PhD Thesis, 2010 - University College Cork

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Literature review forms were retained in the upper soil profile or were transformed into volatile compounds. Typically, artificial nitrogen fertilisers are in the form of nitrates or ammonium salts, or both. Chemically these compounds represent either the most oxidised form of N (in nitrate, NO − 3 , oxidation number of N is +5) or the most reduced form (in ammonium, NH + 4 , oxidation number of N is −3, as in urea, (NH 2 ) 2 CO). This diversity encourages redox reactions facilitated by microbiological organisms as the major route of production of the nitrous oxide in soils. As most of the living organisms are concentrated in the top 10 cm of the soil, it is expected that this layer will be responsible for production of the most quantities of nitrous oxide. It was reported, however, that the concentration of N 2 O at the lower depths (30–50 cm) could reach 10 ppm (Müller et al. 2004; Goldberg and Gebauer 2009). This implies that production might not be restricted to the top soil horizons, although it is not clear how this phenomenon might be reflected in atmosperic emissions of nitrous oxide. Nitrous oxide itself is most frequently a by-product of the transformational processes: two of the most-widely recognized are nitrification and denitrification (Wrage et al. 2001). Nitrification and denitrification are redox processes that describe N-oxidation and reduction, respectively. As such both processes are generally governed by the conditions characteristic for either reduction or oxidation. The reductive environment in the soil typically is an aerobic environment with low-to-mid water content; on the other hand the oxidative environment is anaerobic with average water content 70 to 90% of total porosity. Equations 2.1 and 2.2 show the step-by-step processes describing nitrification and denitrification respectively. 5
Literature review NH 3 → NH 2 OH → NO 2 − → NO 3 − (2.1) NO 3 − → NO 2 − → NO → N 2 O → N 2 (2.2) Each of the steps relies on the catalytic properties of certain enzymes: relevant reductaces in case of the denitrification and ammonia monooxygenase and relevant oxydoreductaces during nitrification. The initial step of nitrification (oxidation of the ammonia to hydroxylamine) requires the presence of oxygen (O 2 ) and free electrons to reduce oxygen to water. These electrons can be obtain during the second step (hydroxylamine oxidation) forming a positive feedback in the chain. Nitrous oxide is thought to be produced as a result of anabiotic transformation (chemodenitrification) from hydroxylamine or nitrite. A number of other processes have been identified as possible source of N 2 O: nitrifier denitrification is a phenomenon in which ammonia is oxidised to nitrite (NO − 2 ) which in turn reduces to molecular nitrogen (N 2 ). As can be seen from the Equation 2.3, ammonia oxidation corresponds to the part of the nitrification processes and the reduction phase of this process is similar to part of the denitrification processes, hence the name. NH 3 → NH 2 OH → NO 2 − → NO → N 2 O → N 2 (2.3) All previously described pathways of N transformation are most commonly performed by autotrophic bacteria. Some heterotrophic bacteria, however, are able to carry out the nitrification (heterotrophic nitrification) and denitrification under aerobic conditions. As the nature of these organisms imply they prefer carbon-rich environments. 6
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Gap-filling techniques for the annu
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General discussion 7. GENERAL DISCU
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Recommendations for further researc
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References REFERENCES Ball, B. C.,
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References Flechard, C. R., P. Ambu
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References grassland in the South E
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References Müller, C., R. J. Steve
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References Yashiro, Y., W. R. Kadir
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PhD Thesis, 2010 - University College Cork

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