Nitrogen volatilization describes a natural process where nitrogen, often applied as fertilizer in agriculture, converts into a gaseous form and escapes into the atmosphere. This transformation results in the loss of valuable nitrogen from the soil, impacting both agricultural productivity and environmental quality. Understanding this phenomenon is important for sustainable land management practices.
The Process of Nitrogen Volatilization
Nitrogen volatilization involves the conversion of ammonium (NH4+) to ammonia gas (NH3), which then escapes into the atmosphere. This process commonly occurs when nitrogen is present in an organic form, such as urea from animal manure, synthetic fertilizers, or decaying plant material.
The initial step, called urea hydrolysis, involves the enzyme urease breaking down urea into ammonium carbonate. This reaction temporarily increases the pH in the immediate vicinity of the fertilizer. As the pH rises, ammonium ions convert into ammonia gas, which readily diffuses from the soil surface into the air. This loss primarily happens from the soil surface or plant residues if the nitrogen source is not incorporated.
Factors Influencing Volatilization
Several factors determine the rate and extent of nitrogen volatilization. Soil pH plays a significant role; higher pH levels (above 7.0) increase the conversion of ammonium to ammonia gas, promoting greater losses. This is because alkaline conditions shift the chemical equilibrium towards the gaseous ammonia form.
Temperature also directly influences volatilization; warmer temperatures accelerate the conversion of urea to ammonia. For example, an increase from 45°F to 60°F can double volatilization loss. Soil moisture is equally impactful; volatilization increases linearly with soil water content until saturation, especially when moist soil surfaces are drying, which facilitates ammonia gas escape.
Increased wind speed enhances the rate at which ammonia gas is carried away from the soil surface, preventing re-dissolution and further driving volatilization. Fertilizer type and placement are also significant. Surface-applied urea or ammonium-based fertilizers are susceptible to volatilization, especially if not incorporated into the soil. Crop residue on the soil surface can exacerbate losses by preventing fertilizer contact, trapping moisture, and providing a high concentration of the urease enzyme, which speeds conversion.
Consequences of Volatilization
Nitrogen volatilization impacts agricultural economics and environmental health. Economically, it represents a direct loss of applied nitrogen fertilizer, diminishing its availability for crop uptake. This reduction in nutrient efficiency can lead to decreased crop yields, requiring more fertilizer application and increasing operational costs. Losses can be substantial; some studies indicate 3-44% of broadcast urea nitrogen can be lost.
Environmentally, ammonia gas contributes to air pollution. Atmospheric ammonia can react with other pollutants to form fine particulate matter, contributing to haze and negatively impacting human respiratory health. While local concentrations of ammonia from volatilization are not high enough to cause direct human harm outdoors, the gas can contribute to acidification and eutrophication in distant ecosystems once redeposited. This indirect impact can lead to biodiversity loss and the formation of oxygen-deficient “dead zones” in aquatic environments.
Strategies for Minimizing Volatilization
Proper fertilizer management can significantly reduce nitrogen volatilization. Incorporating fertilizers into the soil (e.g., through injection, banding, or tillage) is effective as it physically moves nitrogen below the surface, trapping produced ammonia. Deep placement (at least two inches below the surface with well-closed slits) can reduce ammonia emissions by 75-98% compared to surface broadcasting.
Timing of application also minimizes losses. Applying fertilizers just before rainfall or irrigation, or during cooler temperatures, allows nitrogen to move into the soil before significant volatilization. While cooler temperatures were once thought to ensure minimal loss, recent research suggests surface soil moisture at application and subsequent precipitation are more impactful. For instance, 0.5 inches of rain within three hours of urea application can protect it from volatilization on bare soil.
Urease inhibitors are chemical additives that slow the urease enzyme’s activity, delaying urea’s conversion to ammonia gas. These inhibitors, such as N-(n-butyl) thiophosphoric triamide (NBPT), can extend protection from two to ten weeks, allowing more time for fertilizer incorporation by rain or irrigation. This is beneficial in conditions favoring rapid losses, such as poorly buffered soils.
Nitrification inhibitors, primarily designed to slow ammonium-to-nitrate conversion, can indirectly reduce volatilization by prolonging the ammonium form in soil, which is less susceptible to immediate gaseous loss. However, their direct impact on volatilization is less pronounced than urease inhibitors. Choosing less volatile fertilizer types, such as ammonium sulfate or calcium ammonium nitrate, over urea can also reduce volatilization potential.
Immediate irrigation after fertilizer application moves surface-applied nitrogen into the soil. At least 0.5 inches of water on bare or tilled soil, or 1.5 inches on fields with surface residue, can prevent volatilization.
Cover cropping and residue management can influence volatilization. High residue systems increase urease enzyme activity and moisture, potentially leading to higher losses if nitrogen is not adequately incorporated. In such systems, utilizing urease inhibitors or injecting nitrogen sources can be beneficial.