NBPT, or N-(n-butyl) thiophosphoric triamide, is a urease inhibitor used to limit nitrogen losses after urea application. Given current environmental and agronomic challenges, optimizing nitrogen fertilizer use has become a priority. Nitrogen is essential for plant growth, but its volatilization as ammonia represents a loss of efficiency and a source of air pollution. Analyzing nitrogen application rates allows for the precise quantification of these losses and the evaluation of the effectiveness of fertilization practices.
In this context, urease inhibitors, and in particular NBPT, play a key role. This chemical compound improves the efficiency of nitrogen application while limiting its environmental impact. To assess the impact of NBPT on air quality and ammonia volatilization, an analysis of agricultural atmospheric emissions can be carried out under field conditions.
This article will help you understand what NBPT is, from a chemical, regulatory and historical perspective.
Table of Contents
What is the NBPT?
Chemical definition and molecular structure
NBPT, or N-(n-butyl) thiophosphoric triamide, is an organophosphorus molecule used as a urease inhibitor. Its chemical formula is C₄H₁₄N₃PS , with a molar mass of 167.21 g/mol . Structurally, it is a thiophosphoric acid amide with a phosphorus atom tetrahedrally bonded to:
- an atom of sulfur,
- two amido groups (-NH₂),
- and an n-butyl group (-NHC₄H₉).
This white solid has a melting point of 54°C . It is marketed under various names, including Agrotain, and is known for its ability to limit the rapid conversion of urea to ammonia in soils. This property makes it a key additive in urea-based nitrogen fertilizers.
History and regulations
Since the 1970s, numerous compounds have been evaluated as potential urease inhibitors. However, few have met all the requirements: low-dose efficacy, low toxicity, sufficient stability, compatibility with urea, and acceptable cost. NBPT was identified in the 1980s as a particularly promising candidate, initially in the United States, where it was rapidly adopted in fertilizers for intensive agriculture.
In 2008 , NBPT was officially authorized in the European Union as an active substance for nitrogen fertilizers. It is now used in formulations called "urea with urease inhibitor," in which NBPT is incorporated at a level of 0.04% to 0.10% , in accordance with European regulations. These levels ensure an optimal rate of enzyme inhibition (up to 90%).
In addition to its use in large-scale farming, NBPT is now an integral part of sustainable fertilization strategies, supported by European agricultural policies aimed at reducing nitrogen emissions into the environment.
How does a urease inhibitor work?
Urease inhibitors like NBPT play a specific biochemical role in the soil. Their function is to slow down the enzymatic conversion of urea to ammonia, a key step in the nitrogen cycle. This action maximizes the agronomic efficiency of nitrogen fertilizers while limiting losses through volatilization. To understand how NBPT works, it is essential to review the natural function of urease and the mechanism of inhibition it employs.
The role of urease in soil
Urease is an enzyme naturally present in the soil, synthesized by many living organisms: bacteria, fungi, algae, plants, and invertebrates. It catalyzes the hydrolysis of urea (CO(NH₂)₂) into ammonia (NH₃) and carbon dioxide (CO₂), according to the following equation:
CO(NH₂)₂ + H₂O → 2 NH₃ + CO₂
Under typical humidity and temperature conditions, this transformation occurs very rapidly after the application of urea fertilizer, often in less than 48 hours. When ammonia is released faster than it can be absorbed by plants or trapped in the soil, it volatilizes as a gas. This phenomenon generates not only an economic loss (up to 40% of the nitrogen can be lost), but also atmospheric pollution from nitrogen compounds.
Urease activity is strongly influenced by:
- the soil pH
- the temperature (optimum around 30 to 40 °C),
- and the presence of water , which promotes contact between enzyme and substrate.
This type of enzymatic activity can be studied by analyzing sedimentary and mineral soils , in order to assess the conditions favorable to the hydrolysis of urea.
NBPT Mechanism of Action
NBPT is a competitive inhibitor of urease. It acts by reversibly binding to the enzyme's active site, thus preventing urea from accessing it. This temporary inhibition significantly slows the rate of hydrolysis, thereby altering the kinetics of nitrogen transformation in the soil.
