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Ammonia Volatilization from Urea

Urea(46-0-0) accounts for more than fifty percent of the world’s nitrogenous fertilizers usage. It is found in granular or prill form which allows urea to be easily stored, transported and applied in agricultural settings. It is also the cheapest form of 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.

Breakdown of Urea

For plants to absorb the nitrogen from urea it must be first broken down:
$(NH_{2})_{2(gas)}CO+H_{2}O{\stackrel {urease}{\rightarrow }}2NH_{3(gas)}+CO_{2(gas)}$

Urease is a naturally occurring enzyme that catalyzes the hydrolysis of urea to ammonia. The ammonia will likely escape to the atmosphere unless it reacts with water to form ammonium (NH4+) according to this reaction: $NH_{3(gas)}+H_{2}O\rightarrow NH_{4}^{+}+OH^{-}$

This is important because ammonium is a plant available source of nitrogen while ammonia is not. Additionally, the formation of the hydroxide 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 for several hours so it is recommended that urea based fertilizers not be applied or banded with newly seeded corn. 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 water content of the soil. 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 conditions that generally occur in early spring (March and April). However, as spring progresses and surface soils dry, the probability of ammonia volatilization increases greatly. Ideally, a manager should attempt to apply nitrogen immediately before a moderate rain event, allowing ammonia to react with the rain water and infiltrate 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 (pH~8.2 or higher) has proven to increase hydrolysis in urea. One study has shown a 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 increase the activity of urease present. Soils that have high organic matter content also tend to have higher urease concentrations. More urease results in greater hydrolysis of urea and ultimately ammonia volatilization particularly if urea fails to infiltrate the soil.

Urease Inhibitors

Fertilizer is often applied when field conditions are not optimal particularly in large scale operations. Most studies, (why do you say most but only cite one study)indicate that nitrogen losses can be reduced in these situations when urease inhibitor is applied to the fertilizer. There are several inhibitors on the market but currently the most common is NBPT (N-[n-butyl] thiophosphoric triamide) sold worldwide under the trade name, Agrotain. This fertilizer amendment coats urea which prevents the urease enzyme from breaking down the urea for up to fourteen days. This increases the probability that urea will be absorbed into the soil after a rain event rather than volatilized into the atmosphere. This causes the subsequent hydrolyzation to occur below the soil surface and decrease atmospheric losses. The use of inhibitors also decreases the localized zones of high pH common with untreated urea. This decreases the ammonia volatilization by up to ninety percent. The use of NPBT allows urea to be more efficient, cost effective delivery vehicle for nitrogen soil amendments per unit nitrogen with little added cost.

References

the references are repeating themselves

McCarty, G.W., J.M. Bremmer and H.S. Chai. "Effects of N-(n-butyl) thiophosphoric triamide on hydrolysis of urea by plant, microbial and soil urease." Biology and Fertility of Soils (1989): 123-127.

don't forget this last reference, how about that soil fertility text book