Toxicity of Nitrification Inhibitors on Dehydrogenase Activity in Soils

Abstract

--The objective of this research was to determine the effects of nitrification inhibitors (NIs) such as 3,4-dimethylpyrazolephosphate=DMPP, 4-Chlor-methylpyrazole phosphate=ClMPP and dicyandiamide, (DCD) which might be expected to inhibit microbial activity, on dehydrogenase activity (DRA) in three different soils in laboratory conditions. Dehydrogenase activity was assayed via reduction of 2-p-Iodophenyl-3-p-nitrophenyl-5-phenyltetrazoliumchloride (INT). The toxicity and dose response curve of three NIs were quantified under laboratory conditions using a loamy clay, a sandy loam and a sandy soil. The quantitative determination of DHA was carried out spectrophotometrically. In all experiments, the influence of 5-1000 times the base concentration was examined. To evaluate the rate of inhibition with the increasing NI concentrations, dose response curves were presented and no observable effect level =NOEL, as well as effective dose ED 10 and ED 50 (10% and 50% inhibition) were calculated. The NOEL for common microbial activity such as DHA was about 30–70 times higher than base concentration in all investigated soils. ClMPP exhibited the strongest influence on the non target microbial processes in the three soils if it compare to DMPP and DCD. The NOEL,ED 10 and ED 50 values higher in clay than in loamy or sandy soil. The NIs were generally most effective in sandy soils. The three NIs considered at the present state of knowledge as environmentally safe in use.