N2O is generated in soils through various chemical and biochemical pathways. Chemical processes, collectively referred to as chemodenitrification, involve the reaction of nitrite (NO-) with organic matter (Bremner and Nelson, 1968; Nelson, 1982) and hydroxylamine (NH2OH) with organic matter or exchangeable cations (Bremner et al., 1980). However, production of N2O via chemical pathways only proceeds under select circumstances, and is not likely to be a significant source of N2O from soil (Bremner, 1997).
Biochemical processes, in contrast, proceed under most soil environmental conditions because soil microbial communities are remarkably diverse and adaptable. N2O emitted from soils is therefore mostly of biological origin, predominantly from two processes: nitrification and denitrification
(Bremner, 1997). Broadly stated, nitrifiers oxidize ammonium (NH+) to nitrate (NO-), while denitrifiers reduce NO- to N2O or dinitrogen (N2) during anaerobic respiration. Even these very broad categories tend to be blurred as nitrifiers can also denitrify (nitrifier-denitrification) (Wrage et al., 2001). The various microbial groups occupy different ecological niches in soil (Fig. 5.1). Some N2O might also be generated by dis-similatory reduction of NO- to NH+ (DNRA) (DeCatanzaro et al., 1987; Stevens et al., 1998) and other biochemical pathways (Robertson and Tiedje, 1987), but the contribution from these pathways is likely negligible in upland agricultural soils.
Was this article helpful?