Interaction of NBPT with Two Nitrification Inhibitors on Multiple Soils.

Nitrogen (N) stabilizers, such as nitrification/urease inhibitors, are utilized to reduce the amount of N entering the environment while maximizing crop uptake of N.  These products inhibit certain soil biochemical pathways and may have adverse effects on other pathways in the N cycle. The objective of this study was to quantify ammonia volatilization (a N loss pathway) from surfaced applied granular urea treated with two application rates of NBPT (a urease inhibitor) and nitrification inhibitors, DCD and nitrapyrin, from three soils using a laboratory system. The study consisted on nine trials evaluating the urease inhibitor NBPT and nitrification inhibitors DCD and nitrapyrin. Trials one through three evaluated two formulations of NBPT, trials four through six evaluated combinations of NBPT and DCD, and trials seven through nine evaluated combinations of NBPT and nitrapyrin. The application rates for NBPT were 900 and 1,200 mg NBPT per kg urea, 8,400 and 16,000 mg DCD per kg urea, and 1,500 and 3,000 mg nitrapyrin per kg urea. Cumulative N loss as ammonia from untreated granular urea was 40.1%, 44.5%, and 38.2% of applied N for the Raub, Wheeling, and Pella soils, respectively. When the urease inhibitor NBPT was applied alone at 900 mg NBPT kg^-1 urea in the Agrotain^® Ultra formulation, N loss through volatilization during the first three trials was reduced 48.5, 43.3, and 77.9% for the Raub, Wheeling and Pella soil types, respectively. Nitrification inhibitors, DCD and nitrapyrin, alone increased ammonia volatilization losses of N in five out of six trials compared to untreated urea. Soil type significantly influenced the rate and cumulative ammonia loss from surface applied untreated urea and urea treated with the urease/nitrification inhibitors. The results demonstrate that when using a nitrification inhibitor with surface applied granular urea, NBPT also needs to be applied to reduce the amount of N lost as ammonia if conditions are favorable for volatilization. Also, more site/soil specific recommendations for soil types may need to be implemented for these enhanced efficiency N fertilizers to further improve the efficacy of the products.