Ammonia Volatilization from Crop Fields and Mitigation Strategies: A Review of Fertilizer Application and Fertigation Methods

Ammonia (NH3) is a major contributor to the global nitrogen (N) cycle, and it plays an important role in the atmospheric chemistry and climate change. However, excessive NH3 emissions from agricultural activities can cause negative impacts on the environment and human health, such as acidification, eutrophication, biodiversity loss, and respiratory problems. Therefore, reducing NH3 emissions from crop fields is a key challenge for sustainable agriculture.

One of the main sources of NH3 emissions from crop fields is the volatilization of NH3 from applied fertilizers. Volatilization is the process of NH3 gas escaping from the soil surface to the atmosphere, and it is influenced by various factors, such as soil properties, climatic conditions, crop type, and fertilizer type, rate, timing, and placement. Fertigation, which is the application of fertilizers through irrigation systems, is another method of delivering nutrients to crops, and it can also affect NH3 emissions. Fertigation can potentially reduce NH3 emissions by increasing the fertilizer use efficiency and reducing the contact area between fertilizer and soil. However, fertigation can also increase NH3 emissions by enhancing the soil moisture and pH, which favor NH3 formation and volatilization.

The objective of this document is to review the current knowledge on NH3 volatilization from crop fields and the mitigation strategies based on different fertilizer application and fertigation methods. The document will cover the following topics: (1) the mechanisms and factors of NH3 volatilization from fertilizers, (2) the measurement and estimation methods of NH3 emissions from crop fields, (3) the effects of different fertilizer types, rates, timings, and placements on NH3 emissions, (4) the effects of different fertigation systems, frequencies, and durations on NH3 emissions, and (5) the best management practices and recommendations for reducing NH3 emissions from fertilizers and fertigation.

Mechanisms and Factors of Ammonia Volatilization from Fertilizers

Ammonia volatilization from fertilizers occurs when the NH3 concentration in the soil solution exceeds the NH3 concentration in the air above the soil surface, creating a concentration gradient that drives the NH3 diffusion from the soil to the atmosphere. The NH3 concentration in the soil solution depends on the equilibrium between NH3 and ammonium (NH4+), which is governed by the soil pH and temperature. The higher the soil pH and temperature, the more NH3 is formed and volatilized. The NH3 concentration in the air above the soil surface depends on the atmospheric turbulence, humidity, and NH3 deposition. The higher the atmospheric turbulence and humidity, the more NH3 is dispersed and diluted. The higher the NH3 deposition, the more NH3 is removed from the air.

The factors that affect NH3 volatilization from fertilizers can be classified into three categories: fertilizer-related factors, soil-related factors, and environmental factors. Fertilizer-related factors include the type, rate, timing, and placement of fertilizers. Soil-related factors include the soil texture, structure, organic matter, moisture, pH, and microbial activity. Environmental factors include the air temperature, humidity, wind speed, rainfall, and solar radiation. These factors can interact with each other and influence the NH3 volatilization process in complex ways.

Factors: Effect on NH3 volatilization

Fertilizer type: Ammonium-based fertilizers (e.g., urea, ammonium sulfate, ammonium nitrate) have higher potential for NH3 volatilization than nitrate-based fertilizers (e.g., calcium nitrate, potassium nitrate) or organic fertilizers (e.g., manure, compost, biosolids).

Different N fertilizers have different potentials for NH3 volatilization, depending on their chemical forms, solubility, and reaction rates in the soil. Generally, urea-based fertilizers have the highest volatilization potential, followed by ammonium-based fertilizers, while nitrate-based fertilizers have the lowest potential. This is because urea is hydrolyzed to NH4+ and CO2 by the enzyme urease in the soil, and NH4+ can be converted to NH3 by the equilibrium reaction NH4+ + OH-?? NH3 + H2O, especially under alkaline conditions. Ammonium-based fertilizers, such as ammonium sulfate and ammonium nitrate, can also undergo the same equilibrium reaction, but at a lower rate than urea. Nitrate-based fertilizers, such as calcium nitrate and potassium nitrate, do not produce NH4+ in the soil, and thus have negligible volatilization potential.

Fertilizer rate: Higher fertilizer rates increase the NH3 concentration in the soil solution and the NH3 emission rate, but decrease the NH3 emission fraction (the percentage of applied N that is lost as NH3).

Fertilizer timing: Fertilizer application in the morning or evening can reduce NH3 volatilization by avoiding the peak soil temperature and solar radiation. Fertilizer application before or after rainfall can reduce NH3 volatilization by increasing the soil moisture and leaching the fertilizer into the soil.

