Evaluation of the Side Effects of Nitrification-Inhibiting Agrochemicals in Soils

Selected bioassays were used to assess the side effects of agricultural chemicals on microbial activities under laboratory conditions using loamy clay, loam, sandy soil, and the nitrification inhibitors (NI) 3,4-dimethyl pyrazole phosphate (DMPP), 4-chlor-methyl pyrazole (ClMP), and dicyandiamide (DCD). The effect of NI on general microbial activity was assessed at concentrations from 1 to 1000 times the recommended application rate. Also, dehydrogenase (DHA) and dimethyl sulfoxide reductase activity (DRA) on nitrogenase activity (NA) was evaluated. The potential denitrification capacity (PDC), representing the nitrogen cycle specific to soil microbial processes, was examined in incubation experiments in the presence of DMPP, ClMP, and DCD. DHA was estimated spectrophotometrically, and DRA, PDC, and NA gas were quantified using gas chromatography. The morphological changes of a nitrifying bacterial consortium in the presence of the recommended DMPP field application rate and a tenfold-higher concentration were observed under transmission electron microscopy. Inhibition in the presence of increasing NI concentrations was calculated as no effect level (NOEL), effective dose ED10 (10% inhibition), and ED50 (50% inhibition). Dose–response curves expressing the effectiveness of inhibition were most distinct in sandy soils. Most NI-sensitive reacted PDC > DRA > DHA > NA and ClMP added to the three differently structured test soils influenced nontarget microbial process activity at a higher level. Our results clearly indicated that evaluation of agrochemical side effects with soil enzymes is a reliable, sensitive, reproducible, and suitable method to investigate interference with soil microbial activity.