified. N₂O production from NH₂OH played an important role in grassland and cropland soils, as well as partly in the forest soil. In contrast to NO_2^-, the conversion of NH₂OH to N₂O was mostly (>80%) abiotic and was correlated significantly with soil pH, MnO₂ and SOC content. After anoxic incubation, the contribution of NO₂^- to aerobic N₂O production increased, while the contribution of NH₂OH decreased depending on SOC content. <br /> Finally, a close relationship was found between pulse N₂O production after rewetting of air-dried soils and concentration of NO_2^- accumulated in the dry soils. Abiotic processes contributed 10-70% of N₂O production after rewetting of forest soil, but were even considerably higher in the grassland soil after gamma radiation. <br /> In summary, this thesis describes the coupled biotic-abiotic mechanisms of N₂O production during nitrification in detail by studying the processes related to abiotic N₂O production from NH₂OH systematically with a series of experiments, and by exploring the contribution of NO_2^- and NH₂OH to abiotic N₂O production under various environmental conditions and for different soil types. The results of the thesis improve the understanding of the mechanisms as well as the quantification of aerobic N₂O production in soils, and could contribute to developing more effective N₂O mitigation measures, such as increasing soil pH and adding organic soil amendments with appropriate functional groups that can react chemically with NH₂OH.