Fertilizer placement: Subsurface placement (e.g., injection, banding, incorporation) can reduce NH3 volatilization by increasing the contact area between fertilizer and soil, decreasing the NH3 concentration gradient, and enhancing the NH4+ adsorption and nitrification. Surface placement (e.g., broadcasting, spraying, dribbling) can increase NH3 volatilization by exposing the fertilizer to the air and increasing the NH3 concentration gradient.

The fertilizer application method affects the NH3 volatilization by influencing the contact area between fertilizer and soil, the depth of fertilizer placement, and the exposure time of fertilizer to the air. Generally, surface application of fertilizer, especially in solid or granular forms, results in higher NH3 volatilization than subsurface or deep placement, because of the larger contact area and longer exposure time of fertilizer to the air. Incorporation of fertilizer into the soil, either by tillage or irrigation, can reduce NH3 volatilization by increasing the infiltration and retention of N in the soil, and by decreasing the soil pH and temperature near the soil surface. However, incorporation of fertilizer may also increase the soil microbial activity and the urease activity, which may enhance the hydrolysis of urea and the production of NH4+ and NH3 in the soil.

Soil texture: Coarse-textured soils (e.g., sandy soils) have lower NH3 volatilization potential than fine-textured soils (e.g., clayey soils) because they have lower water-holding capacity, lower cation exchange capacity, lower pH, and higher infiltration rate.

Soil structure: Well-structured soils have lower NH3 volatilization potential than poorly-structured soils because they have higher porosity, higher infiltration rate, and lower surface crust formation.

Soil organic matter: Soils with high organic matter content have lower NH3 volatilization potential than soils with low organic matter content because they have higher cation exchange capacity, higher buffering capacity, higher microbial activity, and higher N immobilization.

Soil moisture: Soil moisture has a dual effect on NH3 volatilization. On one hand, high soil moisture can reduce NH3 volatilization by increasing the soil infiltration rate, leaching the fertilizer into the soil, and diluting the NH3 concentration in the soil solution. On the other hand, high soil moisture can increase NH3 volatilization by increasing the soil pH, enhancing the NH3 formation, and reducing the NH3 deposition.

The soil moisture affects the NH3 volatilization by influencing the soil solution concentration, the soil aeration, the soil temperature, and the soil microbial activity. Higher soil moisture can reduce NH3 volatilization by diluting the soil solution and lowering the NH3 concentration, by increasing the soil aeration and enhancing the nitrification of NH4+ to NO3-, and by decreasing the soil temperature and slowing down the hydrolysis of urea and the equilibrium reaction between NH4+ and NH3. However, higher soil moisture can also increase NH3 volatilization by increasing the soil microbial activity and the urease activity, and by creating anaerobic conditions that inhibit the nitrification process. Therefore, the optimal soil moisture for minimizing NH3 volatilization is the field capacity, which is the maximum amount of water that the soil can hold against gravity.

Soil pH: Soil pH is the most important factor affecting NH3 volatilization from fertilizers. High soil pH can increase NH3 volatilization by shifting the equilibrium between NH3 and NH4+ to favor NH3 formation. Low soil pH can reduce NH3 volatilization by shifting the equilibrium to favor NH4+ formation.

The soil pH affects the NH3 volatilization by influencing the equilibrium reaction between NH4+ and NH3 in the soil solution. Higher soil pH favors the formation of NH3, while lower soil pH favors the formation of NH4+. Therefore, alkaline soils tend to have higher NH3 volatilization than acidic soils. The soil pH can be affected by the type of fertilizer, the soil organic matter, the soil texture, the soil moisture, and the crop residue. For example, urea-based fertilizers can increase the soil pH by releasing CO2 and OH- during the hydrolysis process, while nitrate-based fertilizers can decrease the soil pH by releasing H+ during the nitrification process. Soil organic matter can buffer the soil pH by releasing organic acids or bases, depending on the soil conditions. Soil texture can affect the soil pH by influencing the cation exchange capacity and the buffering capacity of the soil. Soil moisture can affect the soil pH by diluting or concentrating the soil solution, and by affecting the soil aeration and the microbial activity. Crop residue can affect the soil pH by releasing organic acids or bases during the decomposition process, and by affecting the soil moisture and the soil temperature.

Soil microbial activity: Soil microbial activity can affect NH3 volatilization by influencing the N transformations in the soil, such as nitrification, denitrification, and immobilization. Nitrification is the oxidation of NH4+ to nitrate (NO3-), which can reduce NH3 volatilization by consuming NH4+ and lowering the soil pH. Denitrification is the reduction of NO3- to nitrous oxide (N2O) or nitrogen gas (N2), which can reduce NH3 volatilization by consuming NO3- and lowering the soil N content. Immobilization is the incorporation of inorganic N into organic N by microorganisms, which can reduce NH3 volatilization by consuming NH4+ and NO3- and lowering the soil N content.

