The microbial processes responsible for N20 formation in soils are discussed in more detail elsewhere in this volume (Perez, Chapter 5). In general, N20 can be formed as a by-product during oxidation of nh3 (nitrification) or during dissimilatory reduction of NO^ (denitrification). Variations on these pathways have been observed as in the case of nitrifier denitrification and methanotrophic nitrification (see Sutka et al., 2003 for summary and additional references). Nitrous oxide itself can serve as an electron acceptor under certain conditions and thus can be reduced before escaping from the soil. The amount of N20 that does escape to the atmosphere from a given soil is determined by several factors including porosity/permeability, soil water content, substrate availability, and the microbial community itself. It is estimated by the Intergovernmental Panel on Climate Change (IPCC) that natural soils contribute 6TgN/yr to the atmosphere as N20, while agricultural soils and livestock contribute approximately 4TgN/yr (Ehhalt et ai, 2001).
Several studies have been conducted to determine the fractionation during production/consumption of N20 by specific nitrifying (Mariotti et al., 1981; Yoshida, 1988; Webster and Hopkins, 1996; Sutka et al., 2003), denitrifying (Mariotti et al, 1981; Webster and Hopkins, 1996; Barford et al., 1999), and nitrogen-fixing bacterial cultures (Yamazaki et al., 1987) and, collectively, have led to the conclusion that denitrification produces N20 with greater 15N enrichment than does nitrification, and that subsequent reduction of N20, either during denitrification or N20 fixation, serves to further enrich residual N20 in both 15N and 180. In theory, these results could be used to decipher the relative contributions of the various processes in a given ecosystem if the initial conditions are known. In practice, however, there are many more bacteria than those that have been investigated, so the modeling of such systems requires the assumption that fractionations remain constant for a given pathway, regardless of mediating organism. In addition, the isotopic content of substrates is highly variable and often not known and the complications of multiple fractionating processes occurring simultaneously make it extremely difficult to apply these factors quantitatively to field data. A number of studies have described the isotopic content of N20 actually emitted from terrestrial systems, both natural and agricultural; a thorough summary of these surveys is provided in Chapter 5 (Perez) of this volume. In general, it can be said that the measured isotopic signature, both for l3N and lsO, of N20 emitted from terrestrial environments is depleted relative to the tropospheric average although anomalously enriched N2O produced within soils has been described (Mandernack et al., 2000).
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