Longevity of NBPT Effectiveness in Agricultural Soils.

A paradigm held by many in the agricultural community is that NBPT, a urease inhibitor in Agrotain® nitrogen stabilizer, is only effective for reducing ammonia volatilization loss from urea for a period of 7-10 days. The objective of this evaluation was to assess the validity of this paradigm by reviewing data from a range of research studies. A volatilization trial was conducted under laboratory conditions on a Wheeling silt loam and an Altavista sandy loam, and kept at constant temperature and moisture. Urea was applied to the surface at the rate of 112 kg N ha^-1 and volatilized ammonia was collected in acid traps under constant airflow for 14 days. Urea volatilized 35% and 23% of applied-N, respectively, while NBPT-treated urea (900 ppm) volatilized 16% of applied N in both soils. A field study conducted in eastern Oregon on wheat using a modified passive flux ammonia trap revealed that surface applied urea at 112 kg ha^-1 with no irrigation volatilized over 55% of the applied nitrogen, while NBPT-treated urea (1500 ppm) volatilized 5% of the applied nitrogen, after 18 days. A field study in Montana on winter wheat was conducted using an integrated horizontal flux approach over 7 to 10 weeks. NBPT-treated (1000 ppm) and untreated urea were surface-applied at the rate of 100 kg N ha^-1 in 12 locations throughout the state. Urea losses over the course of the trials ranged from 6.3 - 44.1% of applied N, and NBPT-treated urea losses ranged from 1.7 - 18.1% of applied N. Multiple incubation and field volatilization studies consistently demonstrate that NBPT-treated urea does not volatilize to the same extent as untreated urea, regardless of the length of the study.