Air temperature: High air temperature can increase NH3 volatilization by increasing the soil temperature, enhancing the NH3 formation, and increasing the NH3 diffusion rate.

Air humidity: High air humidity can reduce NH3 volatilization by increasing the NH3 deposition and decreasing the NH3 concentration gradient.

Wind speed: High wind speed can increase NH3 volatilization by increasing the atmospheric turbulence, enhancing the NH3 diffusion, and decreasing the NH3 deposition.

The wind speed affects the NH3 volatilization by influencing the NH3 concentration gradient and the turbulence intensity between the soil surface and the atmosphere. Higher wind speed can increase NH3 volatilization by increasing the NH3 concentration gradient and the turbulence intensity, which enhance the NH3 diffusion and dispersion from the soil to the air. However, higher wind speed can also reduce NH3 volatilization by increasing the evaporation and the soil moisture, and by decreasing the soil temperature and the soil pH. Therefore, the optimal wind speed for minimizing NH3 volatilization is the one that provides adequate diffusion and dispersion, but also maintains favorable soil conditions.

The soil temperature: The soil temperature affects the NH3 volatilization by influencing the equilibrium reaction between NH4+ and NH3, the soil solution concentration, and the soil microbial activity. Higher soil temperature can increase NH3 volatilization by shifting the equilibrium reaction to the right and increasing the NH3 concentration, by increasing the soil solution concentration and the vapor pressure of NH3, and by increasing the soil microbial activity and the urease activity. However, higher soil temperature can also reduce NH3 volatilization by enhancing the nitrification of NH4+ to NO3-, and by decreasing the soil moisture and the soil pH. Therefore, the optimal soil temperature for minimizing NH3 volatilization is the range of 15-25°C, which is suitable for both plant growth and nitrification.

The crop canopy: The crop canopy affects the NH3 volatilization by influencing the microclimate, the soil surface roughness, and the crop uptake of N. Higher crop canopy can reduce NH3 volatilization by shading the soil surface and lowering the soil temperature, by increasing the soil surface roughness and reducing the wind speed, and by increasing the crop uptake of N and decreasing the N availability in the soil. However, higher crop canopy can also increase NH3 volatilization by increasing the relative humidity and the NH3 concentration in the air, and by creating a stagnant air layer above the soil surface that reduces the NH3 diffusion and dispersion. Therefore, the optimal crop canopy for minimizing NH3 volatilization is the one that provides adequate shading and roughness, but also allows sufficient ventilation and air movement.

Rainfall: Rainfall can have a dual effect on NH3 volatilization. On one hand, rainfall can reduce NH3 volatilization by increasing the soil moisture, leaching the fertilizer into the soil, and increasing the NH3 deposition. On the other hand, rainfall can increase NH3 volatilization by increasing the soil pH, enhancing the NH3 formation, and creating a water film on the soil surface that traps NH3.

Solar radiation: High solar radiation can increase NH3 volatilization by increasing the soil temperature, enhancing the NH3 formation, and drying the soil surface.

Measurement and Estimation Methods of Ammonia Emissions from Crop Fields

There are various methods for measuring and estimating NH3 emissions from crop fields, which can be divided into two categories: direct methods and indirect methods. Direct methods measure the NH3 concentration or flux in the air above the soil surface, while indirect methods estimate the NH3 emission based on the N balance or the N transformation in the soil. A brief description of some of the commonly used methods is given below.

• Direct methods

- Chamber method: This method involves placing a closed or ventilated chamber over the soil surface and measuring the NH3 concentration in the chamber air using a gas analyzer or a passive sampler. The NH3 flux is calculated from the change in NH3 concentration over time and the chamber volume and area. This method is simple, inexpensive, and suitable for small-scale experiments, but it can disturb the microclimate and the NH3 exchange between the soil and the atmosphere.

- Micrometeorological method: This method involves measuring the NH3 concentration and the meteorological variables (e.g., wind speed, air temperature, humidity) at different heights above the soil surface using a gas analyzer or a passive sampler and a meteorological station. The NH3 flux is calculated from the NH3 concentration gradient and the aerodynamic resistance or the eddy covariance. This method is complex, expensive, and suitable for large-scale experiments, but it can capture the spatial and temporal variability of NH3 emissions and the influence of the atmospheric conditions.

• Indirect methods

- N balance method: This method involves measuring the N input and output in the soil-plant system and estimating the NH3 emission as the difference between the N input and output. The N input includes the applied fertilizer, the atmospheric deposition, the irrigation water, and the biological fixation. The N output includes the harvested crop, the leached N, the denitrified N, and the immobilized N. This method is simple, inexpensive, and suitable for long-term experiments, but it can have large uncertainties and errors due to the difficulty of measuring all the N input and output components.

