Development of Green Urease Inhibitors as A Mitigation Tools for Ammonia Volatilization in Urea-Based Fertilizer: Review

Development of Green Urease Inhibitors as A Mitigation Tools for Ammonia Volatilization in Urea-Based Fertilizer: Review

Joshua Asukwo Adam, Ubelejit Uche Adum, Uwem Ekwere Inyang and Innocent Oseribho Oboh

Department of Chemical Engineering, Faculty of Engineering University of Uyo, Uyo Akwa Ibom State, Nigeria

Abstract – Nitrogen fertilizers play a critical role in sustaining global agricultural productivity and food security. Among nitrogen fertilizers, urea is the most widely used due to its high nitrogen content, low cost, and ease of handling. Despite these advantages, the agronomic efficiency of urea is significantly limited by rapid hydrolysis in soil catalyzed by the enzyme urease. This hydrolysis leads to the formation of ammonium carbonate and the subsequent release of ammonia gas, resulting in substantial nitrogen losses through ammonia volatilization. Nitrogen losses from urea fertilizers can range from 10% to 40% of applied nitrogen depending on soil and environmental conditions. These losses not only reduce nitrogen use efficiency but also contribute to environmental pollution, soil degradation, and atmospheric particulate formation. To address this challenge, urease inhibitors have been developed to temporarily suppress urease activity and delay urea hydrolysis. Synthetic inhibitors such as N-(n-butyl) thiophosphoric triamide (NBPT), N-(n-propyl) thiophosphoric triamide (NPPT), and phenyl phosphorodiamidate (PPD) have demonstrated high effectiveness in reducing ammonia volatilization. However, increasing environmental concerns regarding the persistence and toxicity of synthetic chemicals have stimulated research into environmentally friendly alternatives known as green urease inhibitors. These inhibitors are derived from natural sources including plant extracts, essential oils, phenolic compounds, and agricultural residues. Recently, research has focused on green urease inhibitors derived from plant materials, such as neem oil, garlic extract, eucalyptus leaves, and acacia extracts. These plant-based inhibitors are biodegradable, environmentally friendly, and potentially cost-effective alternatives to synthetic chemicals. Several studies report that plant extracts can inhibit urease activity by 2095% depending on the species and extraction method. This review provides a comprehensive evaluation of the mechanisms of ammonia volatilization from urea fertilizers, the role of urease inhibitors in mitigating nitrogen losses, and recent advances in the development of green urease inhibitors derived from plants and natural products. Additionally, a comparison between plant-based and synthetic urease inhibitors is presented to highlight their relative advantages, limitations, and future prospects in sustainable agriculture.

Keywords: Inhibitor, Urea, Fertilizer and volatilization

1.1 INTRODUCTION

Global population growth poses one of the most pressing challenges for modern agriculture. The world population is projected to reach approximately 9.7 billion by 2050, which is expected to outpace the expansion of land that can realistically be allocated for crop production (Yahya, 2018; Matseet al., 2024). Consequently, the amount of arable land available per person is likely to decline, placing immense pressure on agricultural systems to produce more food from

limited resources. In this context, improving nutrient management, particularly nitrogen (N) fertilization, becomes critical to ensuring sustainable crop production. Urea is the most widely used nitrogen fertilizer due to its high N content and ease of application; however, its efficiency is often compromised by significant nitrogen losses. Approximately 60% of applied urea can be lost as ammonia (NH) through volatilization, contributing to air pollution and environmental degradation. During urea hydrolysis, the rapid formation of inorganic ammonium ions (NH) occurs, but the retention of these ions in soil is typically poor because of limited adsorption capacity (Skorupkaet al, 2021; Bhatia et al., 2023). This dual problem of NH volatilization and poor NH retention results in nutrient deficiency for crops and economic loss for farmers. To compensate, farmers frequently apply higher quantities of urea, a practice that is neither cost-effective nor environmentally sustainable (Fu et al., 2020; IPCC, 2021; Sha, et al., 2023).

To address these challenges, research has focused on improving nitrogen use efficiency (NUE) through chemical and biological strategies. Among the most effective approaches are urease inhibitors, which temporarily suppress the activity of the urease enzyme, slowing urea hydrolysis and reducing NH losses (Singh et al., 2019; Wang et al., 2021). Urease inhibitors can be broadly classified into syntheticand natural compounds. Synthetic inhibitors, such as N-(n-butyl) thiophosphorictriamide (NBPT), N-(2-nitrophenyl) phosphoric triamide (2-NPT), and N-(n-propyl) thiophosphorictriamide (NPPT), are widely studied and have demonstrated reductions in ammonia volatilization by up to 6070% under field conditions (Cantarella et al., 2008; Abalos et al., 2014). These compounds act primarily through reversible binding to the urease active site, temporarily preventing urea hydrolysis while allowing gradual conversion to ammonium ions, which can be retained in the soil for plant uptake (Sriraj et al., 2022).

In contrast, natural urease inhibitors are derived from plant sources, including neem, tannin-rich extracts, and flavonoid compounds. Although their inhibition efficiency is often lower than that of synthetic inhibitors, natural compounds offer additional environmental advantages, such as biodegradability, lower toxicity, and potential compatibility with sustainable agriculture practices (Jadon et al., 2018; Rana et al., 2021). The mechanisms of natural inhibitors vary depending on the phytochemical structure: tannins and flavonoids can bind to the urease enzyme and alter its catalytic activity, while neem compounds can form a protective coating around urea granules, slowing hydrolysis and reducing volatilization. The application of both synthetic and natural urease inhibitors has significant implications for NUE (Pan et al., 2016). By delaying urea hydrolysis and improving the retention of ammonium ions in the soil, these inhibitors reduce nitrogen losses, enhance nutrient availability for crops, and minimize the environmental footprint of fertilizer use. Furthermore, integrating urease inhibitors into crop management strategies can reduce the frequency and quantity of urea applications, lowering production costs while maintaining yield (Cowan et al., 2021; BASF, 2016). This highlights the importance of combining chemical innovation with sustainable agricultural practices to meet the growing food demands of a rapidly expanding global population (Guo et al., 2023; Mathialagan et al., 2019; Liu et al., 2029).