In concrete terms, the NBPT:
- delays the transformation of urea into ammonia , allowing for better infiltration of nitrogen into the soil before it is transformed.
- reduces localized pH spikes around fertilizer granules, which normally promote volatilization,
- decreases the concentration of free ammonia in the first few hours after spreading, a critical period for losses.
Field trials have shown that the use of NBPT can:
- ammonia losses in acidic or neutral soils by 60 to 70%
- reduce emissions 40 to 50%
The effect of NBPT is temporary: its inhibitory action lasts a few days, the time it takes for the plant to absorb nitrogen or for it to be fixed by the soil. It is then biologically degraded in the environment, with a half-life of approximately 1 to 2 weeks depending on soil conditions.
Thus, by modulating the release rate of ammonia nitrogen, NBPT improves both the agronomic efficiency of the fertilizer and its environmental compatibility. This technology, easy to integrate into agricultural practices, represents a significant optimization lever for sustainable nitrogen management.
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What are the agronomic advantages of NBPT?
NBPT is much more than a simple chemical additive. It represents a strategic agronomic tool for improving the performance of nitrogen fertilizers, ensuring more efficient fertilization, and reducing the environmental impacts associated with nitrogen losses. This third section details the concrete benefits of NBPT, documented by both scientific studies and field experience.
Reduction of nitrogen losses
One of the main advantages of NBPT lies in its ability to drastically reduce nitrogen losses through ammonia volatilization . In the presence of urease, the rapid hydrolysis of urea causes the release of ammonia gas (NH₃), especially when the fertilizer is applied on the surface without being incorporated into the soil. This gas dissipates into the atmosphere, resulting in a net loss of fertilizer.
Thanks to its inhibitory action, NBPT:
- reduces ammonia losses for solid urea fertilizers by 60 to 70%;
- reduces emissions from nitrogen solutions (e.g., UAN) 40 to 50%
- stabilizes the pH around the fertilizer granules, limiting conditions favorable to volatilization.
This data comes from several trials carried out under real conditions on soils with a pH between 5.5 and 7.5. The effectiveness is maximum when the moisture is sufficient to activate the enzyme, but the fertilizer remains on the surface, without mechanical incorporation.
Environmental gain
Limiting ammonia volatilization has major positive consequences for the environment :
- Reduction of atmospheric nitrogen emissions , responsible for the formation of fine particles (PM10, PM2.5) and air pollution.
- Lesser contribution to the eutrophication of aquatic environments through atmospheric deposition.
- Less indirect greenhouse gas emissions , particularly nitrous oxide (N₂O), from excess mineral nitrogen in the soil.
NBPT is thus part of a low-carbon fertilization , encouraged by the CAP (Common Agricultural Policy) and the sustainability objectives defined by the European Green Deal.
Crop yield improvement
By increasing nitrogen use efficiency (NUE), NBPT promotes better uptake by plants, particularly during critical growth phases (stem elongation, tillering, flowering). This often results in:
- an increase in gross yield (from +5 to +10% depending on the crop),
- improved homogeneity of the plots,
- a reduction in the need for additional units at the end of the cycle.
The benefits are particularly noticeable in nitrogen-sensitive crops such as maize, soft wheat, hay meadows, and rapeseed. NBPT also allows for better synchronization between nitrogen inputs and the plant's actual needs , thus limiting the effects of dilution or transient deficiency.
Many farmers are also observing an improved overall health of crops, with more vigorous plants, greener leaves, and more regular growth, particularly in situations of moderate water stress.
Limitations and precautions for using NBPT
Despite its numerous agronomic and environmental benefits, NBPT is not without its limitations. Like any active ingredient used in agriculture, it requires a thorough understanding of its limitations, chemical interactions, and potential effects on both the plant and the environment. This section details the precautions to take to ensure the safe and effective use of NBPT.
