Ammonia volatilization from urea

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Urea (46-0-0) accounts for more than fifty percent of the world's nitrogenous fertilizers. [1] 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 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. 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.

Contents

Breakdown of urea

For plants to absorb nitrogen from urea it must first be broken down:

[2]


Urease is a naturally occurring enzyme that catalyzes the hydrolysis of urea to unstable carbamic acid. Rapid decomposition of carbamic acid occurs without enzyme catalysis to form ammonia and carbon dioxide. [2] [3] The ammonia will likely escape to the atmosphere unless it reacts with water to form ammonium (NH4+) according to the following reaction:


This is important because ammonium is a plant available source of nitrogen while ammonia is not. [4] 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 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. [5] 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. [6]

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. [1] 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 (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. [7] 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. [8]

Urease inhibitors

Fertilizer is often applied when field conditions are not optimal, particularly in large scale operations. Most studies, [1] [9] 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. [10]

See also

Related Research Articles

Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two –NH2 groups joined by a carbonyl (C=O) functional group.

Fertilizer Substance added to soils to supply plant nutrients for a better growth

A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply one or more plant nutrients essential to the growth of plants. Many sources of fertilizer exist, both natural and industrially produced.

Nitrogen cycle Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

Ammonium Chemical compound

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+
4. It is formed by the protonation of ammonia (NH3). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations (NR+
4), where one or more hydrogen atoms are replaced by organic groups (indicated by R).

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-containing metalloenzymes of high molecular weight.

Nitrification biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate

Nitrification is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil. Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea. This process was discovered by the Russian microbiologist Sergei Winogradsky.

Carbamate

A carbamate is a category of organic compounds that is formally derived from carbamic acid (NH2COOH). The term includes organic compounds (e.g., the ester ethyl carbamate), formally obtained by replacing one or more of the hydrogen atoms by other organic functional groups; as well as salts with the carbamate anion H
2NCOO−
 (e.g. ammonium carbamate).

Nutrient management

Nutrient management is the science and practice directed to link soil, crop, weather, and hydrologic factors with cultural, irrigation, and soil and water conservation practices to achieve optimal nutrient use efficiency, crop yields, crop quality, and economic returns, while reducing off-site transport of nutrients (fertilizer) that may impact the environment. It involves matching a specific field soil, climate, and crop management conditions to rate, source, timing, and place of nutrient application.

Ammonium sulfate

Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen and 24% sulfur.

Sulfamic acid

Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205 °C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen.

<i>Nitrosomonas</i> Genus of bacteria

Nitrosomonas is a genus of Gram-negative bacteria, belonging to the Betaproteobacteria. It is one of the five genera of ammonia-oxidizing bacteria and, as an obligate chemolithoautotroph, uses ammonia as an energy source and as a carbon source in presence of oxygen. Nitrosomonas are important in the global biogeochemical nitrogen cycle, since they increase the bioavailability of nitrogen to plants and in the denitrification, which is important for the release of nitrous oxide, a powerful greenhouse gas. This microbe is photophobic, and usually generate a biofilm matrix, or form clumps with other microbes, to avoid light. Nitrosomonas can be divided into six lineages: the first one includes the species Nitrosomonas europea, Nitrosomonas eutropha, Nitrosomonas halophila, and Nitrosomonas mobilis. The second lineage presents the species Nitrosomonas communis, N. sp. I and N. sp. II, meanwhile the third lineage includes only Nitrosomonas nitrosa. The fourth lineage includes the species Nitrosomonas ureae and Nitrosomonas oligotropha and the fifth and sixth lineages include the species Nitrosomonas marina, N. sp. III, Nitrosomonas estuarii and Nitrosomonas cryotolerans.

Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as NO
x with the aid of a catalyst into diatomic nitrogen, and water. A reductant, typically anhydrous ammonia, aqueous ammonia or urea solution, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. As the reaction drives toward completion, carbon dioxide, CO
2 is produced.

Berthelot's reagent is an alkaline solution of phenol and hypochlorite, used in analytical chemistry. It is named after its inventor, Marcellin Berthelot. Ammonia reacts with Berthelot's reagent to form a blue product which is used in a colorimetric method for determining ammonia. The reagent can also be used for determining urea. In this case the enzyme urease is used to catalyze the hydrolysis of urea into carbon dioxide and ammonia. The ammonia is then determined with Berthelot's reagent.

