THE EFFICACY OF TWO NITRIFICATION INHIBITORS AT HIGH TEMPERATURE IN TWO IOWA SOILS

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Abstract
We studied the efficacy of two nitrification inhibitors, nitrapyrin (NPY) and potassium trithiocarbonate (PTC), applied to two Iowa soils and incubated at high temperature. Samples from a Canisteo sil (fine-loamy, mixed (calcareous), mesic Typic Haplaquoll; 2.6% CaCO3, pH 7.8) and a Nicollet soil (fine-loamy, mixed, mesic Aquic Hapludoll; 0% CaCO3, pH 7.0) were treated with NH4OH (28% NH3), NH4OH plus K2S3C, and NH4OH plus nitrapyrin, applied at rates of 5.7 .mu.g g-1 soil (as CS2) and 0.5 .mu.g g-1 soil, respectively. The samples were incubated at 65% of field capacity for 2, 4, 8, 16, 32, and 64 d in a greenhouse. Daily temperatures fluctuated between 21.7 and 26.7.degree.C. Recovery of NH4+-N was approximately the same in all treatments and decreased over time at about the same rate in the calcareous Canisteo soil. The percentage of recovery of NH4+-N was highest in the samples treated with inhibitors in the noncalcareous Nicollet soil, 57.6, 36.2, and 13.6% at 16 d for the NH4OH + NPY, NH4OH + PTC, and NH4OH treatments, respectively. pH decreased less sharply in the Canisteo soil than in the Nicollet soil. The rate of decrease of pH was similar in all treatments within soils. Estimates of NH4-N half-life ranged from 15.1 to 16.1 d in the Canisteo soil and from 17.3 to 20.3 d in the Nicollet soil. Results from this study indicate that more research is needed on the fate and action of nitrification inhibitors applied at agronomically recommended rates under relatively high temperatures and in the presence of CaCO3.