Chemical stability and compatibility
NBPT has limited chemical stability , which can affect its effectiveness over time if storage or incorporation conditions are not respected:
- Its half-life is less than 6 months at room temperature, depending on humidity and exposure to light or oxygen.
- It is particularly sensitive to the presence of sulfates : tests have shown accelerated degradation of NBPT in the presence of fertilizers such as ammonium sulfate or compound fertilizers containing sulfur.
- It is therefore incompatible with fertilizers containing sulfates : their mixture can greatly reduce the inhibitory activity of NBPT.
To preserve the product's effectiveness:
- It is recommended to use NBPT-urea formulations quickly after manufacture (within 3 to 4 months);
- NBPT-enriched fertilizers should be stored away from heat and moisture , in airtight packaging;
- Mixtures with other fertilizers must be validated beforehand to avoid any harmful interactions.
Potential side effects on crops
Although NBPT is generally well tolerated by plants, some side effects may occur, particularly in cases of overdose , adverse climatic conditions, or specific crop sensitivity :
- chlorosis or marginal necrosis is observed on the leaves, linked to an accumulation of unhydrolyzed urea in plant tissues.
- These symptoms are usually transient and do not have a significant impact on yield if nutritional balance is maintained.
- In some species, a temporary reduction in amino acid content has been observed, suggesting a disruption of nitrogen metabolism by inhibition of endogenous urease.
These effects are generally avoided by:
- respecting the regulatory doses of NBPT (0.04% to 0.10% of the total mass),
- avoiding application in very hot or dry weather
- adapting the type of fertilizer to the specific needs of the crop and its stage of development.
Toxicity and safety of use
NBPT is classified as a hazardous substance in its pure form , according to the criteria of the GHS (Globally Harmonized System of Chemical Classification):
- SGH05: corrosive
- GHS08: sensitizing, mutagenic, reprotoxic
Associated hazard statements include:
- H318: Causes serious eye damage.
- H361: May damage fertility.
when formulated in fertilizers , NBPT is present at very low concentrations (<0.1%), which means that the finished products are not subject to any specific labeling requirements . No toxic risk has been demonstrated for end users when used as directed.
Studies have also shown that:
- NBPT is rapidly mineralized in the soil into CO₂ and other inert by-products,
- Its biodegradability is high , with a half-life of 1 to 2 weeks .
- No significant toxic effects were observed on soil fauna, including earthworms and beneficial microorganisms.
When handling pure NBPT (formulation, laboratory work, etc.), wearing gloves , goggles, and protective equipment is essential. Prevention relies primarily on adhering to usage and storage conditions. To ensure the absence of critical residues, chemical contaminant analysis can be performed on agricultural samples.
Which sectors use NBPT?
NBPT, as a urease inhibitor, is currently used in many industrial sectors where nitrogen management is a major concern. Although its use is primarily associated with agriculture, its scope is gradually expanding into more specialized fields, such as animal nutrition, agronomic research, and environmental management. This section presents the main sectors using NBPT, highlighting their specific objectives and expected benefits.
Conventional and sustainable agriculture
The agricultural sector is by far the main user of NBPT. The objective is clear: to improve the efficiency of nitrogen fertilizers , while reducing losses through volatilization .
The most common types of NBPT-enriched fertilizers are:
- granulated urea,
- UAN (urea-ammonium nitrate solution),
- controlled-release or coated fertilizers .
The cultures involved are varied:
- large-scale crops (wheat, corn, barley, rapeseed),
- industrial crops (sugar beet, potato),
- temporary or permanent grasslands.
The NBPT allows, in particular:
- a reduction in the required nitrogen units (up to -20% depending on conditions),
- a more flexible deferral of contributions , without compromising efficiency,
- an improvement in the nitrogen nutrition index (NNI) measured by the monitoring tools.
reasoned fertilization strategies , with modulation of doses according to the actual needs of the crops, weather conditions, and soil characteristics.
Within the framework of integrated cropping systems, mycotoxin analysis can also be considered to monitor the indirect impact of nitrogen practices on crop health.
