Bacteriostatic action of nitrification inhibitors

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Abstract
Nitrification inhibitors were investigated in an attempt to establish whether such chemicals actually kill ammonium-oxidizing bacteria (bactericidal action) or whether bacteria remain viable but temporarily incapable of nitrification (bacteriostatic action). In laboratory experiments with nitrifying cultures, nitrification was completely inhibited, but numbers of ammonium-oxidizing bacteria were not significantly affected by a 48-h treatment with 2,6-dimethylbenzoquinone, dicyandiamide, or potassium ethyl xanthate applied at the rate of 100 mg inhibitor/L culture medium or by nitrapyrin at 10 mg/L. However, commercial formulations of etridiazole at 100 mg/L and nitrapyrin at 100 mg/L were apparently fatal to ammonium-oxidizing bacteria. In laboratory experiments with soil amended with nitrapyrin or etridiazole, both inhibitors were more bactericidal than in aqueous culture. Nitrification did not resume until 4 to 5 weeks after adding 0.5 mg inhibitor/kg soil and, especially with etridiazole, the population of ammonium oxidizers was drastically diminished and did not fully recover. However, in field experiments, nitrapyrin and etridiazole injected at 1.5 kg/ha with aqueous urea did not affect numbers of ammonium-oxidizing bacteria in cross-sections of the injected band of soil taken after injection in autumn or spring, although the inhibitors were present in concentrations that effectively inhibited nitrification.

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