Table 1.

Enzymes and functional genes involved in microbial production and consumption of N₂O.

Progress	Organism	Enzyme	General reaction	Gene name(s)
Nitrification	AOB, AOA, comammox	Ammonia monooxygenase	NH3 → ${\text{NO}}_2^{-}$	amo
	AOB, comammox, anammox	Hydroxylamine dehydrogenase	NH2OH → NO	hao
	Comammox, NOB	Nitrite oxidoreductase	${\text{NO}}_2^{-}$ → ${\text{NO}}_3^{-}$	nxr
Denitrification	Denitrifiers, denitrifying methanotrophs, ANME-2d	Nitrate reductase (dissimilatory)	${\text{NO}}_3^{-}$ → ${\text{NO}}_2^{-}$	nar
	Denitrifiers, denitrifying methanotrophs, NC10, AOB, AOA, comammox, NOB, anammox	Nitrite reductase (dissimilatory)	${\text{NO}}_2^{-}$ → NO	nirS, nirK
	Denitrifiers, denitrifying methanotrophs, (some) AOB	Nitric oxide reductase	NO → N2O	nor
	Denitrifiers, nondenitrifying N2O reducers	Nitrous oxide reductase	N2O → N2	nosZ