Factors affecting nitric oxide and nitrous oxide emissions from ...

NH 4 +
Ammonium
Mono-oxygenase
NH 2OH
NO
Hydroxylamine
oxidoreductase
(HNO)
H 2O
N 2O
NO 2 -
Figure 2.1 The microbial production of NO and N2O during nitrification
Nitrification, by way of its products, is coupled to denitrification which is the stepwise
reduction of NO3 - to dinitrogen (N2) (Fig. 2.2). Nitrate is reduced by nitrate reductase to NO2 -
which is then reduced to NO and then to N2O by nitrite reductase and nitric oxide reductase
respectively. Unlike nitrification NO and N2O are obligate intermediaries in the denitrification
sequence. Denitrification is completed when N2O is reduced by nitrous oxide reductase to N2.
Denitrifiers are predominantly heterotrophic facultative anaerobes. The proportion of N2O
released during denitrification varies (Wrage et al., 2001): the proportion increases as soils
become more acidic since nitrous oxide reductase is inhibited by acidic pH (Knowles, 1982);
the ratio of N2O: N2 produced increases as the soil NO3 - concentration increases since NO3 - is
preferred as an electron donor to N2O (Schlegel, 1992); as oxygen concentrations in the soil
increase the fraction of N2O produced may also increase since N2O reductase is inhibited by
lower concentrations of oxygen than the other reducing enzymes involved in denitrification
(Knowles, 1982).
NO 3 -
Nitrate
reductase
NO 2 -
Nitrite
reductase
Nitric oxide
reductase
Nitrous
oxide
reductase
NO N2O N2 Figure 2.2 The microbial production of NO and N2O during denitrification
Coupled nitrification-denitrification is not a separate process but merely implies that the
products of nitrification (NO2 - and NO3 - ) can be used by denitrifies. It occurs because suitable
niches for nitrification and denitrification can co-exist in close proximity and NO2 - , NO3 - or
both can diffuse from nitrifying sites to denitrification sites in the soil. This process should
not be confused with nitrifier-denitrification which is considered a nitrification pathway
(Wrage et al., 2001). Nitrifier denitrification is performed by ammonia oxidisers which
convert NH3 to NO2 - which is then reduced to N2O and N2 (Wrage et al., 2001) (Fig. 2.3). It is
a process likely to occur as oxygen starts to become limiting and where carbon levels are low
(Wrage et al., 2001). While nitrifier denitrification has been demonstrated in the laboratory
NO 3 -
(Poth & Focht, 1985), and there are valid reasons as to why it may occur in situ e.g. to
conserve oxygen and to remove toxic NO2 - , its prevalence in situ is not well quantified.
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