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Nitrification of Anhydrous Ammonia Related to Nitrapyrin and Time-Temperature Interactions¹

The effects of time and temperature, with and without nitrification inhibitor, were evaluated on the rate of nitrification for field-applied anhydrous ammonia on a Webster clay loam soil (fine loamy, mixed, mesic Typic Haplaquoll) in central Iowa. The purpose of the study was to document factors influencing the production of oxidized N, forms of N more easily lost from the soil by leaching or denitrification. Anhydrous ammonia at 180 kg N/ha was applied at three application dates in the fall of 1979 (9 October, 27 October, and 14 November) and three application dates in the spring of 1980 (15 April, 2 May, and 25 June). Plots with and without 0.56 kg/ha nitrapyrin were established on each date. Ammonium was found to be concentrated within a 5-cm radius of the injection zone. The accumulation of heat units (HU), calculated by the summation of temperature (°C) multiplied by time (days), was shown to have a negative correlation with recoverable NH₄-N and followed the equation: %NH₄-N = –0.07 HU + 76.4, R2 = 0.84*, significant at the 0.05 level. Thus, much of the variability of recovered NH₄–N could be explained by a time by temperature interaction. This suggests that a grower's decision to apply anhydrous ammonia in the fall when soil temperatures reach a given level, commonly lOT, should be tempered by the date. In early fall, more time is available for nitrification to occur before freezing of the soil, and a greater conversion of NH₄–N to N0₃–N would be expected. In the presence of a nitrification inhibitor, higher percentages of NH₄–N were recovered for a similar heat-unit accumulation. Regression equations predicted an 84% recovery with 233 accumulated heat units in the presence of nitrapyrin, compared with a 60% recovery without nitrapyrin. For an equal percentage of NH₄–N recovery the following spring, a grower could apply anhydrous ammonia earlier in the fall if nitrapyrin was used. According to the equations developed in this study for the 1979-1980 season, a 9 October application of anhydrous ammonia with nitrapyrin would have resulted in approximately a 70% NH⁴–N recovery in the spring of 1980, compared with the same recovery for a 23 November application without nitrapyrin.

Key Words: Nitrogen efficiency • Nitrification inhibition • N-Serve • Typic Haplaquoll

² Former graduate student and associate professor, Dep. of Agronomy, Iowa gate Univ., Ames, IA 50011.

Received for publication March 25, 1983.

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