Ammonia volatilization from urea
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Urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important r ...
(46-0-0) accounts for more than fifty percent of the world's nitrogenous fertilizers.Schwab, G.J. and L.W. Murdock. Nitrogen Transformation Inhibitors and Controlled Release Urea. Extension Report. Lexington, KY: University of Kentucky College of Agriculture, 2005. It is found in granular or
prill A prill is a small aggregate or globule of a material, most often a dry sphere, formed from a melted liquid through spray crystallization. Prilled is a term used in mining and manufacturing to refer to a product that has been pelletized. ANFO e ...
form, which allows urea to be easily stored, transported and applied in agricultural settings. It is also the cheapest form of granular nitrogen fertilizer. Since urea is not an oxidizer at standard temperature and pressure, it is safer to handle and less of a security risk than other common nitrogen fertilizers, such as
ammonium nitrate Ammonium nitrate is a chemical compound with the chemical formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is ...
. However, if urea is applied to the soil surface, a meaningful fraction of applied fertilizer nitrogen may be lost to the atmosphere as ammonia gas; this only occurs under certain conditions.


Breakdown of urea

For plants to absorb nitrogen from urea it must first be broken down: (NH_)_CO + H_O \stackrel NH_ + H_NCOOH \rightarrow2NH_ + CO_
Urease Ureases (), functionally, belong to the superfamily of amidohydrolases and phosphotriesterases. Ureases are found in numerous bacteria, fungi, algae, plants, and some invertebrates, as well as in soils, as a soil enzyme. They are nickel-containin ...
is a naturally occurring enzyme that catalyzes the hydrolysis of urea to unstable
carbamic acid Carbamic acid, which might also be called aminoformic acid or aminocarboxylic acid, is the chemical compound with the formula . It can be obtained by the reaction of ammonia and carbon dioxide at very low temperatures, which also yields an equ ...
. Rapid decomposition of carbamic acid occurs without enzyme catalysis to form ammonia and carbon dioxide. The ammonia will likely escape to the atmosphere unless it reacts with water to form ammonium (NH4+) according to the following reaction: NH_ + H_O \rightarrow NH^_ + OH^ This is important because
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
is a plant available source of
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
while ammonia is not. Additionally, the formation of the
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
ion may cause soils around the applied urea particle to have a pH around 9.0 which increases ammonia volatilization. This area is also highly toxic due to elevated ammonia concentration for several hours so it is recommended that urea based fertilizers not be applied or banded with planted seed at a rate that exceeds 10–20 kg/ha, depending on the crop species.Wells, K.L., L.W. Murdock and H.F. Miller. Urea as a Source of Fertilizer Nitrogen for Crops in Kentucky. Extension Report. Lexington, KY: University of Kentucky College of Agriculture, 1978. It is important that there is adequate moisture because up to thirty percent of the available nitrogen can be lost through atmospheric volatilization within seventy-two hours of application.


Management considerations

Ammonia volatilization reduces the economic efficiency of agricultural cropping systems. Either yield will be reduced or additional costs will be incurred from additional nitrogen fertilizer. The amount of ammonia volatilization depends on several environmental factors, including temperature, pH, and the soil water content. Additionally, the amount of surface residue and time between urea application and precipitation are also critical. Generally speaking, volatilization will be lower when urea is applied during the wetter and cooler conditions that generally occur in early spring (March and April). However, drying surface soil and rising temperatures as spring progresses increases the probability of ammonia volatilization. Ideally, a manager should attempt to apply nitrogen immediately before a moderate rain event (0.1 inch), allowing urea to dissolve and move into the soil. However, this is not always possible. The soil's pH also has a strong effect on the amount of volatilization. Specifically, highly
alkaline soils Alkali, or Alkaline, soils are clay soils with high pH (greater than 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico- ...
(pH~8.2 or higher) have proven to increase urea hydrolysis. One study has shown complete hydrolysis of urea within two days of application on such soils. In acidic soils (pH 5.2) the urea took twice as long to hydrolyze. Surface residues, such as thatch and plant stubble exhibit increased urease activity. Soils that have high organic matter content also tend to have higher urease concentrations. More urease results in greater hydrolysis of urea and ammonia volatilization, particularly if urea fails to move into the soil.


Urease inhibitors

Fertilizer is often applied when field conditions are not optimal, particularly in large scale operations. Most studies, indicate that nitrogen losses can be reduced in these situations when a urease inhibitor is applied to the fertilizer. Urease inhibitors prevent the urease enzyme from breaking down the urea. This increases the probability that urea will be absorbed into the soil after a rain event rather than volatilized into the atmosphere. This causes subsequent hydrolyzation to occur below the soil surface and decreases atmospheric losses. The use of inhibitors also decreases the localized zones of high pH common with untreated urea.Watson, C.J., et al. "Rate and mode of application of the urease inhibitor N-(n-butyl) thiophosphoric triamide on ammonia volatilization from surface-applied urea." Soil Use and Management, British Society of Soil Science (2008): 1-7.


See also

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Nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biologi ...


References

{{Reflist Ammonia Nitrogen cycle Soil chemistry