Controlled-release fertiliser

A controlled-release fertiliser (CRF) is a granulated fertiliser that releases nutrients gradually into the soil. Controlled-release fertilizer is also known as controlled-availability fertilizer, delayed-release fertilizer, metered-release fertilizer, or slow-acting fertilizer. Usually CRF refers to nitrogen-based fertilizers. Slow- and controlled-release involve only 0.15% of the fertilizer market (1995).

Sporosarcina pasteurii formerly known as Bacillus pasteurii from older taxonomies, is a gram positive bacterium with the ability to precipitate calcite and solidify sand given a calcium source and urea; through the process of microbiologically induced calcite precipitation (MICP) or biological cementation. S. pasteurii has been proposed to be used as an ecologically sound biological construction material. It is a commonly used for MICP since it is non-pathogenic and is able to produce high amounts of the enzyme urease which hydrolyzes urea to carbonate and ammonia.

Sporosarcina ureae is a type of bacteria of the genus Sporosarcina, and is closely related to the genus Bacillus. S. ureae is an aerobic, motile, spore-forming, Gram-positive coccus, originally isolated in the early 20th century from soil. S. ureae is distinguished by its ability to grow in relatively high concentrations of urea through production of at least one exourease, an enzyme that converts urea to ammonia. S. ureae has also been found to sporulate when environmental conditions become unfavorable, and can remain viable for up to a year.

Nitrapyrin

Nitrapyrin is an organic compound with the formula ClC5H3NCCl3. It is a widely used nitrification inhibitor in agriculture as well as a soil bactericide and has been in use since 1974. Nitrapyrin was put up for review by the EPA and deemed safe for use in 2005. Since nitrapyrin is an effective nitrification inhibitor to the bacteria Nitrosomonas it has been shown to drastically the reduce NO2 emissions of soil. Nitrapyrin is a white crystalline solid with a sweet odor and is often mixed with anhydrous ammonia for application.

Diamidophosphate

Diamidophosphate (DAP) is the simplest phosphorodiamidate ion, with formula PO2(NH2)2−. It is a phosphorylating ion and was first used for phosphorylation of sugars in aqueous medium. DAP has attracted interest in the area of primordial chemistry.

N-(<i>n</i>-butyl)thiophosphoric triamide

N-(n-butyl)thiophosphoric triamide (NBPT) is the organophosphorus compound with the formula SP(NH2)2(NHC4H9). A white solid, NBPT is an "enhanced efficiency fertilizer", intended to limit the release of nitrogen-containing gases following fertilization. Regarding its chemical structure, the molecule features tetrahedral phosphorus bonded to sulfur and three amido groups.

Phenyl phosphorodiamidate

Phenyl phosphorodiamidate is an organophosphorus compound with the formula C6H5OP(O)(NH2)2. A white solid, it is used as an inhibitor of urease, an enzyme that accelerates the hydrolysis of urea. In this way, phenyl phosphorodiamidate enhances the effectiveness of urea-based fertilizers. It is a component of the technology of controlled release fertilizers.

References

  1. 1 2 3 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.
  2. 1 2 Tisdale, Samuel L.; Nelson, Werner L.; Beaton, James D. (1985), Soil fertility and fertilizers, New York: Macmillan, pp. 161–168, ISBN   0-02-420830-2
  3. ↑ Benini, Stefano, Wojciech R. Rypniewski, Keith S. Wilson, Silvia Miletti, Stefano Ciurli, and Stefano Mangani. 1999. A new proposal for urease mechanism based on the crystal structures of the native and inhibited enzyme from Bacilus pasteurii: why urea hydrolysis costs two nickels. Structure 7:205-216.
  4. ↑ Brady, Nyle C. and Ray R. Weil. The Nature and Properties of Soils. New York: Prentice Hall, 2001.
  5. ↑ 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.
  6. ↑ McInnes, K.J., et al. "Field measurements of Ammonia Loss from Surface Applications of Urea Solution to Bare Soil." Agronomy Journal (1986): 192-196.
  7. ↑ Christianson, C.B., et al. "Microsite Reactions of Urea-nBTPT Fertilizer on the Soil Surface." Soil Biology and Biochemistry (1993): 1107-1117.
  8. ↑ Torello W.A. and Wehner D.J.. “Urease Activity in a Kentucky Bluegrass Turf.” Agronomy Journal (1983): 654-656.
  9. ↑ 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.
  10. ↑ 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.
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