The use of urease inhibitors in agricultural practices has long been explored as one of the strategies to guarantee food supply in enough amounts. The use of urease inhibitors is one of the strategies adopted to improve urea performance in agriculture and mitigate urea driven emission of pollutants (Kavanagh et al., 2021). This is due to the fact that urea, one of the most used nitrogen (N) fertilizers worldwide, rapidly undergoes urease-driven hydrolysis on soil surface yielding up to 70% N losses to environment. Currently, nitrogen fertilizers are utilized to meet 48% of the total global food demand. Nitrogen (N) is a vital soil nutrient essential for good and abundant plant growth (Fathi 2020). The main source of N for the plant comes from the external input application (Masclax-Daubresse et al., 2010).

The demand for nitrogen fertilizers is expected to grow as global populations continue to rise, in view of the world population especially in developing countries like Nigeria. In Nigeria, most farmers used nitrogen-based fertilizer due to the deficiency of nitrogen in soil (Koptevaet al., 2019). Nitrogen (N) fertilizers, and in particular urea-based fertilizers, have been widely used in agriculture, with a projected annual demand increase of 1.5% in the future. Urea is the

most concentrated solid nitrogen fertilizer (46% N), and it is cost-effective and economical in terms of production, making it the leading nitrogen fertilizer product globally (Bremner, 1996). However, its application efficiency is reduced by losses of nitrogen through ammonia volatilization when the urea is not incorporated into the soil under appropriate conditions. High soil pH, temperature, and microbial activity at the soil surface promote rapid hydrolysis of urea to ammonia and carbonate by soil urease enzymes, leading to up to 70% nitrogen loss into the environment. The hydrolysis of urea involves the consumption of protons, which leads to an increase in soil pH around the fertilizer granules. An increase in soil pH from

6.5 to 8.8 after urea application was reported by Overrein and Moe (1967). When urea is applied to the soil surface, it is hydrolyzed to ammonium ions by the urease enzyme, resulting in alkaline soil conditions that favor ammonia loss as gas from the soil surface (Bremner, 1996; Overrein and Moe, 1967). When hydrolysis is delayed, the concentration of NH near the soil surface decreases, which in turn reduces volatilization potential and allows time for rainfall to move urea deeper into the soil (Kiss and Simihaian, 2018).

Figure 1: Urea fertilizer and it structure. Source: Kavanagh et al., 2021

In the presence of water, urea is quickly hydrolyzed to ammonia (NH3), hydroxyl (OH) ions and carbon dioxide by the ubiquitous enzyme urease as seen in Equation (1) (Cancellier et al., 2015). This hydrolysis reaction results in an elevated pH surrounding the fertilizer granule, which switches the ammonium (NH4+)/NH3-equilibrium towards a higher NH3concentration in the soil solution and induces high emissions of NH3 into the atmosphere (Sommer et al. 2004).Part of the emitted NH3 is deposited on vegetation surfaces, where it causes acidification and eutrophication on a regional scale. Its impact is great, especially when deposited in natural and semi-natural ecosystems, and can result in an ecological shift in species diversity (Van Breemenet al. 1982; Bouwman and Van Vuuren 1999). Urea hydrolysis is catalyzed by the enzyme urease, which is produced by soil microorganisms and plant residues. The hydrolysis reaction can be represented in Equation (1) and (2)

(NH2)2CO + H2O2 NH3 + CO2 Equation 1

This reaction results in the formation of ammonia and carbon dioxide. Ammonia then reacts with water to form ammonium ions:

NH3+H2O NH4+ + OH Equation 2

The increase in hydroxyl ions causes a temporary rise in soil pH, promoting the release of ammonia gas. Global ammonia (NH) emissions from nitrogen fertilizer usage are estimated at 1012 Tg yr¹ (Erisman et al., 20013; Sutton et al., 2013).Ammonia emissions in Africa have increased by more than 50% during the past 30 years (Hickman et al., 2021; NASA Earth Science News Team, 2021). These emissions have economic, environmental and national policy implications (van Damme et al., 2022).

Green urease inhibitors (UIs), such as plant extracts (e.g., Vachellia nilotica)(Jadon et al, 2018) and NBPT, 2-NPT, etc. are used as coatings on urea fertilizer to reduce nitrogen loss via ammonia volatilization by up to 70%(Cantarella et al, 2008; Castellano et al., 2019). These compounds block the active site of the soil enzyme urease, slowing down the conversion of urea to ammonia by 12 weeks, allowing time for incorporation into the soil. To mitigate NH3volatilization losses, urease inhibitors can be used. Urease inhibitors are compounds that temporarily block the enzyme urease, slowing down the hydrolysis of urea into ammonium, ammonia, and CO. Several compounds act as urease inhibitors, but only N-(n-butyl) thiophosphorictriamide (NBPT) has been used worldwide, being the most successful in a market that has grown 16% per year in the past 10 years. Urease inhibitor reduces ammonia volatilization and nitrous oxide emission by decreasing both ammonium and nitrate concentrations in soil and hence increase plant N uptake, they enhance nitrogen efficiency and reduce gaseous ammonia losses by 7 to 14 days (Kumar et al., 2020; Castellano et al., 2019). These additives increase fertilizer efficiency and reduce greenhouse gas emissions as seen in Figure (2).

Figure 2: Effect of ammonia volatilization Source: Kumar et al., 2020

Urease inhibitors, such as N-(n-butyl) thiophosphorictriamide (NBPT), are chemical compounds, often formulated with polar organic solvents like glycol or alkyl sulfones, used to slow the enzymatic hydrolysis of urea fertilizers. These inhibitors reduce ammonia volatilization, typically applied at concentration to enhance nitrogen efficiency. NBPT (N-(n-butyl) thiophosphorictriamide), have been used with success and received considerable attention in the past two decades (Liu et al., 2019). NBPT (CAS No. 94317-64-3) with a molecular formula of C4H14N3PS, is a white crystalline solid (boiling point264.0 °C and melting point-59.1 °C) that can be coated to urea. Figure 3 shows the structure of the NBPT molecule. For NBPT to reduce NH3 emission, it needs to act on an enzyme known as urease, which is responsible for the hydrolysis of urea and the consequent NH3 volatilization. NBPT needs to be converted to its oxon analogue (NBPTo) for urease inhibition to occur (Afshar et al., 2018).

