Denitrifying prokaryotes use NO(x) as terminal electron acceptors in response to oxygen depletion. The process emits a mixture of NO, N(2)O and N(2), depending on the relative activity of the enzymes catalysing the stepwise reduction of NO(3)(-) to N(2)O and finally to N(2). Cultured denitrifying prokaryotes show characteristic transient accumulation of NO(2)(-), NO and N(2)O during transition from oxic to anoxic respiration, when tested under standardized conditions, but this character appears unrelated to phylogeny. Thus, although the denitrifying community of soils may differ in their propensity to emit N(2)O, it may be difficult to predict such characteristics by analysis of the community composition. A common feature of strains tested in our laboratory is that the relative amounts of N(2)O produced (N(2)O/(N(2)+N(2)O) product ratio) is correlated with acidity, apparently owing to interference with the assembly of the enzyme N(2)O reductase. The same phenomenon was demonstrated for soils and microbial communities extracted from soils. Liming could be a way to reduce N(2)O emissions, but needs verification by field experiments. More sophisticated ways to reduce emissions may emerge in the future as we learn more about the regulation of denitrification at the cellular level.