Response of Corn to Nitrogen‐15‐Labeled Anhydrous Ammonia with and without Nitrapyrin in Iowa

The response of corn ( Zea mays L.) to 15 N‐labeled anhydrous ammonia applied at 112 and 224 kg N ha− 1 with and without nitrapyrin [2‐chloro‐6‐(trichloromethyl) pyridine] was studied at two sites in Iowa during 1982, 1983, and 1984 to obtain a better understanding of why nitrification inhibitors often do not increase grain yields in the western part of the Corn Belt. Significant increases in grain yields were observed in response to N at both sites, but the response was significant only for the first increment of N at one site. Nitrapyrin had a statistically significant effect on grain yields at only two of the 12 rate‐site years, and this effect was negative. Results showing that nitrapyrin sometimes increased the N content of tissues to above critical levels without increasing yields to plateau levels suggest that this compound sometimes had adverse effects on plant growth when yield decreases were not observed. Application of fertilizer N in excess of plant needs probably is a major reason for lack of positive yield responses to nitrapyrin in this study in which mid‐season moisture stress often limited maximum grain yield and plant need for N. The exact nature of the adverse effects could not be determined, but it seems likely that nitrapyrin may have increased the susceptibility of plants to moisture stress or induced an ammonia toxicity in the plants. Because other researchers also have observed yield depressions, it must be suspected that negative effects of nitrification inhibitors often outweigh or mask the benefits of these compounds. Learning more about the nature of these negative effects and how to avoid them should receive greater attention as a viable strategy for attaining the potential benefits of using nitrification inhibitors during crop production.