Figure 3: Structure of urease inhibitor NBPT. Source: Afshar et al., 2018

The strong urease inhibitory activity of NBPTo results in its binding with the urease active site at three locations, that is, two Ni atoms and a carbamate group. As the addition of supplemental fertilizer N is a cornerstone of many agricultural systems, N lost as NH3-N must be replaced, typically at an economic and environmental cost, to sustain agro ecosystem productivity (Souza et al., 2021). Watson showed that N recovery efficiency and yield of urea treated with NBPT increased by 20 and 8.8% respectively, when compared to yield performance of just urea. Commercially available urease inhibitors (e.g. Agrotain) usually have the active ingredient N-(n-butyl) thiophosphorictriamide (NBPT) in them which is a structural analogue of urea (Souza et al., 2021). NBPT works to reduce gaseous ammonia loss by inhibiting the urease enzyme from degrading urea (Abdo et al., 2020). Many studies have shown that NBPT is effective in reducing ammonia loss from urea as well as its potential to significantly aid in achieving EUs target for GHG emission reductions. Apart from it effects in GHG emission reduction, several other benefits of NBPT coated fertilizershas been reported. For instance, the authors in their paper reported a reduction of the adverse effects of urea fertilizer on seed germination, seedling growth, and early plant growth in soil by amending the fertilizer with as little as 0.01% (w/ w) of N-(n-butyl) thiophosphorictriamide. NBPTs ability to reduce ammonia losses and other GHG emissions as well as several other agricultural benefits, make it an ideal choice for use with urea during farming (Fu et al., 2020).

Despite many disparate NH3 field studies existing for both synthetic and plant-based urease inhibitors, there is presently no review that brings these results together, a significant and important knowledge gap. This review addresses the gap by summarizing the published laboratory and field trial literature on NH3 volatilization mitigation offered by synthetic and plant-based urease inhibitors. This review provides a comprehensive evaluation of the mechanisms of ammonia volatilization from urea fertilizes, the role of urease inhibitors in mitigating nitrogen losses, and the results presented in this review will broaden the understanding of urease inhibitor efficacy in field and laboratory conditions and demonstrate that not all products behave the same in terms of NH3 reduction efficacy and recent advances in the development of green urease inhibitors derived from plants and natural products. Additionally, a comparison between plant-based and synthetic urease inhibitors is presented to highlight their relative advantages, limitations, and future prospects in sustainable agriculture.The aim of this study was to analyze the efficacy of urease inhibitors on NH3emission abatement based on currently available scientific literature. Additionally, the existing gaps in research data were identified.

1. Urease inhibitors

   Urease inhibitors are compounds that temporarily reduce the activity of the urease enzyme, slowing the hydrolysis of urea into ammonia, which reduces nitrogen loss from soils (volatilization) and aids in controlling ammonia-related clinical diseases (Sánchez et al., 2020; Krajewska, 2009). Urea is the most widely used form of nitrogen fertilizer and can

   be formulated as dry granules, prills, or as a fluid alone or mixed with ammonium nitrate (UAN) (Cantarella et al., 2008; Zaman et al., 2008). Urea is also present in animal manures. All these forms of urea have the disadvantage of undergoing considerable losses as ammonia gas if not incorporated into soil soon after application (Zaman et al., 2008; Sanz-Cobena et al., 2012). Once dissolved in water, urea is converted to ammonium bicarbonate within a few days following application by the naturally occurring enzyme urease (Krajewska, 2009). Urease is produced by many soil microorganisms and plants and is present in nearly all soils (e.g., Krajewska, 2009). When urea is hydrolyzed by urease, much of the resulting ammonium is held on soil cation exchange sites. During the conversion, the pH temporarily rises, and ammonia gas is produced. The loss of ammonia, termed volatilization, can range from negligible to over 50% (Sanz-Cobena et al., 2012; Abalos et al., 2014).

   Among the known soil urease inhibitors, N-(butyl) thiophosphorictriamide (NBPT) is currently the most efficient compound (Abalos et al., 2014; Cantarella et al., 2008). In the presence of soil microbiota, NBPT is converted to the respective oxo-analogue called N-(butyl) phosphoric triamide (oxo-NBPT), which exhibits a high capacity for inhibiting urease (Cantarella et al., 2008). Many other substances have been investigated with respect to their potential to inhibit urease activity in soil, but very few were found to be promising for further studies. The great challenge remains to find good candidates that are eco-friendly, nontoxic or of low toxicity to plants, chemically stable, efficient at low concentrations, compatible with urea, and cost-competitive (Kumar et al., 2020; Upadhyay, 2012).

   Several different inhibitors are commercially available globally. The most widely researched urease inhibitors are phosphorodiamide and phosphorotriamide derivatives (Singh et al., 2023). These include N(n-butyl) thiophosphorictriamide (NBPT), N(2-nitrophenyl) phosphoric triamide (2-NPT), and N-(n-propyl) thiophosphorictriamide (NPPT) (Modolo et al., 2018; Song et al., 2022). These phosphoramide inhibitors, NBPT, 2-NPT, and a 3:1 ratio of NBPT

   + NPPT, have been shown to be effective and practically applicable in agricultural field systems. For example, in a field study conducted in New Zealand, Dawar et al. (2011) demonstrated that urea coated with NBPT at 0.1% (w/w) of urea N reduced NH volatilization loss by 67% on average compared to standard urea in a spring application to a perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) sward. Similarly, Matse et al. (2024) reported that application of urea coated with NBPT (0.066% w/w) at two grassland experimental sites in Ireland decreased NH volatilization by 79% on average across the two sites. While research reports show that these urease inhibitors are promising, their widespread application in farm systems has encountered challenges. There is limited information in the literature regarding which urease inhibitor is most effective under various soil types and climatic conditions (Singh et al., 2023; Song et al., 2022).

2. Types of Urease inhibitors

   Approximately 14,000 compounds or combined mixtures of these compounds have been tested to inhibit urea hydrolysis, and many of these are structural analogues of urea (Kiss and Simihaian, 2018; Cantarella et al., 2018). The most common inhibitor among such compounds that has successfully reached the market is N-(n-butyl) thiophosphorictriamide (NBPT), categorized as a phosphoramidate. This compound is traded as Agrotain in the USA and, in different countries, NBPT products are sold under various brand names. In the presence of soil microorganisms, NBPT is converted to its active analogue N-(butyl) phosphoric triamide (oxo-NBPT), which exhibits effective urease inhibition (Cantarella et al., 2018; Kiss andSimihaian, 2002; Kawakami et al., 2012). Different urease inhibitors with varied efficiency, cost, toxicity and stability are available in the market. These inhibitors ranged from elemental ions such as Hg2+ and Ag+, inorganic ions (boric acid) to organic compounds(acetohydroxamic acid and hydroquinone)(Svane et al., 2020). Another importantclass of inhibitors is hydroxyamic acid which is characterized by terminal O-C-NHOHfunctional group and best studied example of this class is acetohydroxamic acid (AHA). This class of inhibitors are highly stable, weakly acidic in nature and highly soluble in water and these properties make it a highly potent urease inhibitor(Arora et al., 2018).

