I. What is nitrous oxide (N2O)?

Atmospheric N2O contributes to both greenhouse effect (Wang et al., 1976) and ozone layer depletion (Crutzen, 1970). N2O possesses a relatively high global warming potential (i.e., 298 times greater than carbon dioxide in a 100-year time horizon;Forster et al., 2007). A change in the N2O mixing ratio from 270 ppb in 1750 to 319 ppb in 2005 caused an increased radiative forcing of 0.16 ± 0.02 W m–2. Of the entire anthropogenic N2O emission (5.7 Tg N2O-N yr−1), agricultural soils provide 3.5 Tg N2O–N yr−1 accounting for approximately 6% of the current global warming (IPCC, 2006).

Source and process
It is estimated that use of N fertilizers and animal manure is the main anthropogenic N2O source and it is responsible for roughly 24% of total annual emissions (Bouwman, 1996; Forster et al., 2007). Nitrous oxide can be mainly produced from

1) aerobic autotrophic nitrification, the stepwise oxidation of ammonia (NH3) to nitrite (NO2−) and to nitrate (NO3−) (e.g., Kowalchuk and Stephen, 2001),

2) anaerobic heterotrophic denitrification, the stepwise reduction of NO3− to NO2−, nitric oxide (NO), N2O and ultimately N2, where facultative anaerobe bacteria use NO3− as an electron acceptor in the respiration of organic material in the condition of insufficient oxygen (O2) (e.g. Knowles, 1982), and

3) nitrifier denitrification, which is carried out by autotrophic NH3 −oxidizing bacteria and the pathway whereby ammonia (NH3) is oxidized to nitrite (NO2−), followed by the reduction of NO2− to nitric oxide (NO), N2O and molecular nitrogen (N2) (e.g., Webster and Hopkins, 1996;Wrage et al., 2001).

4) Other N2O production mechanisms include heterotrophic nitrification (Robertson and Kuenen, 1990), aerobic denitrification by the same heterotrophic nitrifiers (Robertson and Kuenen, 1990), fungal denitrification (Laughlin and Stevens, 2002), and non-biological process chemodenitrification (e.g. Chalk and Smith, 1983; Van Cleemput and Baert, 1984; Martikainen and De Boer, 1993; Daum and Schenk, 1998; Mørkved et al., 2007).

Control factors
Nitrous oxide emissions are reported to be controlled by soil chemical and physical properties such as the availability of mineral N, soil pH, organic matter availability, and soil type, and climate related soil properties such as soil temperature and soil water content (e.g., Mosier, 1994; Bouwman, 1996; Beauchamp, 1997; Yamulki et al. 1997; Dobbie and Smith, 2003; Smith et al. 2003; Dalal et al. 2003).