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.

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N-(n-Butyl)thiophosphoric triamide (NBPT) has been reported to reduce ammonia volatilization from surface-applied urea and urea ammonium nitrate (UAN). A new NBPT formulation (ARM U™, 18% NBPT) that contains a polymer allowing for lower application rate of NBPT was evaluated for its efficacy relative to Agrotain® (30% NBPT) and Arborite® (24% NBPT). Trials consisted of (i) a greenhouse study that compared two rates of ARM U-treated urea (360 and 540 mg NBPT kg−1 urea) with Arborite- or Agrotain-treated urea (480 and 600 mg NBPT kg−1 urea, respectively) and (ii) a field study that compared urea and UAN treated with either ARM U (360 mg NBPT kg−1 urea) or Agrotain (600 mg NBPT kg−1 urea) at two sites. Static chambers fitted with acid-charged discs were used to measure ammonia volatilization at six or seven dates over 28 d. In the greenhouse study, ammonia volatilization was reduced by 96% with either ARM U or Agrotain and 95% with Arborite. In the field study, ARM U and Agrotain reduced ammonia volatilization from urea by 80% and 66%, respectively, across sites. Similarly, ammonia volatilization from UAN was reduced by 46% and 60% with ARM U and Agrotain, respectively. Despite the lower NBPT application rates with ARM U, ammonia reduction by ARM U, Agrotain, and Arborite was not significantly different. The addition of ARM U to urea and UAN enabled lower application rate of NBPT without compromising its efficacy.

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The addition of nitrification inhibitor (NI) with a urease inhibitor, N-(n-butyl) thiophosphoric triamide (NBPT), has been reported to offset the reduction of ammonia volatilization by NBPT. An incubation study was conducted to investigate the interaction between NBPT and NI (3,4-dimethyl pyrazole phosphate) on hydrolysis of urea in five soils with a range of physico-chemical properties. Untreated urea (UR), NBPT treated urea (UR NBPT), or NBPT+NI treated urea (UR DI) were surface-applied (250 kg N ha −1) to each soil. The soils were incubated (21 • C) and destructively sampled nine times during a 22-day period. Urea hydrolysis rate (k; d −1) was measured by the disappearance of urea with time and modeled with a first-order kinetic. The value of k was in the order of UR (0.321) > UR DI (0.183) > UR NBPT (0.151) across the five soils. While the urease inhibitor, NBPT, significantly reduced k in each soil, the addition of a NI with NBPT significantly decreased the ability of NBPT to inhibit urea hydrolysis by an average of 21% across the soils. We found that NI significantly reduced the half-life of urea by about 1 d when compared with NBPT alone. Principal component analysis showed that k did not depend on any of the soil properties, rather, it depended on the type of treatment. Net nitrification rate constant was significantly greater in UR than UR NBPT in loam and clay soils but not different in sandy loam soils. We conclude that the often-reported increase in ammonia volatilization with the double inhibitor relative to NBPT alone may not only be due to the persistence of ammonium but may also be due to an increased rate of urea hydrolysis in the presence of a NI. 1 INTRODUCTION Globally, urea accounts for about 50% of nitrogen (N) fertilizers that are applied to agricultural and horticultural Abbreviations: DI, double inhibitor; NBPT, N (n-butyl) thiophosphoric triamide; NI, nitrification inhibitor; UR DI , double inhibitor treated urea; UR NBPT , NBPT treated urea. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.