N2O from the Oceans and Other Aquatic Regions

According to the IPCC (Ehhalt et al., 2001), the annual mass flux of N2O from the oceans is estimated to be of the order of 3TgN/yr although variability reported in the literature ranges from 1 to 5.8 Tg N/yr (Nevison et al., 1995, 2003 Mosier et al, 1998; Olivier et al, 1998; Kroeze et al, 1999). It has also been noted that river, estuarine, and coastal marine sources of N2O may have increased over time due to dissolved inorganic nitrogen loading resulting from fertilizer N runoff, sewage discharge, and atmospheric deposition; indeed, it has been estimated to account for as much as 20% of the current annual anthropogenic production (Seitzinger and Kroeze, 1998; Seitzinger et al, 2002).

A number of studies have been undertaken to investigate the isotopic signature of these N2O sources with results that are nearly as variable as those from terrestrial soils. Unlike soils, however, the surface oceans are sometimes enriched in both or either 15N and/or lsO relative to tropospheric N20 (Fig. 15.1). While some isotopic data (Kim and Craig, 1990; Dore et al, 1998; Toyoda et al, 2002) and evidence such as the correlation/anti-correlation of N2O with dissolved NO3/O2 (Kim and Craig, 1990) and N2O/O2 emission ratios (Lueker et al, 2003) suggest that nitrification contributes significantly to the N2O flux from a variety of oceanic localities, it is also well established that denitrification plays an important role in oceanic N cycling (Yoshida et al, 1989; Yoshinari et al, 1997; Naqvi et al, 1998; Codispoti et al., 2001). In fact, N2 production can exceed NOg consumption by as much as ~35% under certain circumstances (Codispoti et al, 2001). In this case, it is obvious that reduction of N2O will compete significantly with its production and any depletion of 15N or O content during production through either nitrification or denitrification will be attenuated by subsequent N2O reduction. Indeed, it is likely that such processes account for the minimal overlap of the terrestrial and oceanic fields in Fig. 15.1 even though ostensibly the same pathways of nitrification and denitrification are responsible for N2O production; whereas in the terrestrial environment where soil gas diffusivities can be ~105 greater than diffusion in H2O, gases produced in-situ escape readily to the atmosphere while, in the oceans, the flux to the atmosphere is dependent in large part on seasonal and/or ephemeral mechanical mixing and N2O produced in-situ is long-lived enough to be recycled biologically within the system. It is also important to point out that, as with terrestrial

A^ 0 from the Oceans and Other Aquatic Regions 275

A^ 0 from the Oceans and Other Aquatic Regions 275

Figure 15.1 A plot of S15N vs <5lsO of N9O in different environments. The gray field represents the range that has been observed emitting from terrestrial ecosystems, both natural and human perturbed. The hatched field is the range of values reported for n9o dissolved in surface oceans. The stippled field represents stratospheric measurements. The filled circle is the tropospheric average.

Figure 15.1 A plot of S15N vs <5lsO of N9O in different environments. The gray field represents the range that has been observed emitting from terrestrial ecosystems, both natural and human perturbed. The hatched field is the range of values reported for n9o dissolved in surface oceans. The stippled field represents stratospheric measurements. The filled circle is the tropospheric average.

systems, quantitative estimates of isotopic fluxes between the various N and O reservoirs are dependent on knowledge of the isotopic composition of the substrates undergoing transformation and such details are not available for most of the oceanic N20 isotope data reported to date. Finally, an additional factor that must be kept in mind when considering N20 sea-to-air transfer is that there is fractionation associated with this flux that has been measured as —0.7 and —1.9%o, for N and O, respectively (Inoue and Mook, 1994), the heavier isotopes preferentially remaining in solution.

The isotopic content of N20 emitted from non-ocean aquatic sources has had limited investigation. One study of the Bang Nara River in Thailand has shown that dissolved N20 is highly variable and seasonal with maximums of AN20 = 131 nM (~'20 times saturation') occurring at the onset of the rainy season. Even with these elevated levels, <5l3N and 5lsO were depleted by only ~7 and ~8%o respectively, relative to tropospheric N20 (Boontanon et aL, 2000), indicating that apparently in this system as well, N20 is actively recycled. Again, however, isotopic analysis of substrate was not reported meaning conclusive statements regarding production pathway (s) could not be made.

276 15. Factors Influencing the Stable Isotopic Content of Atmospheric O

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