- N transformation method: This method involves measuring the N transformation in the soil, such as nitrification, denitrification, and immobilization, and estimating the NH3 emission as the difference between the applied N and the transformed N. The N transformation can be measured using isotopic or chemical tracers, soil incubation, or soil sampling and analysis. This method is complex, expensive, and suitable for short-term experiments, but it can provide insight into the mechanisms and factors of NH3 volatilization and the interactions between different N processes.

Effects of Different Fertilizer Types, Rates, Timings, and Placements on Ammonia Emissions

The type, rate, timing, and placement of fertilizers are the main factors that can be manipulated by farmers to reduce NH3 emissions from crop fields. The effects of these factors on NH3 emissions have been studied extensively in the literature, and some of the general findings and recommendations are summarized below.

• Fertilizer type

- Among the ammonium-based fertilizers, urea has the highest potential for NH3 volatilization, followed by ammonium sulfate and ammonium nitrate. Urea is hydrolyzed to NH4+ and bicarbonate (HCO3-) by the enzyme urease in the soil, which increases the soil pH and favors NH3 formation. Ammonium sulfate and ammonium nitrate can also increase the soil pH, but to a lesser extent than urea, because they are partially or fully nitrified to NO3-, which consumes H+ and lowers the soil pH. Therefore, using nitrate-based fertilizers or organic fertilizers can reduce NH3 emissions compared to using ammonium-based fertilizers, especially urea.

- However, nitrate-based fertilizers and organic fertilizers have other drawbacks, such as higher cost, lower N availability, higher leaching potential, and higher greenhouse gas emissions. Therefore, using urease inhibitors or nitrification inhibitors can be an alternative option to reduce NH3 emissions from ammonium-based fertilizers. Urease inhibitors can delay or inhibit the hydrolysis of urea, which reduces the soil pH increase and the NH3 formation. Nitrification inhibitors can delay or inhibit the oxidation of NH4+ to NO3-, which reduces the NH4+ consumption and the NH3 volatilization. However, these inhibitors can also have negative effects on the crop yield, the soil microbial activity, and the water quality, depending on the soil and climatic conditions and the application rate and frequency.

• Fertilizer rate

The effect of fertilizer rate on NH3 emissions is not linear, but rather depends on the soil N demand and the fertilizer N efficiency. Higher fertilizer rates can increase the NH3 emission rate, but decrease the NH3 emission fraction, because the excess N is more likely to be volatilized, but the proportion of the applied N that is lost as NH3 is lower. Therefore, applying the optimal fertilizer rate that matches the crop N demand and the soil N supply can reduce NH3 emissions and increase N use efficiency. The optimal fertilizer rate can be determined by using soil testing, crop modeling, or decision support tools.

• Fertilizer timing

The effect of fertilizer timing on NH3 emissions is related to the crop growth stage and the environmental conditions. Applying fertilizer at the early or late stages of crop growth can increase NH3 emissions, because the crop N uptake is low and the fertilizer N is more exposed to the volatilization factors. Therefore, applying fertilizer at the peak or critical stages of crop growth can reduce NH3 emissions and increase N use efficiency. The peak or critical stages of crop growth can be identified by using crop phenology, leaf color, or plant sensors.

Applying fertilizer in the morning or evening can reduce NH3 emissions, because the soil temperature and solar radiation are lower, which reduces the NH3 formation and volatilization. Applying fertilizer before or after rainfall can reduce NH3 emissions, because the soil moisture is higher, which increases the soil infiltration and leaching of the fertilizer N. However, applying fertilizer during or immediately before heavy rainfall can increase NH3 emissions, because the soil surface can be flooded and the NH3 can be trapped in the water film.

• Fertilizer placement

The effect of fertilizer placement on NH3 emissions is related to the contact area and depth between the fertilizer and the soil. Subsurface placement, such as injection, banding, or incorporation, deep placement, can reduce NH3 emissions, because the fertilizer N is more in contact with the soil, which increases the NH4+ adsorption and nitrification, and less exposed to the air, which decreases the NH3 concentration gradient and diffusion. Surface placement, such as broadcasting, spraying, or dribbling, can increase NH3 emissions, because the fertilizer N is more exposed to the air, which increases the NH3 concentration gradient and diffusion, and less in contact with the soil, which decreases the NH4+ adsorption and nitrification.

However, subsurface placement can also have disadvantages, such as higher cost, higher energy consumption, higher soil disturbance, and higher greenhouse gas emissions. Therefore, using controlled-release fertilizers or polymer-coated fertilizers can be another option to reduce NH3 emissions from surface placement. Controlled-release fertilizers or polymer-coated fertilizers can release the N gradually, which reduces the NH3 formation and volatilization. However, these fertilizers can also have drawbacks, such as higher cost, lower N availability, and lower crop yield, depending on the release rate and pattern and the crop N demand.

