What is greenhouse gas nitrous oxide?

Atmospheric N₂O contributes to both the greenhouse effect (Wang et al. 1976) and plays a precursor role in the ozone layer depletion (Crutzen 1970). Nitrous oxide has a relatively high global warming potential (i.e., 298 times greater than carbon dioxide in a 100-year time horizon; IPCC 2006; Forster et al. 2007). Use of N fertilizers and animal manure is recognized as the main anthropogenic source of N₂O with roughly 24% of total annual emissions (Bouwman 1996; Forster et al. 2007). Nitrous oxide emissions from natural lands comprise 55% of the total global N₂O emissions (Forster et al. 2007). 

Soil N₂O can be produced and consumed via different biological pathways. Aerobic autotrophic nitrification is the stepwise oxidation of ammonia (NH₃) to hydroxylamine (NH₂OH) as first intermediate, subsequently to nitrite (NO₂⁻) as second intermediate, and finally to nitrate (NO₃⁻) (Kowalchuk and Stephen 2001). Production of N₂O can occur because of enzymatic decompositions of the substrates NH₂OH and NO₂⁻ resulting in production of N₂O and other gaseous N species (Arp and Stein 2003; Baggs 2011). Heterotrophic denitrification is the stepwise reduction of NO₃⁻ to NO₂⁻, to nitric oxide (NO), to N₂O, and ultimately to dinitrogen (N₂), where NO₃⁻ as an electron acceptor in the respiration of organic material under low oxygen conditions (Knowles 1982). Although denitrification in soils is typically associated with anaerobiosis (Bremner 1997), denitrification and the associated enzymatic activity have been detected where aerobic conditions are reestablished after an anaerobic phase (Patureau 2000). In close association with denitrification, microbial co-denitrification can source N from organic co-substrates for increased N₂O production. Additional biosyntheses of the intermediate NO₂⁻ as substrate for soil N₂O production can also happen during nitrate ammonification (Baggs 2011) and nitrifier denitrification (Wrage et al. 2001) processes. Conversely, the respiratory reduction of N₂O to N₂ represents a biological consumption pathway, which in certain cases could result in soil uptake of atmospheric N₂O (Bremner 1997). These several N transformation processes can be coupled simultaneously or sequentially in soils and a wide variety of microbes typically participates.