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 10˚C, 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.