Effects of Different Fertigation Systems, Frequencies, and Durations on Ammonia Emissions

Fertigation is the application of fertilizers, soil amendments, or other water-soluble products through an irrigation system. It is a widely used technique in agriculture, especially for crops that require frequent and precise nutrient and water management, such as fruits, vegetables, and ornamentals. Fertigation can improve crop yield and quality, reduce fertilizer and water use, and enhance nutrient use efficiency. However, fertigation also poses some environmental challenges, such as the potential for nutrient leaching, runoff, and volatilization. Among these, ammonia (NH3) volatilization is one of the major sources of nitrogen (N) loss from fertigated soils, which can reduce N availability for crops, increase greenhouse gas emissions, and contribute to acid rain and eutrophication.

The amount of NH3 volatilized from fertigated soils depends on various factors, such as the type and concentration of fertilizer, the soil properties, the climatic conditions, and the fertigation system, frequency, and duration. These factors can interact with each other and affect the NH3 emission rate and the total NH3 loss. Therefore, it is important to understand how different fertigation practices influence NH3 volatilization and to identify the best management practices to minimize NH3 emissions while maintaining crop productivity and quality.

Effects of Fertigation Systems

Fertigation systems can be classified into two main types: surface and subsurface. Surface fertigation refers to the application of fertilizer solutions on the soil surface, either by sprinklers, drip emitters, or micro-sprinklers. Subsurface fertigation refers to the application of fertilizer solutions below the soil surface, either by buried drip lines or micro-tubes. The choice of fertigation system depends on the crop type, the soil characteristics, the water availability, and the economic feasibility.

Several studies have compared the effects of surface and subsurface fertigation on NH3 emissions from different crops and soils. The results are not consistent, as some studies reported higher NH3 emissions from surface fertigation, while others reported higher NH3 emissions from subsurface fertigation, or no significant difference between the two systems. The variation in the results may be attributed to the differences in the experimental conditions, such as the fertilizer type and rate, the soil type and moisture, the crop type and canopy, the weather conditions, and the measurement methods.

Some of the possible mechanisms and factors that may explain the effects of fertigation systems on NH3 emissions are:

- Fertilizer placement: Surface fertigation exposes the fertilizer solution to the air, which increases the NH3 volatilization potential. Subsurface fertigation reduces the contact area between the fertilizer solution and the air, which decreases the NH3 volatilization potential. However, subsurface fertigation may also create anaerobic conditions in the soil, which may enhance the microbial production of NH3 from organic N sources, such as crop residues and soil organic matter. Moreover, subsurface fertigation may increase the soil temperature and pH, which may also increase the NH3 volatilization potential.

- Fertilizer movement: Surface fertigation may cause more lateral and vertical movement of the fertilizer solution in the soil, which may dilute the fertilizer concentration and reduce the NH3 volatilization potential. Subsurface fertigation may cause less movement of the fertilizer solution in the soil, which may increase the fertilizer concentration and the NH3 volatilization potential. However, the movement of the fertilizer solution also depends on the soil texture, structure, and moisture, as well as the irrigation rate and frequency. For example, sandy soils may have more leaching and less NH3 volatilization than clayey soils, regardless of the fertigation system.

- Crop canopy: Surface fertigation may increase the NH3 volatilization potential by creating a humid and warm microclimate under the crop canopy, which may enhance the NH3 diffusion from the soil to the air. Subsurface fertigation may decrease the NH3 volatilization potential by reducing the humidity and temperature under the crop canopy, which may inhibit the NH3 diffusion from the soil to the air. However, the crop canopy may also affect the NH3 volatilization potential by altering the wind speed, the solar radiation, and the leaf area index, which may have different effects on the NH3 exchange between the soil and the air.

Effects of Fertigation Frequencies

Fertigation frequency refers to the number of fertigation events per unit time, such as per day, per week, or per crop cycle. The choice of fertigation frequency depends on the crop nutrient demand, the soil water holding capacity, and the irrigation scheduling. Fertigation frequency can affect the NH3 volatilization potential by influencing the fertilizer concentration, the soil moisture, and the soil temperature.

Several studies have compared the effects of different fertigation frequencies on NH3 emissions from different crops and soils. The results are also not consistent, as some studies reported higher NH3 emissions from higher fertigation frequencies, while others reported higher NH3 emissions from lower fertigation frequencies, or no significant difference between different frequencies. The variation in the results may be attributed to the differences in the experimental conditions, such as the fertigation system, the fertilizer type and rate, the soil type and temperature, the crop type and canopy, the weather conditions, and the measurement methods.

