Nitrogen (N) losses in the form of ammonia volatilization limit the efficiency of urea-based fertilizers world-wide. Urease inhibitor, N-(n-butyl) thiophosphoric triamide (NBPT) is a promising additive to reduce this N loss. Studies were conducted to test the efficacy of a relatively new NBPT formulation, ARM U (18% NBPT m/v), containing a proprietary polymer that allows for a low NBPT application rate in reducing ammonia volatilization from urea-based fertilizers. The results showed that ARM U reduced ammonia volatilization by an average of 85% across three soils and its effectiveness was not significantly different from other commercial NBPT formulations whose NBPT concentrations were 33-67% greater than ARM U.

Further studies showed that delaying N fertilizer application until late-fall, as recommended on the Canadian prairies, did not stop ammonia volatilization from urea-based fertilizers. Across 4 site-year, total ammonia volatilization (% of applied N) from untreated urea-based fertilizers applied in the fall and spring were 14 and 16%, respectively. We found that the reduction of ammonia volatilization by NBPT was significantly greater in the fall (65%) than in the spring (40%) across 4 site-year and the addition of NI with NBPT (double inhibitor, DI) reduced the efficacy of NBPT in decreasing ammonia volatilization from spring-applied urea by 27%. The conserved N by NBPT and DI improved the crop N use efficiency at one of the two sites in this study.

Investigation on the interaction between NBPT and NI on urea hydrolysis in five soils at 21 oC and six soils at 5, 15, and 25 oC showed that while NBPT reduced the rate of urea hydrolysis across soils and temperatures, the addition of NI reduced the inhibitory effect of NBPT on urea hydrolysis. Hence, reduction in ammonia volatilization is less with DI than only NBPT.

Overall, the research shows that (i) NBPT effectively reduced ammonia volatilization even at a low concentration (ii) significant ammonia volatilization could occur from urea-based fertilizers even at soil temperature below 3 oC (iii) use of NBPT to bridge crop urea use efficiency is site-specific (iv) NI impaired the effectiveness of NBPT in suppressing urea hydrolysis and ammonia volatilization.