Recent analyses that include the full impact of agriculture on the nitrous oxide budget (Mosier et al, 1998; Kroeze el al, 1999) bring the total mass balance of N20 fluxes and atmospheric accumulation into much better agreement than earlier IPCC assessments (Prather et al, 1995) which estimated that there was an unaccounted for source of ~1.5TgN/yr. The latter, along with the poorly understood isotopic evidence found in the 1990s, such as the observed 170 excess (Cliff and Thiemens, 1997; Cliff et al, 1999; Rockmann et al, 2001c) and the stratospheric enrichments of 15N and lsO (Kim and Craig, 1993; Rahn and Wahlen, 1997), led to numerous efforts to find a 'missing source' of nitrous oxide. As described above, the stratospheric enrichments are now well understood in terms of the fractionations associated with photolysis and photooxidation (Johnston et al, 1995; Yung and Miller, 1997; Rahn et al, 1998; Umemoto, 1999; Rockmann etal., 2000, 2001b; Johnson etal., 2001; Kaiser 2002a; Blake et al, 2003) and there are several candidate mechanisms for the observed
170 excess (Rockmann et al, 2001c; Estupinan et al, 2002; Blake et al, 2003; McLinden et al, 2003; Michalski et al, 2003; Kaiser et al, 2004). The laboratory work investigating the fractionation resulting from nitrous oxide photochemistry has led to the unusual circumstance that, in the N2O system, the isotopic systematics of the stratosphere are far better understood than those influencing the terrestrial and oceanic biospheres. Continued studies of stratospheric N2O have further refined our understanding of its photochemical fractionation and show potential to lend insight into cross-tropopause transport and stratospheric mixing (Griffith et al, 2000; Toyoda et al, 2001; Parks et al, 2004).
The temporal and spatial variability of the natural and anthropogenic sources of N2O combined with the wide range of their reported isotopic content (Fig. 15.1), make it practically impossible to assign specific values for these terms in a forward model. Rahn and Wahlen (2000) circumvented this problem when trying to develop a global model by employing the tactic of utilizing the better known terms of the budget to solve for the isotopic content of the terrestrial source. They concluded that although additional sources of N2O could not be discounted, such sources are not necessary for closure of the isotopic budget within the constraints of the known parameters. While this may not be the most satisfying result, it may well be that a tightly closed isotopic budget of N2O is an unrealistic goal. This is not to say that measurements of atmospheric N2O are a futile exercise; on the contrary, isotopic measurements of N2O in discrete environments will always provide details of the processes affecting its production and loss. For instance, when combined with isotopic measurements of N substrates, N2O isotopes can reveal temporal details of how nutrients cycle in different ecosystems under varying conditions (Perez et al, 2001), and in the stratosphere, the details of how mixing processes take place can be elucidated (Rahn et al, 1998; Griffith et al, 2000; Kaiser et al, 2002a). A common reference material with absolute calibration of the intramolecular distribution of N isotopes in particular will go a long way toward advancing these goals.
Finally, results of N2O isotopic analysis from air occluded in ice cores have the potential to reveal details of N cycling during glacial and interglacial times. Results of analyses of air in unconsolidated polar snow (firn) (Sowers et al, 2002; Rockmann et al, 2003a) have already shown that the pre-anthropogenic values of <515N and <5lsO were enriched by •~2%o and 1 %o respectively relative to present times, verifying the temporal decrease attributed to isotopically light agricultural emissions first predicted by Rahn and Wahlen (2000). Subsequent measurements of 24 samples spanning the past 35 k year in ice from Taylor Dome, Antarctica have led to the conclusion that, with the exception of the Younger Dryas, the ratio of terrestrial to marine N2O emissions has remained relatively constant over that time (Sowers et al, 2003). As methodologies improve and more records become available (including 1:>N position measurements), there will be much to learn about N20 and the cycling of nitrogen.
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