Summary Results from the pioneering research on the interactions between pH and denitrification in soil from the 1950s to the present are reviewed, the changing perceptions of this complex relationship are discussed, and the current status of the subject is assessed. Facets of this relationship that are analysed in detail include the direct or indirect influence of pH on overall denitrification rates in soils, changes in the composition of gaseous products that depend on pH, methods for measuring the process, the concept of an optimum pH for denitrification, and the adaptation of microbial denitrifying communities to acidic environments. The main conclusions to be drawn are as follows. Total gaseous emissions to the atmosphere (N2O, NO and N2) have repeatedly been shown to be less in acidic than in neutral or slightly alkaline soils. This may be attributable to smaller amounts of organic carbon and mineral nitrogen available to the denitrifying population under acid conditions rather than a direct effect of low pH on denitrification enzymes. Numerous laboratory and field studies have demonstrated that the ratio N2O:N2 is increased when the pH of soils is reduced. The relation between soil pH and potential denitrification as determined by various incubation methods remains unclear, results being influenced both by original conditions in soil samples and by unknown changes during incubation. The concept of an optimum pH for denitrification has been frequently proposed, but such a term has little or no meaning without reference to specific attributes of the process.

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The Lough Neagh catchment area covers about one‐third of the land area of Northern Ireland. This report documents NO2− concentrations in the major rivers entering Lough Neagh, which are frequently in the range of 100 to 150 μg N L−1 and exceed the European Community (EC) water quality guide values. The contribution of land drainage to NO2− loads carried by these rivers was estimated to be about 40%. The remaining 60% of NO2− appears to originate from N transformations at the sediment‐water interface of the river system. The available evidence suggests that NH+4 originating from agricultural pollution provides the N substrate for nitrification by Nitrosomonas to NO2−. What is anomalous is why this NO2− is not further oxidized rapidly to NO−3 by Nitrobacter. A possible mechanism is that the Nitrobacter is retarded by the presence of free ammonia concentrations that can be predicted to be present in the range of 65 to 76 μg N L−1.

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