Nitrous oxideoxygen gas relationship

The relationship between oceanic N2O production/consumption and dissolved O2 concentrations is shown schematically in Figure 3.5. While the influence of O2 concentrations on the N2O production via nitrification is still lacking a mechanistic explanation, the influence of O2 on denitrification and thus N2O production results from two factors: (1) the redox potential of NO3~ respiration favours denitrification under reduced O2 concentrations (see for example Falkowski et al, 2008) and (2) the enzyme involved in N2O consumption, N2O reductase, is sensitive to O2 concentrations (Firestone and Tiedje, 1979). For example, Naqvi et al (2000) attributed the accumulation of N2O off West India to the onset of denitrification at low O2 concentrations, with the assumption that the activity of the N2O reductase could not be established because of frequent aeration of the shallow shelf waters (so-called stop-and-go denitrification).



Figure 3.5 N,0 production versus 0, saturation in the ocean

Note:The approximate regimes of nitrification and denitrification are indicated. Note that there is no clear threshold between nitrification and the onset of denitrification; nitrification and denitrification can occur at the same O2 saturation levels.

Source: Modified from the original figure in Codispoti et al (1992)

Figure 3.5 N,0 production versus 0, saturation in the ocean

The apparent oxygen utilization (AOU) is a measure of the amount of O2 consumed during organic matter remineralization (oxidation) in the ocean. Because nitrification is part of the organic matter oxidation sequence, plots of AN2O versus AOU have been used to identify the prevailing formation and consumption processes of N2O in the water column. The overwhelming majority of the Cat. I profiles (see above) show positive linear AN2O/AOU relationships, suggesting that nitrification is the main N2O formation process in most parts of the oceans (Bange and Andreae, 1999). This is supported by the fact that in most oxic water columns N2O is positively correlated with dissolved nitrate (NO3-), the final product of nitrification (see for example Walter et al, 2006a). However, there are caveats against a straightforward interpretation of the linear AN2O/AOU relationship as an indicator for N2O formation via nitrification because a linear AN2O/AOU relationship may not necessarily result from nitrification alone: most recently, based on N2O isotopomer data (see below), Yamagishi et al (2005) argued that net N2O formation in the oxygen minimum zone (OMZ) of the western North Pacific Ocean mainly results from denitrification with only a small contribution from nitrification. They showed that this N2O, when diffusing into deep waters, produces a reasonably linear AN2O/AOU relationship. Moreover, by applying a two-end-member mixing model, Nevison et al (2003) showed that isopycnal mixing of water masses with different preformed N2O and O2 concentrations can result in a linear AN2O/AOU relationship, which can mask the 'true' biological N2O production. They state:

we find that the biological N2O yield per mole O2 consumed cannot be calculated with great confidence from cross-plot correlation slopes. The essential problem is that the N2O yield is spatially variable. As a result, strong mixing gradients exist in the data that can overwhelm more subtle N2O production terms.

A linear AN2O/AOU relationship does not exist in sub-oxic and anoxic water masses (i.e., Cat. II and Cat. III profiles, see above) indicating a complex interplay between N2O formation and consumption during denitrification and/or a coupling of nitrification and denitrification at the upper boundary of the sub-oxic zones (see for example Bange et al, 2005; Walter et al, 2006b; Westley et al, 2006; Farias et al, 2007; Yamagishi et al, 2007).

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