CONTRASTS OF NITRAPYRIN, DICYANDIAMIDE, AND ISOBUTYLIDENE DIUREA EFFECTS ON TOTAL INORGANIC SOIL-NITROGEN

While much research has been done with nitrification inhibitors, little information exists on their effects on total inorganic soil nitrogen (N) (TISN) (NH4+-N + NO3--N). The objective of this study was to compare the effects of 2-chloro-6-(trichloromethyl)-pyridine (nitrapyrin) and dicyandiamide (DCD) on TISN concentrations and to compare the effects of the nitrification inhibitors with those of IBDU, a slow release N source. Effects of the three N-amendments were compared in soil planted to potato (Solanum tuberosum L. cv. Atlantic). Treatments were combinations of N at 67, 134, and 202 kg/ha; and five N-amendments, nitrapyrin at 0.56 and 1.12 kg/ha, DCD at 5.6 and 11.2 kg/ha, and isobutylidene diurea (IBDU) applied as one-third of fertilizer N. Studies were conducted on an Arenic ochraqualf, a Grossarenic Paleudult, and a Grossarenic Paleaquult. Concentrations of TISN were influenced by nitrapyrin rate in five of sixteen year- location-sampling date combinations. Of the five, all involved interactions with N rate effects. With two of these five, TISN concentration was increased by an increase in nitrapyrin rate with 67 kg N/ha, with one other, TISN concentration was decreased with an increase in nitrapyrin rate with 67 kg N/ha. Differences between nitrapyrin and DCD in their effects on TISN were few. Thus, at the rates applied in this study, the two inhibitors had similar effects on soil N. IBDU was more effective than the nitrification inhibitors, for maintaining plant available N in the rooting zone during the middle and latter portions of the growing season.