**3. Results**

#### *3.1. NBPT Recovery*

The interaction of treatment and time did not significantly affect % NBPT recovered, except in Soil 2 (Table 2). The significant interaction in Soil 2 was because of greater NBPT recovered in NBPT only with DI on 0.5 and 2 d (Figure 1). The % of NBPT recovered immediately after treatment application (time = 0 d) was less than 70% in all the soils except in Soil 1 (Figure 1). The low recovery of NBPT on 0 d might be due to other NBPT species (e.g., [M + Na]+) of the ion chromatograms that were not accounted for. As expected, the % of NBPT recovered significantly decreased with time in an exponential decay order (Figure 1). The persistence of NBPT was shortest in Soil 1, with the NBPT recovery reaching the lowest point (3%) by 2 d in both inhibitor treatments. In contrast, the % of NBPT recovered from neutral to alkaline soils on 2 d ranged from 22 to 44% in both inhibitor treatments (Figure 1). By 7 d, NBPT was below the detection limit in all soils. The NBPT recovery was well predicted by the exponential decay function as indicated by the Nash–Sutcliffe model efficiency, which ranged from 0.93 to 0.99 across the soils.


**Table 2.** Effect of inhibitor treatment and time on % of NBPT recovered in soils.

Probability values are significant at <0.05. df, degree of freedom.

**Figure 1.** The percentage recovery of NBPT in soils. Error bars are standard errors of the mean (*n* = 3). NBPT and NBPT(p) are measured and predicted % of NBPT recovered in NBPT inhibitor treatment; DI and DI(p) are measured and predicted % of NBPT recovered in DI inhibitor treatment. *N*-(*n*-butyl) thiophosphoric triamide; DI, double inhibitor [NBPT + 3,4-dimethyl pyrazole phosphate]; Y, % of applied NBPT recovered; RMSE, Root mean square error, ME, Nash–Sutcliffe model efficiency.

#### *3.2. Kinetics of NBPT Degradation*

The NBPT degradation rate constant was not significantly affected by inhibitor treatment or the interaction of soil and inhibitor treatment (Table 3). As such, the half-life of NBPT in each soil was not affected by the type of inhibitor treatment (NBPT only versus DI; Table 3). Averaged across soils, the half-life of NBPT in either inhibitor treatment was 1.3 d. The lack of a significant difference in the half-life of NBPT between the two inhibitor treatments did not agree with our hypothesis. Our previous study had found that NI reduced the half-life of NBPT-treated urea in soils by 1 d at 21 ◦C [15].

**Table 3.** Effect of inhibitor treatment and soil on degradation rate constant (k) and half-life (t1/2) of NBPT.


Note. Means with different letters within a column are significantly different at a probability value of <0.05 using Fisher protected LSD.

Unlike the inhibitor treatment, there was a significant effect of soil on the half-life of NBPT in soils. Soil 1, which was the acidic soil, had the shortest half-life (0.4 d), while Soil 4, which was slightly alkaline, had the longest half-life (2.1 d) when averaged across inhibitor treatments (Table 3). NBPT is *N*-(*n*-butyl) thiophosphoric triamide, DI is double inhibitor [*N*-(*n*-butyl) thiophosphoric triamide + 3,4-dimethyl pyrazole phosphate]. The shortest half-life of NBPT in Soil 1 was consistent with previous studies that found that the NBPT degradation rate was faster in acidic than alkaline soils [16,17]. Similarly, the shortest half-life of the inhibitor treatments in Soil 1 corroborated our previous studies that used the same soils and found the half-life of urea treated with either NBPT or DI to be shorter in Soil 1 than in other soils [14,15]. Additionally, other studies had also reported a lower NBPT inhibition of urea hydrolysis in acidic than alkaline soils [26,27]. The lack of NI on NBPT degradation in this study was probably because of the absence of urea. This is because the soil pH around applied urea changes during the hydrolysis of urea and the nitrification process. This implies that NI did not affect the persistence of NBPT in soil but rather impaired the inhibitory effect of NBPT on urea hydrolysis. With no effect of NI on NBPT degradation, the observed inhibition of NBPT to reduce urea hydrolysis by NI, as noted in the studies [14,15] might have been because of the soil acidification during nitrification [28]. While hydrolysis of NBPT in soils to form NBPTO and NBPD is required to inhibit the process of urea hydrolysis [1,7], rapid hydrolysis of NBPT, as shown in the case of acidic soil, may be counter-effective. For example, NBPTO and phenyl phosphorodiamidate are potent urease inhibitors with greater inhibition of urease than NBPT under a buffered solution, but their reduced persistence in soils makes them less effective in reducing ammonia volatilization when compared to NBPT [16,29–31].

Stepwise regression analysis showed that soil pH, organic matter, and urease activities accounted for 91% of the variation in the half-life of NBPT in soil. Of these soil properties, soil pH was the most predictive factor of NBPT half-life, as indicated in Equation (4). The persistence of NBPT in soils increased as the soil pH increased from strongly acidic to neutral soil pH and then decreased from neutral soil pH to slightly to moderately alkaline soil pH (Figure 2; pH classification based on USDA). An earlier study had shown that the half-life of NBPT in acidic soil (pH = 4.9) could be extended by 2.5 d when the soil pH was increased to neutral pH (6.9) using calcium hydroxide [16]. Despite the reported reduced persistence of NBPT in acidic than alkaline soils, the reduction of ammonia volatilization by NBPT relative to untreated urea is not always lower in acidic than alkaline soils [32].

Half-life = −5.874 + 1.221(pH) − 0.0141(urease activity) − 0.1163(organic matter) (4)

**Figure 2.** Relationship between soil pH and NBPT half-life. Y is NBPT half-life in soil.

## **4. Conclusions**

The urease inhibitor, NBPT, plays an important role in conserving applied urea-N in the soil. While NI is known to impair the inhibitory effect of NBPT on urea hydrolysis, our study showed that NI did not interfere with the persistence of NBPT in soil. Instead, the persistence of NBPT in soil was mainly influenced by soil pH. We found that the degradation of NBPT was two to four times greater in acidic than neutral to alkaline soils. The half-life of NBPT was 0.4 d in acidic soil and 1.3 to 2.1 d in neutral to alkaline soils. As such, N management with NBPT may be more suitable for alkaline than acidic soils, and alkaline soils thereby provide more flexibility in precipitation or irrigation scheduling to incorporate urea into the soil while reducing N losses. Future studies will need to evaluate how the interaction between urea, NI, and NBPT affect the persistence of NBPT over a wide range of soils and environmental conditions.

**Author Contributions:** Conceptualization, A.A.L.; Data curation, A.A.L.; Formal analysis, A.A.L.; Funding acquisition, O.O.A.; Investigation, A.A.L.; Methodology, A.A.L.; Project administration, O.O.A.; Supervision, O.O.A.; Validation, A.A.L.; Visualization, A.A.L.; Writing—original draft, A.A.L.; Writing—review and editing, A.A.L. and O.O.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data available upon request.

**Acknowledgments:** The authors appreciate the technical support of Emily Komatsu in method development for measuring NBPT with the HPLC-MS.

**Conflicts of Interest:** The authors declare no conflict of interest.
