EFFECTS OF CONCENTRATION, INCUBATION-TEMPERATURE, AND REPEATED APPLICATIONS ON DEGRADATION KINETICS OF DICYANDIAMIDE (DCD) IN MODEL EXPERIMENTS WITH A SILT LOAM SOIL

The kinetics of dicyandiamide (DCD) decomposition were studied (at 80% water-holding capacity) in pretreated and non-pretreated soils, using model experiments. DCD was added in different concentrations (6.7, 16.7, and 33.3-mu-g DCD-N g-1 dry soil) and incubated at various temperatures (10-degrees, 20-degrees, and 30-degrees-C). Additionally, DCD decomposition was examined in sterile soil (with or without Fe2O3) after inoculation with a DCD-enrichment culture. In the sterile variants (30-degrees-C) the applied dicyandiamide concentration remained constant, even after 36 days. In the sterilized and reinoculated variant, DCD disappeared within 7 days. Addition of Fe2O3 powder to the sterilized soil had no effect on DCD degradation. In the pretreated soils, DCD mineralization started immediately at all temperatures and concentrations without a lag phase. A temperature increase of 10-degrees-C doubled the mineralization rate. The mineralization rates were independent of the initial concentrations. In the non-pretreated soils (except at 30-degrees-C with 16.7 and 33.3-mu-g DCD-N g-1 dry soil) DCD decreased only after a short (30-degrees-C) or a long (10-degrees-C) lag phase. These results suggest that an inducible metabolic degradation occurred, following zero-order kinetics.