   Hydroquinone and quinones are known to inhibit urease by interacting with key residues at or near the enzymes active site (Mazzei et al., 2022). Benzoylthiourea derivatives have been investigated as urease inhibitors, showing mixed-type inhibition that suggests interactions with active or allosteric sites of urease (Rego et al., 2018; Costa et al., 2015). Coumarinyl-pyrazolinyl thioamide compounds have been synthesized and demonstrated significant non-competitive urease inhibition in vitro (e.g., Inhibition of urease by coumarinylpyrazolinyl thioamide derivatives, 2018). Phenolic aldehyde derivatives such as protocatechuic aldehyde, syringaldehyde, and vanillin have been studied for their ability to inhibit urease and serve as molecular scaffolds for more active inhibitors (Horta et al., 2016). Several organic compounds continue to be investigated for urease inhibition due to their structural features and potential to reduce enzyme activity (Modolo et al., 2018).

3. Leveraging Plant-Based Urease Inhibitors: Implications of Previous and Recent Research

Green urease inhibitors have emerged as a cornerstone of sustainable agriculture, specifically engineered to suppress urease enzyme activity and thereby mitigate urea hydrolysis and ammonia (NH) volatilization from urea-based fertilizers, which globally result in 2070% nitrogen (N) losses depending on soil conditions (Cantarella et al., 2008; Zaman et al., 2008; Sanz-Cobena et al., 2012). Unlike persistent synthetic inhibitors such as NBPT (N-(n-butyl) thiophosphorictriamide), green variants prioritize natural, plant-derived, or biodegradable compounds like polyphenols, flavonoids, and saponins, which chelate nickel ions in urease’s active site or disrupt its catalytic triad while degrading rapidly in soil to avoid long-term ecological disruption (Modolo et al., 2015; Krajewska, 2009). Green urease inhibitors therefore offer a promising, eco-friendly approach to tackling urea hydrolysis and ammonia volatilization in urea-based fertilizers. These compounds target the urease enzyme, slowing urea breakdown and nitrogen loss while minimizing environmental harm compared to syntheti alternatives (Abalos et al., 2014; Chen et al., 2008). Plant-based urease inhibitors, such as extracts from Vachellia nilotica, Eucalyptus camaldulensis, garlic (allicin), and cumin, serve as natural and sustainable alternatives to synthetic inhibitors like NBPT, reducing nitrogen loss from urea fertilizers. These natural compounds effectively inhibit soil urease activity, significantly increasing nitrogen use efficiency and reducing ammonia volatilization (Ramli et al., 2014; Ferreiraa et al., 2016; Rana et al., 2021).

There are currently several alternatives to minimize nitrogen losses from urea fertilizers and improve nitrogen uptake by crops. Slow-release nitrogen fertilizers consist of fertilizers coated with hydrophobic materials that create a physical barrier against water, thereby promoting the gradual release of urea into the soil solution (Chen et al., 2008). Another strategy is the use of nitrification inhibitors, which delay NH oxidation by nitrifying bacteria, thereby preventing NO formation and reducing nitrogen leaching from soils (Abalos et al., 2014). Urease inhibitors remain one of the most widely used approaches for overcoming nitrogen losses in agricultural systems because they delay urea hydrolysis, increasing the chances of urea incorporation into soil through rainfall, irrigation, or mechanical incorporation (Zaman et al., 2008; Sanz-Cobena et al., 2012).

Green variants emphasize natural or biodegradable sources, such as plant extracts (e.g., from tannin-rich plants like Acacia or Eucalyptus) and microbial-derived compounds, thereby avoiding persistent synthetic chemicals (Modolo et al., 2015; Ferreiraa et al., 2016). Research has also highlighted modified phenylphosphorodiamidate (PPD) derivatives and bio-based NBPT analogs that degrade faster in soil, thereby reducing residue risks while maintaining high ammonia volatilization reduction efficiencies (Krajewska, 2009; Abalos et al., 2014). These compounds can outperform traditional NBPT formulations in stability tests, particularly when combined with improved formulations such as stabilized coatings that enhance persistence and efficiency under field conditions (Chen et al., 2008).

2.3. Mechanisms Underlying the Effectiveness of Plant-Based Inhibitors

Plant-based urease inhibitors typically contain bioactive phytochemicals that interfere with urease enzyme activity through several mechanisms. Many plant phytochemicals interact directly with the urease enzyme by binding to its active site. Flavonoids and tannins are particularly known for their ability to chelate the nickel ions present in the urease catalytic center, thereby preventing the enzyme from hydrolyzing urea (Modolo et al., 2015; Ogawa and Yazaki 2018; Rana et al., 2021). Polyphenolic compounds found in plants can form complexes with proteins, including urease enzymes. This interaction alters the structural conformation of the enzyme and reduces its catalytic activity (Modolo et al., 2015; Ferreiraa et al., 2016). Some plant compounds inhibit the activity of urease-producing microorganisms in soil, thereby reducing the overall rate of urea hydrolysis (Ramli et al., 2014; Jadon et al., 2018). The effectiveness of these mechanisms explains why many plant extracts have demonstrated strong urease inhibition in laboratory studies (Kumar et al., 2020; Liu et al., 2019) as could be seen in Figure 4.

Figure 4: Green urease inhibitor Source: Kumar et al., 2020

1. Urea Hydrolysis Mechanism

   Urea (CO(NH)), the most widely used nitrogen fertilizer globally, undergoes rapid enzymatic hydrolysis by soil urease. Urea fertilizers rapidly hydrolyze via soil urease into ammonia, which volatilizes as NH gas, especially in alkaline or high-pH soils. This process can cause 2050% nitrogen lossin surface-applied urea, reducing crop uptake and contributing to air pollution and eutrophication (Krajewska, 2009; Bremner and Douglas, 1971). The reaction produces ammonia and carbamic acid, which decomposes to CO and more NH (Krajewska, 2009; Bremner and Douglas, 1971):

   CO(NH2)2 + H2O 2(NH3) +CO2 Equation 3

   In alkaline soils (pH >7) or surface-applied scenarios, up to 50% of applied N volatilizes as NH within days, leading to N loss, soil acidification, and atmospheric pollution that contributes to eutrophication

2. Previous Studies on Urease Inhibitors

   Table 1: Major Studies on Synthetic Urease Inhibitors for Reducing Ammonia Volatilization and Their Knowledge Gaps

