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 mai ntained for 8 yr in a 2-yr wheat-fallow rotation. Nitrapyrin [2-chloro-6-(t richloromethyl)pyrindine] was applied with anhydrous ammonia (NH3) in the f all preceding wheat growth to retard nitrification, Our objective was to de termine the effects of long-term, biennial application of anhydrous NH3 wit h and without nitrapyrin on soil nitrification. We were particularly intere sted in the potential residual effects of the long-term repeated applicatio ns of anhydrous NHS and nitrapyrin. Nitrification potentials were measured in control (no added N) soil, or soil fertilized with anhydrous NHS with or without nitrapyrin for both rotation phases. Nitrification potentials were higher in soils receiving anhydrous NH3 than in the control soil during th e cropped rotation. Nitrification potentials in soils receiving anhydrous N H3 with nitrapyrin were similar to those of the control soils during the en tire wheat fallow rotation period. Further, nitrification potentials in soi ls with a history of nitrapyrin use were significantly lower than in soils without nitrapyrin use when measured after 2 yr, We observed a transient in crease in nitrification potentials with the application of anhydrous NH3 th at did not last in the fallow gear, suggesting that the long-term, biennial application of anhydrous NH3 at a rate of 50 kg ha(-1) had no residual eff ect on soil nitrifrer population size. In contrast, our results suggest tha t the long-term, biennial application of nitrapyrin did have a residual eff ect on soil nitrifier populations that lasted at least 2 yr.