Potency of nitrification inhibitors following their repeated application to soil

Summary Nitrification inhibitors were applied to a field experiment on loamy sand soil each autumn for 4 years, immediately prior to sowing winter cereals. Laboratory experiments demonstrated that repeated application of the inhibitor dicyandiamide (DCD) to a soil had little effect either on the rate of DCD decomposition or the ability of DCD to inhibit nitrification. Repeated field application of the inhibitors DCD, nitrapyrin or etridiazole resulted in increased sensitivity of ammonium-oxidizing bacteria to nitrapyrin or etridiazole, but not to DCD. The rate of decomposition of etridiazole was unaffected by four annual applications of this inhibitor, but decomposition of nitrapyrin was somewhat slower in soil that had received nitrapyrin annually for 4 years than in soil that had never been treated with an inhibitor.