Response of Winter Wheat to Inhibiting Nitrification of Fall-Applied Nitrogen

Large overwinter leaching and denitrification losses of fall-applied N are common in many Corn Belt soils cropped to wheat (Triticum aeslivum L.). These losses decrease the utilization efficiency of N fertilizers, lower crop yields and quality, necessitate multiple fertilizer applications, and predispose wheat to several soil-borne diseases. This study was conducted to determine if inhibiting nitrification of fall-applied ammoniacal fertilizers would reduce N losses and be a practical alternative to multiple fertilizer applications in maintaining crop yield, quality, and tolerance to disease. We tested the effectiveness of a specific nitrification inhibitor, Z-chloro-6(trichloromethyl)-pyridine (nitrapyrin), in reducing losses of fall-applied NH₄⁺ sources of N in four Indiana soils of varying texture. The field experiment was conducted over a 5-year period by comparing the growth, disease incidence, grain protein, and yield of wheat in field plots receiving anhydrous ammonia, urea, or ammonium sulfate plus nitrapyrin with wheat production in plots treated with these N sources without the inhibitor. Calcium nitrate was used in some studies to provide a source of N exhibiting high potential for loss. Inorganic N analyses of soil showed that severe loss of N frequently occurring as a result of fall application of N may be markedly reduced by using a nitrification inhibitor. Wheat yields were increased as much as 40% (average increase was 14.7%) as a result of applying 0.55 kg of nitrapyrin/ha with fall-applied NH₄⁺ fertilizers. Yields obtained with fall-applied N plus nitrapyrin were similar to those observed with a split-application (25% fall—75% spring) of N without the inhibitor. Fall application at 44 kg N/ha plus nitrapyrin resulted in yield similar to those with 88 kg N/ha without the inhibitor. Inhibition of nitrification of fall-applied NH₄⁺ also increased protein content, minimized N loss from the up per soil profile, and reduced the severity of fungal root and crown rots. Nitrification inhibitors added to fallapplied NH₄⁺ should provide a practical alternative to multiple fertilizer applications for maintaining the yield, quality, and disease tolerance of wheat grown in the Corn Belt.