*Article* **Degradation of** *N***-(***n***-butyl) Thiophosphoric Triamide (NBPT) with and without Nitrification Inhibitor in Soils**

**Ahmed A. Lasisi \* and Olalekan O. Akinremi**

Department of Soil Science, University of Manitoba, Winnipeg, MB R3T 5H3, Canada; wole.akinremi@umanitoba.ca

**\*** Correspondence: greatahmlas@gmail.com

**Abstract:** Recent studies have shown that nitrification inhibitor (NI) impairs the efficacy of urease inhibitor, *N*-(*n*-butyl) thiophosphoric triamide (NBPT), in reducing ammonia volatilization and urea hydrolysis rate. A laboratory study was conducted to evaluate the influence of NI (specifically 3,4-dimethyl pyrazole phosphate) on the degradation of NBPT in six soils. Soils were amended with either NBPT (10 mg NBPT kg−<sup>1</sup> soil) or NBPT plus NI (DI; 10 mg NBPT + 2.5 mg NI kg−<sup>1</sup> soil), incubated at 21 ◦C, and destructively sampled eight times during a 14-day incubation period. The degradation of NBPT in soil was quantified by measuring NBPT concentration with highperformance liquid chromatography-mass spectrometry, and the degradation rate constant was modeled with an exponential decay function. The study showed that the persistence of NBPT in soil was not influenced by the presence of NI, as the NBPT degradation rate constant across soils was 0.5 d−<sup>1</sup> with either NBPT or DI. In contrast, the degradation rate constant was significantly dependent on soils, with values ranging from 0.4 to 1.7 d<sup>−</sup>1. Soil pH was the most important variable affecting the persistence of NBPT in 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. The faster degradation of NBPT in acidic soils may explain its reduced efficacy in such soils.

**Keywords:** NBPT; nitrification inhibitor; half-life; degradation rate constant

#### **1. Introduction**

Globally, urea is the predominant form of granular nitrogen (N) fertilizer used to supplement soil N availability to crops. It is relatively safe to handle and contains high N content (46%). When urea is applied to soils, it becomes hydrolyzed in the presence of the urease enzyme into one bicarbonate ion and two molecules of ammonia [1]. The process of urea hydrolysis increases the soil pH around urea, which drives the equilibrium between ammonia and ammonium toward ammonia, thereby resulting in the volatilization of ammonia. Ammonia volatilization from urea when it is surface-applied without incorporation could be greater than 20% of applied N and is one of the reasons for low urea-N use efficiency [2]. Apart from being an economic loss to the farmers, ammonia volatilization has a deleterious effect on the environment and human health [3].

*N*-(*n*-butyl) thiophosphoric triamide (NBPT) is a urease inhibitor that has been widely reported to decrease ammonia volatilization from urea-based fertilizers under different soil and environmental conditions [2,4,5]. The NBPT is usually used to coat urea granules or mixed with liquid urea-based fertilizers, such as urea ammonium nitrate. The NBPT reduces ammonia volatilization by suppressing the activity of urease enzymes responsible for the hydrolysis of urea [6]. To suppress urease activity, NBPT becomes converted to either *N*-(*n*-butyl) thiophosphoric diamide (NBPD) or *N*-(*n*-butyl) phosphoric triamide (NBPTO) in soils [1,7]. The NBPD and NBPTO become hydrolyzed into monoamido thiophosphoric acid and diamido phosphoric acid, respectively. These then block the two nickel ions' active sites of urease enzymes via two oxygen atoms and one amide group [1]. The global

**Citation:** Lasisi, A.A.; Akinremi, O.O. Degradation of *N*-(*n*-butyl) Thiophosphoric Triamide (NBPT) with and without Nitrification Inhibitor in Soils. *Nitrogen* **2022**, *3*, 161–169. https://doi.org/10.3390/ nitrogen3020012

Academic Editor: Jacynthe Dessureault-Rompré

Received: 19 February 2022 Accepted: 31 March 2022 Published: 2 April 2022

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efficiency of NBPT in reducing ammonia volatilization from urea relative to untreated urea has been estimated to be 52% [2].

Several studies have reported a reduction in the efficiency of NBPT when combined with a nitrification inhibitor (NI) in reducing ammonia volatilization from urea [8–13]. The decrease in NBPT efficiency when used as NBPT plus NI (double inhibitor, DI) was attributed to the persistence of ammonium by the NI. Recent studies to elucidate the mechanism of the reduced NBPT efficacy in decreasing ammonia volatilization with DI showed that NI, specifically 3,4-dimethyl pyrazole phosphate, significantly impaired the inhibitory effect of NBPT on urea hydrolysis rates across several soils and temperatures [14,15]. For example, NI was found to reduce the inhibitory effect of NBPT on the urea hydrolysis rate by 21% across five soils at 21 ◦C [14]. The effectiveness of NBPT in reducing the urea hydrolysis rate decreases as temperature increases [15]. The action of NBPT on urea has been shown to be more effective in reducing ammonia volatilization during fall than spring seasons on the Canadian prairies [13]. Moreover, studies have also shown that the rate of NBPT degradation in soils was greater in acidic than alkaline soils [16,17]. However, there is a lack of information on the influence of NI on the degradation of NBPT with or without urea in soils. This study was conducted to evaluate the influence of NI, particularly 3,4-dimethyl pyrazole phosphate, on the degradation rate of NBPT without urea in six soils. We hypothesized that NI would interfere with the persistence of NBPT in soils.