   Author(s)/year	Inhibitor Name	Type	Soil/Crop System	Key Findings	Knowledge Gap
   Bremner and Douglas 1971	Phenyl phosphorodiamidate (PPD)	Synthetic	Laboratory soil	Demonstrated inhibition of soil urease activity	Field-scale validation not conducted
   Watson et al. 1994	NBPT	Synthetic	Grassland soils	Reduced ammonia volatilization from urea fertilizer	Effects on soil microbial communities not studied
   Byrnes, 2000	NBPT	Synthetic	Agricultural soils	Improved nitrogen use efficiency	Limited long-term environmental assessment
   Grant et al. 2010	Hydroquinone	Synthetic	Field crop soils	Reduced urease activity and ammonia loss	Toxicity risks not evaluated
   Cantarella et al. 2008	NBPT	Synthetic	Tropical soils	Up to 60% reduction in NH volatilization	Economic feasibility not studied
   Zaman et al. 2008	NBPT	Synthetic	Pasture soils	Increased nitrogen retention	Impact on soil nitrogen cycling not assessed
   Abalos et al. 2014	NBPT	Synthetic	Mediterranean soils	Reduced ammonia volatilization and NO emissions	Did not compare with plant inhibitors
   Sanz-Cobena et al. 2012	NBPT	Synthetic	Wheat cropping system	Reduced ammonia emissions and improved NUE	Long-term soil health effects not evaluated
   Chen et al. 2008	NBPT + DMPP	Synthetic	Paddy soil	Reduced nitrogen losses and improved yield	Limited climatic condition comparisons

   Castellano et al., 2019	NBPT	Synthetic	Field soil experiment	Reduced NH emission by about 62%	Lack of sustainable alternatives explored
   Guardia et al. 2017	NBPT	Synthetic	Maize cropping system	Reduced ammonia volatilization	No comparison with organic inhibitors
   Pan et al. 2016	NBPT	Synthetic	Agricultural soil	Increased nitrogen use efficiency	Did not examine environmental toxicity

   One of the earliest studies investigating urease inhibition in soils was conducted by Bremner and Douglas (1971). Their research focused on the inhibitory effects of phenyl phosphorodiamidate (PPD) on soil urease activity. Using laboratory incubation experiments, they demonstrated that PPD significantly slowed the hydrolysis of urea by blocking th active site of the urease enzyme. Their findings provided the first evidence that chemical inhibitors could effectively regulate the rate of urea hydrolysis in soil systems. However, the study was limited to controlled laboratory conditions and did not evaluate the performance of the inhibitor under field conditions where environmental factors such as temperature, rainfall, and soil microbial diversity influence urease activity.

   Further research was conducted by Watson et al. (1994), who investigated the effectiveness ofN-(n-butyl) thiophosphorictriamide (NBPT) as a urease inhibitor in grassland soils. Their results indicated that NBPT significantly reduced ammonia volatilization following urea application. The study reported improved nitrogen use efficiency and increased nitrogen retention in soil systems treated with NBPT. The authors concluded that urease inhibitors could play a critical role in reducing nitrogen losses from agricultural systems. Despite these positive findings, the study did not examine the long-term ecological impact of NBPT on soil microorganisms, which are essential for nutrient cycling.Similarly, Byrnes (2000) examined the agronomic efficiency of urease inhibitors in agricultural soils. The study confirmed that NBPT effectively delayed urea hydrolysis, resulting in lower ammonia emissions and improved nitrogen availability for plant uptake. Byrnes emphasized that the use of urease inhibitors could increase fertilizer efficiency while reducing environmental pollution associated with nitrogen loss. However, the study primarily focused on short-term nitrogen dynamics and did not assess the potential accumulation or degradation of inhibitor residues in soil ecosystems.

   Research conducted by Grant et al. (2010) explored the use ofhydroquinone as an alternative urease inhibitor. Hydroquinone was found to reduce urease activity and decrease ammonia volatilization from urea fertilizers. The authors reported that hydroquinone could effectively slow down the conversion of urea to ammonium, thereby improving nitrogen retention in the soil. However, concerns regarding the potential toxicity of hydroquinone to soil organisms were not fully addressed in the study.

   1. Field-Based Evaluations of Synthetic Urease Inhibitors

      A significant advancement in the study of urease inhibitors was reported by Cantarella et al. (2008), who conducted field experiments to evaluate the effectiveness of NBPT in tropical soils. Their findings showed that the application of NBPT reduced ammonia volatilization by approximately 5060% compared with untreated urea fertilizers. The study also demonstrated improved nitrogen use efficiency and enhanced crop productivity in soils treated with NBPT. These results highlighted the practical benefits of urease inhibitors in agricultural systems. Nevertheless, the economic feasibility of widespread NBPT use among smallholder farmers was not examined.Similarly, Zaman et al. (2008) investigated the performance of NBPT in pasture soils. Their study demonstrated that the inhibitor significantly reduced nitrogen losses and increased nitrogen availability for plant uptake. The authors observed that the effectiveness of NBPT varied depending on environmental conditions such as soil moisture and temperature. This finding suggests that the efficiency of synthetic inhibitors may depend on site-specific conditions. However, the study did not explore the effects of NBPT on soil microbial diversity or long-term soil health. Further work by Sanz-Cobena et al. (2012) evaluated the impact of urease inhibitors on ammonia emissions in wheat cropping systems. Their research showed that NBPT significantly reduced ammonia emissions from urea fertilizers while improving nitrogen use efficiency. Additionally, the study reported a reduction in greenhouse gas

      emissions associated with nitrogen fertilizer use. Although the results demonstrated the environmental benefits of urease inhibitors, the long-term sustainability of continuous NBPT application was not investigated.Abalos et al. (2014) conducted experiments in Mediterranean agricultural soils to assess the effectiveness of NBPT in reducing nitrogen losses. Their findings indicated that urease inhibitors not only reduced ammonia volatilization but also decreased nitrous oxide emissions. These results highlight the potential role of urease inhibitors in mitigating greenhouse gas emissions from agriculture. However, the study focused primarily on synthetic inhibitors and did not compare their performance with plant-based alternatives.

      Research by Pan et al. (2016) further confirmed the effectiveness of NBPT in improving nitrogen use efficiency. Their study demonstrated that the application of NBPT significantly reduced ammonia emissions and enhanced nitrogen uptake by crops. However, the potential environmental risks associated with long-term application of synthetic inhibitors were not addressed.

