Nitrous oxide emissions, from soil denitrification

Although the estimated source strengths remain uncertain, emissions from soils appear to dominate the nitrons oxide bndget (IPCC, 1990). The main biogenic sources of nitrous oxide, NO, and N2 in soils are the microbial processes of denitrification and nitrification. 

It is thought that little net NO is produced in denitrification, it being readily reduced to N2O, and nitrification is therefore the main source of NO (Anderson and Levine, 1986 Skiba et al, 1993). Nitrous oxide is also produced in both nitrification and denitrification. At low O2 concentrations in otherwise aerobic soil, small amounts of N2O are formed as a by-product of nitrification, N2O not itself being reduced to NO,. In denitrification, the proportion of N2O produced relative to N2 increases as the availability of O2 increases and that of carbon decreases (Tiedje, 1988). In general only a small fraction of the N nitrified or denitrified in these pathways is released as NO or N2O. The emission is therefore sensitive to the amount of mineral N in the system, which is driven principally by additions of nitrogen fertilizers and deposition of nitrogen from the atmosphere. 

Nitrous oxide is important not only as a greenhouse gas but, as discussed in Chapter 12, as the major natural source of NC/ in the stratosphere, where it is transported due to its long tropospheric lifetime (Crutzen, 1970). The major sources of N20 are nitrification and denitrification in soils and aquatic systems, with smaller amounts directly from anthropogenic processes such as sewage treatment and fossil fuel combustion (e.g., see Delwiche, 1981 Khalil and Rasmussen, 1992 Williams et al., 1992 Nevison et al., 1995, 1996 Prasad, 1994, 1997 Bouwman and Taylor, 1996 and Prasad et al., 1997). The use of fertilizers increases N20 emissions. For pastures at least, soil water content at the time of fertilization appears to be an important factor in determining emissions of N20 (and NO) (Veldkamp et al., 1998). 

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