Effect of Long‐Term, Biennial, Fall‐Applied Anhydrous Ammonia and Nitrapyrin on Soil Nitrification

Long‐term dryland wheat plots were established in northwestern Utah and maintained for 8 yr in a 2‐yr wheat‐fallow rotation. Nitrapyrin [2‐chloro‐6‐(trichloromethyl)pyrindine] was applied with anhydrous ammonia (NH 3 ) in the fall preceding wheat growth to retard nitrification. Our objective was to determine the effects of long‐term, biennial application of anhydrous NH 3 with and without nitrapyrin on soil nitrification. We were particularly interested in the potential residual effects of the long‐term repeated applications of anhydrous NH 3 and nitrapyrin. Nitrification potentials were measured in control (no added N) soil, or soil fertilized with anhydrous NH 3 with or without nitrapyrin for both rotation phases. Nitrification potentials were higher in soils receiving anhydrous NH 3 than in the control soil during the cropped rotation. Nitrification potentials in soils receiving anhydrous NH 3 with nitrapyrin were similar to those of the control soils during the entire wheat fallow rotation period. Further, nitrification potentials in soils with a history of nitrapyrin use were significantly lower than in soils without nitrapyrin use when measured after 2 yr. We observed a transient increase in nitrification potentials with the application of anhydrous NH 3 that did not last in the fallow year, suggesting that the long‐term, biennial application of anhydrous NH 3 at a rate of 50 kg ha −1 had no residual effect on soil nitrifier population size. In contrast, our results suggest that the long‐term, biennial application of nitrapyrin did have a residual effect on soil nitrifier populations that lasted at least 2 yr.