Effect of the nitrification inhibitors dicyandiamide,nitrapyrin and thiourea onNitrosomonas europaea

Effects of the inhibitors dicyandiamide (DCD), nitrapyrin (N-Serve, NS), and thiourea (TU) on growth and metabolism ofNitrosomonas europaea were tested under laboratory conditions. Growth of a pure culture was completely suppressed by 10 ppm NS and 0.5 ppm TU; inhibition by 300 ppm DCD was 83%. Ammonia oxidation and respiration of Nitrosomonas cell suspensions were reduced by 93% (10 ppm NS), 95% (0.5 ppm TU), and 73% (300 ppm DCD).Hydroxylamine oxidoreductase was not affected by high concentrations of inhibitors (200 ppm DCD, 100 ppm NS, 100 ppm TU). Cytochrome oxidase activity was increased by 10% with 200 ppm DCD, not affected by 100 pm TU, and inhibited by 52% with 100 ppm NS.The modes of action thus differed among the tested inhibitors. The merely bacteriostatic effect of dicyandiamide was demonstrated by the reversibility of growth inhibition.