Nitrous oxide (N O) is emitted during microbiological nitrogen (N) conversion processes, when N O production exceeds N O consumption. The magnitude of N O production vs. consumption varies with pH and controlling net N O production might be feasible by choice of system pH. This article reviews how pH affects enzymes, pathways and microorganisms that are involved in N-conversions in water engineering applications. At a molecular level, pH affects activity of cofactors and structural elements of relevant enzymes by protonation or deprotonation of amino acid residues or solvent ligands, thus causing steric changes in catalytic sites or proton/electron transfer routes that alter the enzymes' overall activity. Augmenting molecular information with, e.g., nitritation or denitrification rates yields explanations of changes in net N O production with pH. Ammonia oxidizing bacteria are of highest relevance for N O production, while heterotrophic denitrifiers are relevant for N O consumption at pH > 7.5. Net N O production in N-cycling water engineering systems is predicted to display a 'bell-shaped' curve in the range of pH 6.0-9.0 with a maximum at pH 7.0-7.5. Net N O production at acidic pH is dominated by N O production, whereas N O consumption can outweigh production at alkaline pH. Thus, pH 8.0 may be a favourable pH set-point for water treatment applications regarding net N O production.

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