3. Studies on Plant-Based Urease Inhibitors

   Author(s) /Year

   Plant Source

   Inhibit or Compo und

   Metho d Used

   Inhibit ors

   Param eters Measu red

   Key Result s

   Duratio n of NH Loss Suppre ssion

   Environ mental Effect

   Type of Inhibiti on & Kinetic Mecha nism

   Properti es of Inhibito rs & Impact to Soil

   Key Findin gs

   Knowl edge Gap

   Phytoch emical Properti es & Active Sites

   Modolo et al., 2015

   Variou s plant extract s

   Polyph enols

   Enzym e assay

   Polyph enols

   Urease activity

   Signifi cant inhibit ion

   Short-term (hours)

   Biodegrad able; minimal toxicity

   Non-competi tive; binds urease allosteri c site

   Plant-derived; water-soluble; minimal soil impact

   Polyph enols inhibite d urease activity

   No field trials

   Polyphen olic compoun ds; hydroxyl groups interact with allosteric sites on urease enzyme

   Ramli et al., 2014

   Garlic

   Allicin

   Soil incuba tion

   Allicin

   Urease activity, soil N loss

   Signifi cant urease inhibit ion

   ~57 days

   Biodegrad able; low toxicity

   Non-competi tive; interact s with urease active site

   Plant-derived; organos ulfur compou nd; minimal soil effect

   Effectiv e natural urease inhibito r

   Persiste nce in soil not evaluat ed

   Organosu lfur compoun d; binds to urease active site cysteine residues

   Kumar et al., 2020

   Chamo mile

   Flavon oids

   Labora tory

   Flavon oids

   Urease activity

   45% urease

   ~57 days

   Biodegrad able;

   Non- competi

   Plant- derived;

   Effectiv e plant-

   Crop yield

   Flavonoid structure

   assay

   inhibit ion

   minimal environm ental toxicity

   tive; flavono ids bind urease away from active site

   flavonoi d-rich; water-soluble; minimal soil impact

   based urease inhibito r

   respons e not studied

   with hydroxyl groups; bind to allosteric sites modifyin g enzyme conformat ion

   Rana et al., 2021

   Vachell ia nilotica

   Tannins

   Soil enzym e analysi s

   Tannins

   Urease activity

   70% urease inhibit ion

   ~57 days

   Biodegrad able; low toxicity

   Mixed-type inhibiti on; tannins bind urease and form enzyme -phenol comple xes

   Plant-derived; polyphe nolic; water-soluble; minimal soil effect

   Strong urease inhibiti on

   Field validati on lacking

   Polyphen olic tannins; multiple hydroxyl groups interact with urease active site and peripheral regions

   Growing concerns about the environmental impact of synthetic inhibitors have led to increased research on plant-derived urease inhibitors. Modolo et al. (2015) investigated the urease inhibitory properties of various plant extracts containing polyphenolic compounds. Their laboratory experiments demonstrated that plant polyphenols effectively inhibited urease activity by interacting with the enzymes active site. These findings suggest that natural plant compounds could serve as environmentally friendly alternatives to synthetic inhibitors. However, the study was limited to enzyme assays and did not evaluate the performance of plant extracts under soil conditions. Similarly, Ramli et al. (2014) studied the inhibitory effects of garlic extract on urease activity. The researchers identified allicin, a sulfur-containing compound present in garlic, as the primary inhibitor of urease activity. Their results showed that garlic extract significantly slowed the hydrolysis of urea in soil incubation experiments. This finding indicates that plant-derived sulfur compounds may play an important role in urease inhibition. However, the persistence of these compounds in soil and their long-term effectiveness were not evaluated. Research conducted by Kumar et al. (2020) investigated the urease inhibitory activity of chamomile extract. The study demonstrated that chamomile extracts containing flavonoids reduced urease activity by approximately45%. The authors suggested that flavonoids interact with the nickel ions present in the active site of the urease enzyme, thereby inhibiting its catalytic activity. While the results are promising, the study did not assess the impact of chamomile extract on crop growth and nitrogen use efficiency in field conditions.

4. Research on Tannin-Based Urease Inhibitors

   Tannins are naturally occurring polyphenolic compounds that have been reported to inhibit urease activity. Rana et al. (2021) investigated the urease inhibitory potential of tannin extracts obtained from Vachellia nilotica. Their study reported that the tannin extract inhibited urease activity by approximately 70% in soil enzyme assays. The authors attributed this effect to the ability of tannins to bind with urease proteins and alter their structure. These findings highlight the potential of tannin-rich plant materials as natural urease inhibitors. However, the study did not evaluate the long-term stability of tannins in soil or their influence on crop yield.

   Similarly, Ogawa and Yazaki (2018) examined the urease inhibitory properties of Acacia bark extract, which is also rich in tannins. Their findings indicated that the extract significantly reduced urease activity in laboratory assays. Despite these promising results, the study did not investigate the mechanisms by which tannins interact with the urease enzyme.

5. Studies on Other Plant-Derived Inhibitors

   Several studies have explored the urease inhibitory properties of other plant extracts. Ferreiraa et al. (2016) investigated the effects of eucalyptus leaf extract on soil urease activity. The study found that phenolic compounds present in eucalyptus leaves effectively inhibited urease activity in laboratory experiments. However, the potential effects of eucalyptus extracts on soil nutrient cycling were not examined. Similarly, Khan et al. (2018) studied the urease inhibitory properties of mint extract. Their results indicated that menthol, the primary compound in mint, exhibited moderate urease inhibition in soil incubation experiments. While these findings suggest that mint extracts could serve as natural inhibitors, large-scale field trials have not yet been conducted. Research by Sultana (2024) investigated the inhibitory effects of onion extract on urease activity. The study identified organosulfur compounds in onions as the primary inhibitors of urease. These compounds were found to significantly reduce urea hydrolysis in laboratory experiments. However, the effect of onion extracts on crop productivity and soil microbial activity was

   not evaluated. Similarly, Shah et al. (2020) reported that ginger extract containing gingerol delayed urea hydrolysis in soil systems. Their findings suggest that ginger-derived compounds could potentially serve as natural urease inhibitors. However, further research is needed to determine the practical application of these compounds in agricultural systems. Another plant-derived compound investigated as a urease inhibitor is curcumin, which is found in turmeric. Riaz et al. (2021) reported that curcumin exhibited significant urease inhibition in enzyme assays. The authors suggested that curcumin interacts with the active site of the urease enzyme, thereby preventing urea hydrolysis. Despite these promising findings, field validation studies are still required.

   Plant-derived urease inhibitors have shown promising results in laboratory studies, particularly those containing tannins, flavonoids, and sulfur compounds. Nevertheless, several challenges remain, including variability in inhibitor effectiveness, limited field validation, and lack of standardized extraction methods.