Some of the possible mechanisms and factors that may explain the effects of fertigation frequencies on NH3 emissions are:

- Fertilizer concentration: Higher fertigation frequencies may reduce the NH3 volatilization potential by applying smaller amounts of fertilizer per fertigation event, which may lower the fertilizer concentration and the NH3 equilibrium pressure in the soil solution. Lower fertigation frequencies may increase the NH3 volatilization potential by applying larger amounts of fertilizer per fertigation event, which may increase the fertilizer concentration and the NH3 equilibrium pressure in the soil solution. However, the fertilizer concentration may also depend on the fertilizer movement and the crop uptake, which may vary with the soil texture, structure, and moisture, as well as the irrigation rate and duration.

- Soil moisture: Higher fertigation frequencies may increase the NH3 volatilization potential by maintaining higher soil moisture levels, which may enhance the NH3 diffusion from the soil to the air. Lower fertigation frequencies may decrease the NH3 volatilization potential by allowing lower soil moisture levels, which may inhibit the NH3 diffusion from the soil to the air. However, the soil moisture may also depend on the soil water holding capacity, the evaporation, and the transpiration, which may vary with the soil type, the weather conditions, and the crop type and canopy.

- Soil temperature: Higher fertigation frequencies may decrease the NH3 volatilization potential by reducing the soil temperature, which may lower the NH3 equilibrium pressure and the NH3 diffusion coefficient in the soil solution. Lower fertigation frequencies may increase the NH3 volatilization potential by increasing the soil temperature, which may increase the NH3 equilibrium pressure and the NH3 diffusion coefficient in the soil solution. However, the soil temperature may also depend on the solar radiation, the wind speed, and the crop canopy, which may have different effects on the heat exchange between the soil and the air.

Effects of Fertigation Durations

Fertigation duration refers to the length of time that the fertilizer solution is applied per fertigation event, such as minutes, hours, or days. The choice of fertigation duration depends on the irrigation rate, the irrigation volume, and the fertilizer concentration. Fertigation duration can affect the NH3 volatilization potential by influencing the fertilizer distribution, the soil moisture, and the soil temperature.

Few studies have compared the effects of different fertigation durations on NH3 emissions from different crops and soils. The results are also not consistent, as some studies reported higher NH3 emissions from longer fertigation durations, while others reported higher NH3 emissions from shorter fertigation durations, or no significant difference between different durations. The variation in the results may be attributed to the differences in the experimental conditions, such as the fertigation system, the fertigation frequency, the fertilizer type and rate, the soil type and temperature, the crop type and canopy, the weather conditions, and the measurement methods.

Some of the possible mechanisms and factors that may explain the effects of fertigation durations on NH3 emissions are:

- Fertilizer distribution: Longer fertigation durations may reduce the NH3 volatilization potential by distributing the fertilizer solution more evenly and deeply in the soil, which may dilute the fertilizer concentration and reduce the contact area between the fertilizer solution and the air. Shorter fertigation durations may increase the NH3 volatilization potential by concentrating the fertilizer solution in a smaller and shallower soil volume, which may increase the fertilizer concentration and the contact area between the fertilizer solution and the air. However, the fertilizer distribution may also depend on the fertigation system, the fertigation frequency, the soil texture, structure, and moisture, and the crop root distribution.

- Soil moisture: Longer fertigation durations may increase the NH3 volatilization potential by maintaining higher soil moisture levels, which may enhance the NH3 diffusion from the soil to the air. Shorter fertigation durations may decrease the NH3 volatilization potential by allowing lower soil moisture levels, which may inhibit the NH3 diffusion from the soil to the air. However, the soil moisture may also depend on the fertigation frequency, the soil water holding capacity, the evaporation, and the transpiration, which may vary with the soil type, the weather conditions, and the crop type and canopy.

- Soil temperature: Longer fertigation durations may decrease the NH3 volatilization potential by reducing the soil temperature, which may lower the NH3 equilibrium pressure and the NH3 diffusion coefficient in the soil solution. Shorter fertigation durations may increase the NH3 volatilization potential by increasing the soil temperature, which may increase the NH3 equilibrium pressure and the NH3 diffusion coefficient in the soil solution. However, the soil temperature may also depend on the solar radiation, the wind speed, and the crop canopy, which may have different effects on the heat exchange between the soil and the air.

Best Management Practices for Reducing NH3 Emissions from Fertilizers and Fertigation

Nitrogen (N) is an essential nutrient for crop growth and yield, but it can also be a source of environmental pollution when applied in excess or in inappropriate forms. One of the main pathways of N loss from agricultural systems is ammonia (NH3) volatilization, which occurs when N fertilizers or manures are applied to the soil surface or through irrigation water (fertigation). NH3 volatilization not only reduces the efficiency and profitability of N fertilization, but also contributes to acidification and eutrophication of terrestrial and aquatic ecosystems, as well as to greenhouse gas emissions and human health problems.

