Ammonia volatilization is a natural process where nitrogen, an important nutrient, escapes from soil or water into the atmosphere as ammonia gas. This phenomenon is a significant concern in fields such as agriculture and environmental science. It represents a loss of valuable nitrogen, which can affect plant growth and contribute to broader environmental issues. Understanding this process is important for managing nutrient cycles effectively.
The Process of Ammonia Volatilization
The process begins with nitrogen in forms like ammonium ions (NH4+) found in soil or water, often from sources such as animal manure, urea fertilizers, or decaying plant material. Under specific conditions, ammonium ions (NH4+) convert into ammonia gas (NH3).
Once converted, ammonia gas readily escapes from the soil surface into the atmosphere. This transformation is often facilitated by the enzyme urease, which breaks down urea into ammonia and carbon dioxide. This gaseous escape results in a direct loss of nitrogen from the soil system.
Conditions That Promote Ammonia Volatilization
Several environmental and application-related factors significantly increase the rate at which ammonia volatilizes from the soil. High soil pH, especially above 7.0, is a primary factor. The chemical equilibrium between ammonium and ammonia shifts towards the gaseous ammonia form in more alkaline conditions, promoting its escape. For instance, the breakdown of urea can temporarily raise the pH around the fertilizer application site to around 9.0, even in neutral or acidic soils, which increases volatilization risk.
Elevated temperatures also accelerate ammonia volatilization. Warmer soil and air temperatures increase the kinetic energy of ammonia molecules, making them more likely to convert to gas and escape into the atmosphere. This is particularly true when temperatures are above 67°F (19°C).
Soil moisture content also plays a role; low soil moisture or drying cycles after fertilizer application can enhance volatilization. As the soil surface dries, the concentration of ammonium ions near the surface increases, and the lack of water to dissolve the ammonia gas allows it to escape more easily. This occurs when fertilizers are applied to moist, warm soil surfaces.
Increased wind speed further promotes volatilization by continuously removing the ammonia gas from the air directly above the soil surface. This removal creates a steeper concentration gradient, encouraging more ammonia to escape from the soil to maintain equilibrium.
The type of fertilizer and its application method also heavily influence volatilization losses. Urea and ammonium-based fertilizers are particularly susceptible to ammonia loss, especially when they are applied to the soil surface without immediate incorporation. Surface application allows the ammonia gas to escape directly into the atmosphere.
Consequences of Ammonia Loss
The primary negative impact of ammonia volatilization is the substantial loss of nitrogen, a nutrient plants need in large quantities for healthy growth. When nitrogen escapes as ammonia gas, it is no longer available for plant uptake, directly reducing the effectiveness of applied fertilizers. This inefficiency means a significant portion of fertilizer investment is lost, leading to diminished returns for agricultural operations.
This loss of nitrogen can result in lower crop yields because plants do not receive enough of this essential nutrient to reach their full growth potential. Farmers may need to apply more fertilizer to compensate for these losses, increasing costs and resource consumption. Beyond the direct agricultural impact, ammonia volatilization contributes to air pollution. The released ammonia gas can react with other atmospheric compounds, forming fine particulate matter that can affect air quality and human health.
Minimizing Ammonia Loss
Implementing specific management strategies can significantly reduce ammonia volatilization and improve nitrogen use efficiency in agricultural systems. One effective method is the immediate incorporation of fertilizers into the soil after application. Burying urea or ammonium-based fertilizers, either through tillage or irrigation, physically places the nitrogen below the soil surface, where it is less exposed to the conditions that promote gaseous loss. When manure is incorporated within one day of application, nitrogen retention is significantly higher.
Proper timing of fertilizer application is another important strategy. Applying fertilizers when environmental conditions are less conducive to volatilization, such as during cooler temperatures or immediately before a predicted rainfall event, can greatly reduce losses. Cooler temperatures slow down the chemical conversion of ammonium to ammonia, while rainfall washes the fertilizer into the soil, ensuring its incorporation.
The use of urease inhibitors is a chemical approach to minimize ammonia loss. Urease inhibitors are substances that temporarily block the activity of the urease enzyme, which is responsible for breaking down urea into ammonia. By slowing this breakdown, these inhibitors allow more time for urea to be moved into the soil by rain or irrigation before significant volatilization can occur. This approach helps maintain higher nitrogen availability for plants and reduces localized pH increases.
Selecting appropriate fertilizer types can also play a role. While urea and ammonium-based fertilizers are common, some enhanced-efficiency fertilizers are designed to release nitrogen more slowly or in less volatile forms. Post-application irrigation, even light watering, can help dissolve surface-applied fertilizers and move them into the soil profile, where they are less prone to volatilization. This practice helps to mitigate losses, especially when immediate mechanical incorporation is not feasible.