Animal health and ruminant nutrition
More recently, NBPT has been studied and partially integrated into animal nutrition , particularly in ruminants (cattle, sheep).
The goal is to optimize the use of dietary urea :
- By limiting the excessively rapid hydrolysis of urea in the rumen (a medium very rich in urease), NBPT allows a more gradual release of ammonia , better synchronized with the energy needs of digestive microorganisms.
- This results in better conversion of nitrogen into microbial proteins , better nitrogen balance , and a reduction in urinary excretion .
Studies have shown that NBPT, administered in low doses in feed, has no negative impact on animal health, and leaves no detectable residues in milk or tissues after a period of 28 days (study by van de Ligt et al., 2019).
Emissions management in agricultural environments
The NBPT is also used as a tool for reducing agricultural gaseous emissions , particularly within the framework of voluntary or regulated environmental approaches (ammonia emission reduction plan, LCA-type climate indicators).
Several experiments have shown that NBPT allows:
- N₂O (nitrous oxide) emissions in certain soil types (up to -30% depending on the conditions);
- a reduction in emissions of volatile nitrogen compounds in surface spreading (meadows, intercrops);
- an improvement in the environmental balance of fertilizers within the framework of a life cycle analysis (LCA).
These effects are particularly sought after in:
- farms subject to environmental performance objectives (HVE, carbon certifications),
- projects to reduce inputs in vulnerable areas , or near sensitive areas (watercourses, Natura 2000 areas).
Laboratory analyses of NBPT: techniques and standards
The development and use of NBPT in fertilizers require precise and reliable analyses to verify its concentration, regulatory compliance, and to assess its behavior in different environments (soil, plant, food, water). Analytical laboratories play a central role in this process. This section presents the main analytical techniques used for NBPT, the applicable standards, and the quality requirements to be met during testing.
Objective of the analyses
NBPT's analyses aim to achieve several objectives depending on the context:
- to control the NBPT content in nitrogen fertilizers in order to comply with the regulatory thresholds set (0.04% to 0.10%),
- to verify the homogeneity of the formulation in industrial batches,
- monitor the stability of NBPT during storage or after mixing with other substances (e.g., sulfates),
- detect the presence of residues in agricultural products, water, animal matrices (milk, meat), or soils as part of environmental or toxicological studies.
In all cases, an appropriate analytical method that is sensitive, reproducible and compliant with good laboratory practices is essential.
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Standardized analytical methods
High-performance liquid chromatography coupled with a diode detector (HPLC-DAD) is now the reference method for measuring NBPT in technical products and environmental samples.
- Technique : HPLC-DAD
- Standard method : EN 15688
- Sampling conditions : sample taken from a waterproof and inert support , protected from moisture and light
The EN 15688 method allows for the precise quantification of NBPT in solid or liquid fertilizers, even at low concentrations. It is particularly well-suited for quality control in formulations and for marketing authorization dossiers.
For more complex matrices (milk, plants, animal tissues, etc.), advanced methods such as UHPLC-MS/MS can be used to improve sensitivity and selectivity. These techniques are described in the scientific literature, particularly for applications in animal nutrition and residue analysis.
In some cases, additional analysis by atomic absorption spectrometry can also be used to measure the metallic elements related to the stability of NBPT.
Quality standards and accreditations
NBPT analyses must be performed in accordance with international quality standards to ensure the reliability and traceability of results. The main regulatory frameworks are as follows:
- ISO 17025 : the reference standard for the competence of testing laboratories. It guarantees that methods are validated, equipment is calibrated, and results are reliable.
All of YesWeLab are selected based on their level of specialization , their experience in nitrogen compound analysis , and their accreditation records . This network ensures comprehensive coverage of analytical needs, whether for:
- the production control of an NBPT fertilizer,
- the search for residues in plants or animal products,
- the assessment of the environmental degradation of NBPT in the soil.
The results can be incorporated into a regulatory compliance file , an agronomic performance report , or a traceability audit .