6. Comparative Studies on Synthetic and Plant-Based Inhibitors

Although numerous studies have investigated either synthetic or plant-based urease inhibitors, relatively few studies have directly compared the effectiveness of these two groups of inhibitors (Modolo et al., 2015; Ogawa and Yazaki 2018; Abalos et al., 2014). Synthetic inhibitors such as NBPT generally exhibit higher and more consistent urease inhibition under field conditions (Cantarella et al., 2008; Zaman et al., 2008; Sanz-Cobena et al., 2012; Abalos et al., 2014). However, plant-derived inhibitors offer several advantages, including biodegradability, environmental safety, and lower cost (Modolo et al., 2015; Ogawa and Yazaki 2018; Ramli et al., 2014). Research indicates that plant-based inhibitors may reduce ammonia volatilization by 2070%, whereas synthetic inhibitors may achieve reductions of 5070%, depending on soil conditions (Abalos et al., 2014; Cantarella et al., 2008; Rana et al., 2021). Despite these differences, plant-based inhibitors represent promising alternatives for sustainable agriculture, particularly in regions where synthetic inhibitors are expensive or unavailable (Modolo et al., 2015; Ogawa and Yazaki 2018; Ramli et al., 2014). However, several knowledge gaps remain. Most studies on plant-based inhibitors have been conducted under laboratory conditions, and there is limited information regarding their performance in field environments (Rana et al., 2021; Jadon et al., 2018; Kumar et al., 2020). Additionally, the mechanisms of urease inhibition by many plant compounds remain poorly understood (Ogawa and Yazaki 2018; Modolo et al.,

2015).

Table 3: Comparative Characteristics of Plant-Based and Synthetic Urease Inhibitors

One of the earliest comprehensive field studies on synthetic urease inhibitors was conducted by Cantarella et al. (2008), who evaluated the effectiveness of NBPT (N-(n-butyl) thiophosphorictriamide) in tropical agricultural soils. The authors conducted field experiments measuring ammonia volatilization following urea application and considered parameters such as soil nitrogen retention and urease activity. NBPT was applied as a reagent mixed with urea fertilizer, and its effect was compared to untreated urea. Their findings demonstrated that NBPT reduced ammonia volatilization by approximately 5060%, a significant reduction attributed to the temporary inhibition of urease activity. This delay in urea hydrolysis allowed more time for ammonium ions to be adsorbed into the soil, reducing gaseous losses. The results of this study are consistent with previous research demonstrating the efficacy of NBPT in diverse soil types, confirming its role as a reliable synthetic inhibitor. However, the study did not investigate plant-derived or natural urease inhibitors, leaving unanswered questions about the relative performance of synthetic versus natural options under tropical field conditions.

Similarly, Zaman et al. (2008) examined the performance of NBPT in pasture soils under variable environmental conditions. Their research included field applications of urea with and without NBPT, with measurements focusing on ammonia volatilization, nitrogen retention, and the influence of environmental parameters such as soil moisture, temperature, and rainfall. They found that NBPT reduced ammonia losses by approximately 4055% compared with untreated urea. The study highlighted that while urease inhibitors significantly enhance nitrogen use efficiency, their effectiveness can fluctuate depending on soil and climatic conditions. These findings align closely with Cantarella et al. (2008), reinforcing that synthetic inhibitors are effective but may show variable performance under different environmental contexts.

Further evidence supporting the effectiveness of NBPT was provided by Sanz-Cobena et al. (2012), who evaluated its impact on ammonia emissions in wheat cropping systems. Field experiments measured ammonia volatilization, nitrogen availability, and fertilizer efficiency after NBPT application. The results showed that NBPT reduced ammonia volatilization by approximately 50% relative to untreated urea and enhanced nitrogen availability for crops, contributing to improved fertilizer efficiency. This confirms that synthetic urease inhibitors not only mitigate nitrogen losses but also improve crop nutrient uptake, consistent with prior studies in tropical and pasture soils. However, similar to earlier research, this study did not assess the long-term environmental effects of repeated NBPT use, particularly concerning soil microbial communities, which remains an important area for future investigation. Overall, these studies consistently demonstrate that NBPT can reduce ammonia volatilization by 4060% across different agricultural soils and cropping systems. The methods typically involved field-based measurements of ammonia emissions, with urea as the primary nitrogen source and NBPT as the inhibitor reagent. While the results corroborate

previous work confirming the high effectiveness of synthetic inhibitors, they also underscore the need for further research into plant-derived alternatives, long-term soil impacts, and the influence of environmental factors on inhibitor performance. The collective evidence positions NBPT as a reliable tool for improving nitrogen management, yet highlights that sustainability considerations and natural inhibitor comparisons are still underexplored.

Table 4: Studies Evaluating Natural Compounds as Urease Inhibitors

td>

~57 days

Another significant contribution to the literature is the study by Abalos et al. (2014), which investigated the effects of the synthetic urease inhibitor NBPT on nitrogen losses in Mediterranean agricultural soils. Using field experiments, the authors measured ammonia volatilization and nitrous oxide emissions following urea application. Their results showed that NBPT reduced ammonia volatilization by approximately 45% and also contributed to reductions in nitrous oxide emissions, confirming the effectiveness of synthetic inhibitors in mitigating nitrogen losses. These findings are consistent with previous research demonstrating the high efficiency of NBPT in controlling urea hydrolysis. However, the study did not explore alternative natural inhibitors derived from plant sources, leaving a gap in sustainable approaches to urease inhibition.

In contrast, several studies have examined plant-based inhibitors, which offer the advantage of being biodegradable and environmentally friendly. Jadon et al. (2018) evaluated neem-coated urea in field trials and found that it reduced ammonia volatilization by approximately 27.5% compared with conventional urea. Although this reduction was lower than that achieved with synthetic inhibitors such as NBPT, the results highlight the potential of neem as a sustainable urease inhibitor suitable for agricultural systems. Similarly, Mohanty et al. (2022) investigated neem extracts in soil systems and reported a reduction in ammonia volatilization of approximately 35%. The study emphasized that the effectiveness of plant-derived inhibitors can vary depending on extraction methods and environmental conditions, but overall supports the use of neem as a viable alternative to synthetic chemicals.Other plant-based approaches have also demonstrated strong inhibitory effects. Rana et al. (2021) studied tannin extracts from Vachellia nilotica in laboratory and soil incubation experiments, observing reductions in ammonia volatilization of 6070%, comparable to some synthetic inhibitors. The authors attributed the strong effect to the high tannin content of the extract, which interacts with urease enzymes and suppresses their catalytic activity. Likewise, Kumar et al. (2020) examined chamomile plant extracts under laboratory soil conditions and found a 45% reduction in ammonia volatilization, attributing the inhibitory effect to flavonoid compounds in the plant. While these studies demonstrate the potential of natural inhibitors, they are primarily limited to controlled experimental conditions, and long-term effects on crop productivity and soil microbial dynamics remain largely untested.