The amount of NH3 volatilized from fertilizers and fertigation depends on several factors, such as the type and rate of N source, the timing and method of application, the soil properties, the crop type and stage, the weather conditions, and the irrigation management. Therefore, the adoption of best management practices (BMPs) that minimize NH3 losses and maximize N use efficiency is crucial for sustainable and profitable crop production. This document provides an overview of the main BMPs and recommendations for reducing NH3 emissions from fertilizers and fertigation, based on the current scientific knowledge and practical experience.

BMPs for reducing NH3 emissions from fertilizers

The choice of the N source is one of the most important factors affecting NH3 volatilization. Generally, NH3 losses are higher from urea-based fertilizers than from ammonium-based or nitrate-based fertilizers, because urea undergoes hydrolysis by the enzyme urease, producing NH4+ and OH-, which increase the soil pH and favor NH3 formation. Therefore, the following BMPs are recommended for reducing NH3 emissions from urea-based fertilizers:

- Use urease inhibitors, which are chemicals that temporarily inhibit the activity of urease, delaying the hydrolysis of urea and reducing the peak of NH3 production. Examples of urease inhibitors are N-(n-butyl) thiophosphoric triamide (NBPT) and N-(n-propyl) thiophosphoric triamide (NPPT).

- Use controlled-release or slow-release urea, which are urea granules coated with materials that regulate the release of urea to the soil solution, such as sulfur, polymers, or resins. These products reduce the rate and extent of urea hydrolysis and NH3 volatilization, as well as the risk of N leaching and denitrification.

- Incorporate urea into the soil as soon as possible after application, either mechanically or by irrigation. This reduces the exposure of urea to urease and the contact of NH3 with the air, enhancing the retention of N in the soil. The incorporation depth should be at least 5 cm for optimal results.

- Apply urea when the soil moisture is adequate but not excessive, and avoid applying urea on wet or flooded soils. This ensures that urea dissolves quickly and moves into the soil, reducing the surface area for NH3 volatilization. However, too much water can also increase NH3 losses by creating anaerobic conditions and enhancing urease activity.

- Apply urea when the soil temperature is low, preferably in the early morning or evening. This reduces the rate of urea hydrolysis and NH3 volatilization, as well as the evaporation of water from the soil surface.

- Apply urea when the wind speed is low, preferably less than 5 km/h. This reduces the dispersion of NH3 from the soil surface to the atmosphere, increasing the chance of NH3 reabsorption by the soil or the crop canopy.

- Apply urea in small and frequent doses, rather than in large and infrequent doses. This matches the N supply with the crop demand, reducing the accumulation of urea and NH3 in the soil and the potential for N losses.

For ammonium-based fertilizers, such as ammonium sulfate, ammonium nitrate, or ammonium phosphate, the main BMPs for reducing NH3 emissions are:

- Incorporate ammonium-based fertilizers into the soil as soon as possible after application, either mechanically or by irrigation. This reduces the exposure of NH4+ to nitrification, which produces H+ and lowers the soil pH, favoring NH3 formation.

- Apply ammonium-based fertilizers when the soil pH is low, preferably below 7.0. This reduces the equilibrium concentration of NH3 in the soil solution, enhancing the retention of NH4+ in the soil.

- Apply ammonium-based fertilizers when the soil moisture is adequate but not excessive, and avoid applying ammonium-based fertilizers on wet or flooded soils. This ensures that NH4+ moves into the soil, reducing the surface area for NH3 volatilization. However, too much water can also increase NH3 losses by creating anaerobic conditions and enhancing nitrification.

- Apply ammonium-based fertilizers when the soil temperature is low, preferably in the early morning or evening. This reduces the rate of nitrification and NH3 volatilization, as well as the evaporation of water from the soil surface.

- Apply ammonium-based fertilizers when the wind speed is low, preferably less than 5 km/h. This reduces the dispersion of NH3 from the soil surface to the atmosphere, increasing the chance of NH3 reabsorption by the soil or the crop canopy.

- Apply ammonium-based fertilizers in small and frequent doses, rather than in large and infrequent doses. This matches the N supply with the crop demand, reducing the accumulation of NH4+ and NH3 in the soil and the potential for N losses.

For nitrate-based fertilizers, such as calcium nitrate or potassium nitrate, the main BMPs for reducing NH3 emissions are:

- Apply nitrate-based fertilizers when the soil pH is high, preferably above 7.0. This reduces the equilibrium concentration of NH3 in the soil solution, enhancing the retention of NO3- in the soil.

- Apply nitrate-based fertilizers when the soil moisture is adequate but not excessive, and avoid applying nitrate-based fertilizers on wet or flooded soils. This ensures that NO3- moves into the soil, reducing the surface area for NH3 volatilization. However, too much water can also increase NH3 losses by creating anaerobic conditions and enhancing denitrification, which produces N2O and N2.

