Nitrification is the oxidation of ammonium, NH4+, to NO3- via hydroxyl-amine, NH2OH, and nitrite, NO2-. For details about nitrification see the overview article by Ward (2008). Autotrophic nitrification represents the final step of the remineralization of nitrogen containing organic matter and is performed in two steps by ammonium-oxidizing bacteria (AOB) (for example Nitrosomonas, Nitrosospira and Nitrosococcus) and nitrite-oxidizing bacteria (NOB) (for example Nitrobacter and Nitrospira), respectively:
NO is not known to be an obligatory intermediate during ammonium oxidation. It can be produced by AOB but the mechanism is not well understood. During autotrophic nitrification N2O can be formed by AOB either via the pathways NH2OH ^ N2O and NO ^ N2O or via the pathway NO2- ^ NO ^ N2O (the latter is part of the so-called nitrifier-denitrification process). Nitrification is an aerobic process; however, under low-oxygen (sub-oxic) conditions, N2O yields are enhanced. Alternatively, N2O can be formed during heterotrophic nitrification (i.e. nitrification linked to aerobic denitrification) via the reaction NO2- ^ NO ^ N2O as well, but the enzymes involved in the heterotrophic reaction sequence are different from those involved in the autotrophic pathway. Under oxic conditions, N2O yields from heterotrophic nitrification are higher than those from autotrophic nitrification. However, the relevance of heterotrophic nitrification in the marine environment is not known yet.
Both nitrification and denitrification as sources and sinks of oceanic N2O have been described in the water column, in the sediments and in association with suspended particles (for example Schropp and Schwarz, 1983; Seitzinger, 1990; Michotey and Bonin, 1997; Nevison et al, 2003; Codispoti et al, 2005). N2O yields from nitrification range from 0.004 per cent to 0.4 per cent, whereas the N2O yield from the denitrifying sub-oxic zone in the Arabian Sea was estimated to be about 2 per cent (see overview in Bange, 2008). Culture studies with strains of nitrifiers revealed that the N2O yield from nitrification is significantly enhanced (up to 10 per cent) under sub-oxic conditions (Goreau et al, 1980). N2O yields from sedimentary denitrification range from 0.1 per cent to 0.5 per cent, with values up to 6 per cent in nutrient-rich regions (see overview in Seitzinger and Kroeze, 1998).
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