Urease in the soil: a key element in the nitrogen cycle

Urease is an enzyme that participates in the hydrolysis process of urea added to the soil or native, the main function of this enzyme is to act on Carbon-Nitrogen bonds, in most soils nitrate is the most abundant form, however, in acidic soils ammonium is the predominant ion, because nitrification is strongly limited. The main forms of nitrogen that plants absorb are nitrate (NO3-) and ammonium (NH4+) ions.

- Nitrification: Nitrification is an important stage in the nitrogen cycle in soils caused by the biological oxidation of ammonium with oxygen into nitrite followed by the oxidation of those nitrites into nitrates, in clearer words nitrification is part of the mineralization process, which refers to the complete decomposition of organic matter, with the release of nitrogen compounds available to plants.

In this same sense, urease is a crucial enzyme that plays a fundamental role in soil health and fertility. It is present in both living organisms and soil organic matter, and its main function is the decomposition of urea into ammonium. This conversion is essential for the nitrogen cycle, as it allows plants to assimilate this essential nutrient for their growth.

To understand what is mentioned in the previous paragraph we must know that urea is an organic compound composed of nitrogen and carbon, which is produced as a waste product of the metabolic activity of living organisms. Both plants and animals excrete urea in their urine, being an important source of nitrogen in the soil. But it is also important to keep in mind that there is an inorganic fertilizer known as urea commercially, it usually comes granulated, which is obtained by combining ammonia with CO2 under pressure to form ammonium carbamate, which decomposes into urea and water.

For the application of granulated urea on agricultural surfaces, some producers in the southern area of Lago de Maracaibo, Zulia State Venezuela, use as a method of application alvoleo, that is, they spread it above the soil surface, this can cause a greater loss by volatization, because, when urea comes into contact with the soil water, a hydrolyzation process begins with the help of the urease enzyme that is produced by plants, animals and microorganisms, acting as a catalyst for said process. At the moment that the hydrolysis of urea begins, a compound known as ammonium carbonate (NH4)2CO3 is formed, as this compound is generated, the pH around the fertilizer granule increases, often reaching values higher than eight (08), an action that favors the volatilization of ammonium (NH4+) being lost in the form of ammonia gas (NH3), since as the pH of the soil increases, volatilization increases. In flooded soils, the aforementioned also occurs when urea is applied, because the hydrolysis of it would increase the pH of the water, generating the ideal conditions for the release of ammonia.

Urea
Public domain image taken from

Wikimedia Commons

It is necessary to highlight that the acceleration of hydrolysis happens on the surface of the soils because it is the area where there is the greatest activity of the urease enzyme that catalyzes the hydrolysis of urea, decreasing its action in the deepest areas of the soil, therefore soil moisture has an important role, because urea moves freely with water and can reach the interior of the soil.

Another optimal way to incorporate urea into the soil would be with tillage methods (plow, harrow), if some tillage method is not implemented due to not having the necessary machinery and implements, it is recommended to apply it in surface bands in a concentrated way, that is, in rows throughout the crop concentrating a large amount of fertilizer, this with the aim of minimizing volatilization, since concentrating urea in the bands would be exceeding the soil's capacity to completely hydrolyze it, allowing more time for the nitrogenous element to be incorporated into the interior of the soil where it cannot be volatized.

On the other hand, it is recommended that producers do not apply urea where high contents of vegetable residues are accumulated, due to the high activity of the urease enzyme in said residues, in addition to not allowing direct contact with the soil, for all this it becomes necessary to know what type of nitrogen element to apply with fertilizers, since fertilizers such as ammonium nitrate or ammonium sulfate do not have significant losses due to volatilization.

In acidic soils, there are low rates of mineralization of the soil organic matter, since it contains approximately 5% by weight of nitrogen and only 2 to 5% of the total is released annually by decomposition, in the form of ammonium. This process is faster in warm well-aerated and moist soils (Vernimmen et al., 2007). However, another important form of ammonium release from organic matter is the activity of enzymes from the group of amidohydrolases of which urease is the most important and thus nature has solved the problem of N deficiency in soils with reduced bacterial activity

Urease is one of the different enzymes that exist in the soil, the activity of this enzyme is the result of the reserve or accumulation of it, excretion of the roots and the proliferation of microorganisms with the ability to synthesize it (Bachmeier et al., 2002). Urease intervenes in the hydrolysis of urea-type substrates due to its microbial origin. Its activity is extracellular. This enzyme can form very stable complexes (urease-humus) with soil colloids (Pascual et al., 2002).

The importance of urease in soil
  1. Nitrogen Cycle: Urease is a vital link in the nitrogen cycle, allowing the transformation of urea into a form assimilable by plants.
  2. Soil fertility: Urease activity contributes to soil fertility by increasing the availability of nitrogen for plants.
  3. Ecosystem health: Urease plays an important role in the overall health of the ecosystem by promoting plant growth and the decomposition of organic matter.
Factors that affect the activity of urease
  • Temperature: The activity of urease increases with temperature, being more efficient in hot climates.
  • Humidity: Urease requires a humid environment to function properly.
  • Soil pH: The activity of urease is optimal in soils with a slightly acidic to neutral pH.
Final considerations
Dear readers, urease is an essential enzyme for the nitrogen cycle, soil fertility and the general health of the ecosystem, as it could be observed its activity is affected by various environmental factors and its measurement can serve as an indicator of soil quality. Understanding the role of urease is fundamental for the sustainable management of soils and agriculture.

Bibliographic references
  • Bachmeier, K., Williams A., Warmington J y Banks S. (2002). Urease activity in microbiologically induced calcite precipitation. Journal of Biotechnology 93 171–18

  • Bravo, S. (2000). Chemistry of soils. Barinas: Editorial Board UNELLEZ.

  • Pascual, J., Moreno J., Hernández T, and García C. (2002). Persistence of immobilized and total urease and phosphatase activities in a soil amended with organic wastes. Bioresource and Technology. 82: 73-78.

  • Urbano, P. (2002). Phytotechnics Plant production engineering. Madrid: Mundi-prensa.

  • Vernimmen, R., Verhoef H., Verstraten J., Bruijnzeel L., Klomp N., Zoomer H., Wartenberg P. (2007).Nitrogen mineralization, nitrification and denitrification potential in contrasting lowland rain forest types in Central Kalimantan, Indonesia. Soil Biology & Biochemistry 39:2992–3003.

Sources

- Photography and images: The image referring to the public domain urea whose link is located below the image and the rest of the photographs is the property of the author @ amestyj
- Agrotecnia banner: made by the author @amestyj with own images
- Hive Banner: Designed by the author @amestyj with image owned by hive.



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