29.3.3 Microbial Ureases

Several bacteria, fungi, yeasts, algae, and even plants produce the enzyme urease (Follmer, 2008). This enzyme hydrolyzes urea fertilizer into NH₃ and CO₂ with a rise in soil pH; free N is then released into the atmosphere through NH₃ volatilization (Fazekasova, 2012). It has been reported that urease synthesis takes place constitutively in some microorganisms (Mobley et al., 1995). However, its synthesis is inhibited when cells grow in the media that have an N source such as NH^4 + (Geisseler et al., 2010). Furthermore, its production is increased in the presence of alternative N sources such as urea (Mobley et al., 1995). Soil urease activity has great importance as it modulates the uptake of nitrogen to the plants after the application of urea as fertilizer. The urease enzyme is an ideal candidate that can be widely used to explore the changes in soil fertility, as urease activity increases with the use of organic fertilizers and decreases with soil tillage (Saviozzi et al., 2001). The sources of urease present in the soil are microorganisms and plants, found as both intra- and extracellular enzymes. Alternatively, urease can be extracted from plants or microbes instantly hydrolyzed in the soil by the action of proteolytic enzymes. Urease activity in the soil is affected by various factors such as organic matter content, cropping history of the soil, soil depth, soil amendments, heavy metals, pH, and temperatures (Yang et al., 2006). It is also reported that toxic heavy metals decrease urease activity (Yang et al., 2006). Additionally, urease activity increases with a rise in temperature. As urease has a vital role in the hydrolysis of urea fertilizer, it is important to explore factors that decrease the efficiency of this enzyme in the soil microenvironment.