- Apply nitrate-based fertilizers when the soil temperature is low, preferably in the early morning or evening. This reduces the rate of denitrification and NH3 volatilization, as well as the evaporation of water from the soil surface.

- Apply nitrate-based fertilizers when the wind speed is low, preferably less than 5 km/h. This reduces the dispersion of NH3 from the soil surface to the atmosphere, increasing the chance of NH3 reabsorption by the soil or the crop canopy.

- Apply nitrate-based fertilizers in small and frequent doses, rather than in large and infrequent doses. This matches the N supply with the crop demand, reducing the accumulation of NO3- and NH3 in the soil and the potential for N losses.

BMPs for reducing NH3 emissions from fertigation

Fertigation is the application of fertilizers through irrigation water, which can be an efficient and convenient way of delivering N to crops. However, fertigation can also increase the risk of NH3 volatilization, especially when using urea or ammonium-based fertilizers, because the irrigation water can increase the soil pH and the surface area for NH3 formation and dispersion. Therefore, the following BMPs are recommended for reducing NH3 emissions from fertigation:

- Use nitrate-based fertilizers for fertigation, rather than urea or ammonium-based fertilizers. This reduces the equilibrium concentration of NH3 in the soil solution, enhancing the retention of NO3- in the soil.

- Use acidifying agents, such as sulfuric acid or phosphoric acid, to lower the pH of the irrigation water and the soil, especially when using urea or ammonium-based fertilizers. This reduces the formation and volatilization of NH3, as well as the precipitation of calcium carbonate or magnesium carbonate, which can reduce the solubility and availability of N and other nutrients.

- Use nitrification inhibitors, such as dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP), to reduce the nitrification of NH4+ to NO3-, especially when using ammonium-based fertilizers. This reduces the production of H+ and the lowering of soil pH, which can increase the formation and volatilization of NH3.

- Use low-pressure and low-angle sprinklers, or drip irrigation, rather than high-pressure and high-angle sprinklers, for fertigation. This reduces the exposure of the irrigation water and the fertilizer to the air, as well as the drift and evaporation of the water droplets, which can increase the concentration and volatilization of NH3.

- Apply fertigation when the soil moisture is adequate but not excessive, and avoid applying fertigation on wet or flooded soils. This ensures that the irrigation water and the fertilizer move into the soil, reducing the surface area for NH3 volatilization. However, too much water can also increase NH3 losses by creating anaerobic conditions and enhancing nitrification or denitrification.

- Apply fertigation when the soil temperature is low, preferably in the early morning or evening. This reduces the rate of nitrification, denitrification, and NH3 volatilization, as well as the evaporation of water from the soil surface.

- Apply fertigation when the wind speed is low, preferably less than 5 km/h. This reduces the dispersion of NH3 from the soil surface to the atmosphere, increasing the chance of NH3 reabsorption by the soil or the crop canopy.

- Apply fertigation in small and frequent doses, rather than in large and infrequent doses. This matches the N supply with the crop demand, reducing the accumulation of NH4+, NO3-, and NH3 in the soil and the potential for N losses.

NH3 volatilization is a major pathway of N loss from agricultural systems, which can reduce the efficiency and profitability of N fertilization, as well as cause environmental and health problems. The adoption of BMPs that minimize NH3 losses and maximize N use efficiency is crucial for sustainable and profitable crop production. The main BMPs for reducing NH3 emissions from fertilizers and fertigation include the choice of the N source, the use of additives, the incorporation of fertilizers into the soil, the timing and method of application, the soil and weather conditions, and the irrigation management. These BMPs should be tailored to the specific conditions and needs of each farm and crop, and evaluated for their cost-effectiveness and feasibility.

Conclusion

Ammonia volatilization from crop fields is a major source of NH3 emissions, as well as a loss of N from the soil-plant system. It is influenced by various factors, such as fertilizer type, rate, timing, and placement, fertigation method, irrigation system, soil properties, climatic conditions, and crop type and growth stage. Ammonia volatilization can be mitigated by adopting appropriate fertilizer and fertigation practices, such as using nitrification inhibitors, urease inhibitors, or controlled-release fertilizers, reducing the fertilizer rate or concentration, applying the fertilizer or fertigation in split doses, timing the fertilizer or fertigation according to the soil and weather conditions, placing the fertilizer below the soil surface or incorporating it into the soil, using drip irrigation or sprinkler irrigation with large droplets, and avoiding or washing off the fertigation on the plant foliage. However, these mitigation options may have different effectiveness, limitations, and trade-offs, depending on the local conditions and the system objectives. Therefore, it is important to evaluate the costs and benefits of each option, and to optimize the fertilizer and fertigation management for each specific situation.