Nitrogen volatilization is a process that reduces the effectiveness of nitrogen-based fertilizers and manures. This process involves the transformation of nitrogen compounds, primarily urea or ammonium, from their solid or dissolved state in the soil into ammonia gas (\(text{NH}_3\)). Once converted to this gaseous form, the nitrogen escapes from the soil surface and is lost into the atmosphere. This gaseous loss represents a direct reduction in the amount of usable nitrogen intended for crop uptake, making the fertilizer application less efficient.
The Chemical Process of Ammonia Loss
The chemical conversion of fertilizer nitrogen into a volatile gas begins with the breakdown of urea, the most common form of nitrogen fertilizer. When urea is applied to the soil surface, it is quickly dissolved by soil moisture, initiating hydrolysis. This reaction is dependent on the urease enzyme, a biological catalyst produced by soil microorganisms and found abundantly on plant residue and in organic matter.
The urease enzyme breaks the urea molecule down into unstable carbamic acid, which rapidly decomposes into ammonium (\(text{NH}_4^+\)) and carbon dioxide (\(text{CO}_2\)). The formation of ammonium releases hydroxide ions (\(text{OH}^-\)), resulting in a temporary, localized increase in the soil \(text{pH}\) surrounding the fertilizer granule. This temporary rise can push the soil in the immediate vicinity to a \(text{pH}\) of 9.0 or higher.
This localized alkalinity drives the final step of volatilization. In the soil solution, ammonium (\(text{NH}_4^+\)) and ammonia (\(text{NH}_3\)) exist in an equilibrium highly dependent on \(text{pH}\). When the \(text{pH}\) increases due to urea hydrolysis, the equilibrium shifts, converting the plant-usable ammonium ion into gaseous ammonia. Since ammonia is a gas, it readily escapes from the soil surface into the atmosphere, often peaking in the first few days to two weeks after surface application.
Environmental and Economic Impacts
The loss of nitrogen through volatilization has significant consequences for both agricultural economics and environmental quality. Globally, the average loss of applied nitrogen is estimated to be around 17.6%, though this can range from near zero to over 40% under unfavorable conditions. This loss represents a direct financial cost for farmers, wasting investment in fertilizer and often leading to lower crop yields due to nutrient deficiency.
The environmental repercussions extend beyond the field. Once in the atmosphere, ammonia gas can react with compounds like sulfuric and nitric acids, forming tiny solid particles known as atmospheric particulate matter (\(text{PM}_{2.5}\)). These fine particles are a known cause of respiratory issues and contribute to general air pollution.
The ammonia gas and particulate matter eventually return to the earth through wet or dry deposition, contributing to acid rain and the acidification of soils. This redeposition also acts as an unintended fertilizer in natural ecosystems, causing the over-fertilization of nitrogen-sensitive habitats. This excess nitrogen input can negatively alter plant species composition and reduce biodiversity by favoring common, fast-growing species that out-compete native plants.
Key Factors Driving Volatilization Rates
Several environmental and soil conditions interact to accelerate the chemical conversion and loss of nitrogen. High soil \(text{pH}\) is a primary driver, as alkaline conditions directly favor the conversion of ammonium to volatile ammonia gas. While most agricultural soils may not be highly alkaline, the localized \(text{pH}\) spike caused by the urea hydrolysis reaction is enough to initiate significant losses.
Elevated temperatures increase the rate of urease enzyme activity, speeding up the entire hydrolysis process. Studies show that an increase in temperature from \(45^circtext{F}\) to \(60^circtext{F}\) can double the volatilization loss when other factors remain constant. Warm soil also holds less dissolved ammonia, which facilitates its escape from the soil solution into the air.
Moisture is also a factor, as a small amount is required to dissolve the urea granule and activate the urease enzyme. The worst-case scenario occurs when fertilizer is applied to a moist soil surface, followed by warm, dry, and windy conditions without sufficient rain to wash the urea into the soil. Surface application without incorporation is another major factor, as it leaves the urea exposed to the highest concentrations of urease enzyme found in surface residue and thatch.
Strategies for Reducing Nitrogen Loss
Implementing specific management practices can reduce the potential for nitrogen volatilization. The most direct approach is to incorporate the fertilizer into the soil immediately after application, either mechanically through tillage or naturally with rainfall or irrigation. Incorporating the fertilizer moves the urea away from the high urease concentration on the soil surface and allows the ammonium to be bound by the soil’s cation exchange sites, stabilizing it against loss.
Timing the fertilizer application to coincide with an expected rain event is another strategy, as even a small amount of rain, such as \(0.1\) inches, can dissolve and move the nitrogen into the soil profile. Applying fertilizer during cooler periods, such as early spring or late fall, also slows the urease activity and reduces the rate of conversion to ammonia gas.
Chemical additives offer an effective mitigation tool, particularly the use of urease inhibitors. These specialized compounds, such as \(text{N-(n-butyl) thiophosphoric triamide (NBPT)}\), work by temporarily blocking the active site of the urease enzyme. By delaying the hydrolysis of urea, urease inhibitors provide a window of several days for the fertilizer to be incorporated into the soil by rain or irrigation before the rapid conversion to ammonia begins.