INFLUENCE OF TEMPERATURE ON MINERALIZATION KINETICS WITH A NITRIFICATION INHIBITOR (MIXTURE OF DICYANDIAMIDE AND AMMONIUM THIOSULFATE)

The influence of temperature on the action of a dicyandiamide nitrification inhibitor was studied during a laboratory incubation after the addition of ammonium sulphate labelled with N-15. In the control treatment, nitrification was only slightly affected by temperature and was rapid; on the 42nd day, two-thirds of the N-15 was incorporated into the nitrate fraction while no further tracer was found in ammoniacal form. With the addition of dicyandiamide, the process was slowed down considerably when the temperature was maintained at 10-degrees-C, and only about 10% of the N-15 was nitrified in 6 months. After 1 month of incubation at 10-degrees-C, a temperature increase to 15-degrees-C for 4 weeks modified the nitrification kinetics only slightly. However, as soon as the temperature reached 20-degrees-C, the beginning of dicyandiamide decomposition and an increase in the quantity of NO3--N was observed. The inhibition was measured by the nitrification index, which was greater than 80% as long as the temperature did not exceed 15-degrees-C, and decreased to 10% after 6 months; this value was reached only after 1 year in soil maintained at 10-degrees-C. The half-life of the NH4+ was decreased by raising the temperature. In the experimental conditions described, nitrification was inhibited by the dicyandiamide for at least 6 months provided the temperature did not exceed 15-degrees-C.