Finally, more recent studies continue to underscore the high performance of synthetic inhibitors. Castellano et al., (2019) evaluated modern urease inhibitor technologies in agricultural soils and reported ammonia volatilization reductions of up to 62% following the application of NBPT-treated urea fertilizers. These results reinforce the superior effectiveness of synthetic inhibitors in reducing nitrogen losses. However, the study did not explore plant-derived alternatives or the integration of synthetic and natural inhibitors, indicating an opportunity for future research to combine efficacy with sustainability.Overall, the literature indicates that while synthetic urease inhibitors such as NBPT are highly effective in reducing ammonia volatilization and other nitrogen losses, plant-based inhibitorssuch as neem, tannin, and chamomile extractsshow promising results with additional environmental and economic benefits. The main limitations of natural inhibitors are the variability of their effectiveness under field conditions and the lack of long-term studies on soil health and crop productivity. Integrating both synthetic and plant-based inhibitors, or developing optimized natural formulations, represents a potential path forward for sustainable nitrogen management in agriculture.

Table 5: Global Studies on Ammonia Volatilization Reduction Using Urease Inhibitors

3.0 Future prospects

To achieve a better understanding and adoption of the urease inhibitors in agriculture field settings, the following area of research need to be strengthened. Despite these promising advantages, several challenges must be addressed before plant-based urease inhibitors can be widely adopted in agricultural systems. One of the primary challenges is the variability in phytochemical composition among plant species. Factors such as plant genetics, environmental conditions, and extraction methods can influence the concentration of active compounds in plant extracts, leading to inconsistent inhibitor performance. In addition, many plant-derived compounds may degrade rapidly in soil environments due to microbial activity, temperature fluctuations, and exposure to sunlight. Another significant limitation identified in the literature is the lack of extensive field-based studies evaluating the effectiveness of plant-based inhibitors under real agricultural conditions. While many laboratory experiments have demonstrated strong urease inhibition, relatively few studies have assessed the long-term impacts of these inhibitors on crop yield, soil fertility, and microbial dynamics in field settings. Addressing this knowledge gap will be critical for advancing the practical application of plant-based urease inhibitors.

Furthermore, the commercialization of green urease inhibitors faces technological and economic challenges. Large-scale production of plant extracts requires efficient extraction methods, stable formulations, and reliable supply chains for plant materials. Advances in technologies such as microencapsulation, nano-formulation, and bio-based fertilizer coatings may help improve the stability and performance of plant-derived inhibitors. Overall, the findings presented in this review highlight the considerable potential of plant-based urease inhibitors as sustainable alternatives to synthetic chemical inhibitors. By reducing ammonia volatilization and improving nitrogen use efficiency, these natural compounds could play a significant role in promoting environmentally responsible agricultural practices.

Future research should focus on identifying new plant species with strong urease inhibitory properties, optimizing extraction and formulation techniques, and conducting long-term field trials to evaluate their effectiveness under diverse agricultural conditions. Additionally, interdisciplinary collaborations between agronomists, soil scientists, chemists, and agricultural industries will be essential for translating laboratory discoveries into commercially viable products.

The degradation of urease inhibitors during storage is an important factor that will affect their ultimate field efficacy. Therefore, more studies are needed on the degradation characteristics of these urease inhibitors during fertilizer product storage. Some studies have been done with NBPT (Lasisi et al., 2020; Sha et al., 2020) but limited or no previous studies have been published showing degradation rates with synthetic and plant based urease inhibitors. A layer of confidence is provided in Europe where regulatory minimum levels exist which define the inhibitor levels that must be present on the fertilizer at the point of sale. Assurance regarding inhibitor content is important for farmers and highlights the importance of inhibitor degradation studies and regulation in the sector. Field research studies are needed to understand the effect of soil, environmental and management factors on the efficacy of urease inhibitors. More research is needed to develop advanced models that can simulate the efficacy of urease inhibitors at global scale based on environmental factors, management practices and soil properties.

Future research should focus on identifying new plant sources of urease inhibitors, optimizing extraction techniques, and conducting large-scale field experiments to evaluate their effectiveness under real agricultural conditions. Such approaches could significantly reduce ammonia volatilization while maintaining soil health and agricultural productivity.

4.0 CONCLUSION

Nitrogen fertilizers are vital for maintaining global agricultural productivity and meeting the food demands of a growing population. Among them, urea is the most widely used because of its high nitrogen content, affordability, and ease of application. However, its efficiency is often reduced by nitrogen losses through ammonia volatilization after soil application. This occurs when the urease enzyme rapidly hydrolyzes

urea, producing ammonia gas that escapes into the atmosphere. As a result, nitrogen use efficiency declines while environmental problems such as air pollution, soil acidification, and eutrophication of water bodies increase. Research shows that ammonia volatilization is affected by several soil and environmental conditions, including soil pH, temperature, moisture, fertilizer placement, and microbial activity. To address this issue, urease inhibitors have been developed to slow down the activity of the urease enzyme, delaying the breakdown of urea and allowing nitrogen to move deeper into the soil. Synthetic inhibitors such as NBPT, PPD, and hydroquinone have proven effective, often reducing ammonia losses by 4070% and improving nitrogen uptake by crops. Despite these benefits, concerns remain about their high cost, possible environmental persistence, and potential effects on soil microbial communities, particularly in regions where access may be limited for smallholder farmers.

As a sustainable alternative, researchers have increasingly focused on plant-based or green urease inhibitors derived from natural sources. Extracts from plants such as neem, garlic, chamomile, eucalyptus, acacia, onion, ginger, and turmeric contain bioactive compounds; including polyphenols, flavonoids, tannins, alkaloids, and sulfur-containing compounds, that can inhibit urease activity. These compounds interfere with the enzyme through various biochemical mechanisms, such a binding to its active sites or altering its structure. Because plant-derived inhibitors are biodegradable, environmentally friendly, and often locally available, they offer a promising approach to improving fertilizer efficiency, reducing nitrogen losses, and supporting more sustainable agricultural practices.

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