Efficiency of fall versus spring applied urea-based fertilizers treated with urease and nitrification inhibitors I. Ammonia volatilization and mitigation by NBPT

Mid- or late-fall N fertilization is usually recommended on the Canadian prairies to reduce N losses from fall-applied N. Yet, N fertilizers are less efficient when applied in mid- or late fall than in spring on crop yield. Ammonia volatilization from N fertilizers is one of the reasons for the low N use efficiency. We conducted a 2-yr study to quantify and contrast the efficacy of a urease inhibitor, N-(n-butyl) thiophosphoric triamide (NBPT), with and without a nitrification inhibitor, 3,4- dimethylpyrazole phosphate (NI), in reducing NH3 volatilization from fall- and spring-applied urea-based fertilizers in two contrasting soils. Treatments consisted of surface-applied urea and urea-NH4NO3 (UAN) with and without NBPT or NBPT + NI (double inhibitor, DI). We measured NH3 volatilization with acid-charged disks during 21 (fall) or 28 (spring) d. Total NH3 volatilization (percentage of applied N) from untreated urea and UAN across sites ranged from 7 to 22% with fall application and from 3 to 29% with spring application. Addition of NBPT only or DI to urea or UAN reduced NH3 volatilization by 61 to 74% in the fall and 4 to 69% in the spring across site-years. We found that the reduction of NH3 volatilization by NBPT was significantly greater in the fall (65%) than in the spring (40%) and with urea (61%) than with UAN (43%). Our study showed that (a) significant NH3 volatilization could occur from surface-applied urea-based fertilizers even under late-fall temperatures and (b) NBPT reduced NH3 volatilization from fall- as well as spring-applied urea-based fertilizers. Therefore, farmers are encouraged to treat their urea-based fertilizers with NBPT when considering surface application.