Soil, urease

Jackbean urease was immobilized on kaolinite and montmorillonite [98]. The amounts of urease required for maximum immobilization were 70 and 90 mg g 1 of kaolinite and montmorillonite, respectively. The Km values of immobilized urease (25.1-60.8 mM) were of the same order of magnitude as that of free urease (29.4 mM) but one order of magnitude higher than those of soil urease (1.77-2.90 mM). Immobilization of urease on clay surfaces leads to increases in the kinetic constants. 

Fig. 1. Dynamics of urease, acid phosphatase and dehydrogenase activity in soil under Cd pollution (Soil urease activity is expressed as mg NH3-N g 1 dry soil 24 h-1, Soil phosphatase activity is expressed as the mg phenol produced g-1 dry soil 24 h 1, Soil dehydrogenase activity is expressed as mgTPF g-1 dry soil 24 h 1, from Akmal et al. 2005b).

Urea is expected to have very high mobility in soil. Urea is not expected to volatilize from dry soil surfaces based upon its vapor pressure. Various field and laboratory studies have demonstrated that urea degrades rapidly in most soils. Urea is rapidly hydrolyzed to ammonium ions through soil urease activity, which produces volatile gases, that is, ammonia and carbon dioxide. However, the rate of hydrolysis can be much slower depending upon the soil type, moisture content, and urea formulation. 

The reaction is written to illustrate the formation of the ionic species most prevalent at soil pHs. While urea is extremely stable at ambient conditions, soil urease converts urea rapidly often urea fertilizer additions are completely hydrolyzed in 2 to 3 days, a very rapid reaction in soil terms. The rate of hydrolysis depends on the soil temperature, moisture, how dispersed the urea is, and the amount of urease present. Very cold or dry soils hydrolyze urea more slowly than warmer or moist soils. The enzyme activity can be significantly different in different zones of a particular soil. High enzyme activities can develop on a moist soil surface because of algal and other microbial growth on the soil surface. 

As yet there is no direct evidence that in soils urea is hydrolysed even in part via the allophanate pathway. Nevertheless there is a risk in assuming that urea-hydrolysing activities of soils are due entirely to the action of soil ureases. Soils differ appreciably in the stability of their urea-hydrolysing enzymes following incubation or storage and in their activity responses to air-drying, or to toluene addition, or to irradiation to induce soil sterilization. Such treatments may actually increase rather than decrease activities in some soils. Clearly the situation in unfractionated soils is complex even for urea hydrolysis, and may involve different enzymes and enzymic mechanisms as well as enzymes acting in different microenvironments. 

Shaking soil suspensions during assay decreased Km values and increased Vmax values for soil ureases, phosphatases and arylsulphatases . Frankenberger and Tabatabai showed that Km and Vmax values for soil amidases, active against formamide, acetamide and propionamide, ranged with soil type as well as assay substrate. Km values also varied with pH of assay, being lowest at the pH optimum (pH 8.5). 

For soil ureases. Km values decreased as the proportions of soil activity due to the microbial biomass increased following substrate addition, and later increased as the contributions from intact cells diminished. Km values for cellular and adsorbed ureases of a soil were estimated to be 57 mM and 252 mM respectively ". ... 

Some of the most detailed studies of the properties of extracted soil enzymes has been devoted to soil ureases. Early studies by Briggs and Segal reported the cyrstallization of urease-active material (N content, 8.87o) by acetone precipitation from a phosphate buffer extract of a surface forest soil. Ultracentrifugation separated the preparation into three components. Urease activity was maximal at pH 7.1, and was inhibited by p-chloromercuribenzoate and by heavy metal ions (Ag, Hg, Cu ). 

Burns et extracted from soil, clay-free material with urease activity equivalent to about 20% of that of unextracted soil. Partial separation of brown-coloured humic compounds from the extracted ureases could be achieved with little loss of enzymic activity. The extracted soil urease retained activity when incubated for 24h with Pronase, whereas this proteinase was sufficiently stable in the presence of soil to demonstrate the destruction of added jack bean urease. Pronase action also markedly decreased the activities of complexes of bentonite and jack bean urease but was ineffective against a bentonite-urease-lignin complex. The persistence of urease activity in soils was attributed to the formation of exocellular urease-organic colloidal complexes, which permitted reaction between enzyme and substrate and diffusion of products, but which protected ureases from attack by soil proteinases. Further studies by Pettit et compared the properties of extracted ureases, soil ureases and jack bean urease respectively. Km values were influenced by the type of buffer used. Ureases of soil extract had the highest Km value and were the most stable to thermal... 

BURNS R.G., PUKITE A.H. and McLAREN A.D. 1972. Concerning the location and persistence of soil urease. Soil Science Society of America Proceedings, 308-311. 

Relationships between substituted urea herbicides and soil urease activity. Weed Research, 1 365-368. 

MAY P.B. and DOUGLAS L.A. 1976. Assay for soil urease activity. Plant and Soil, 301-305. 

PAL S. and CHHONKAR P.K. 1979. Thermal sensitivity and kinetic properties of soil urease. Journal of the Indian Society of Soil Science, 43-47. 

PAULSON K.N. and KURTZ L.T. 1970. Michaelis constant of soil urease. 

Moreno J.L, Garcia C., Landi L., Falchini L., Pietramellara G., Nannipieri P. The ecological dose value for assessing Cd toxicity on ATP content and DHA and urease activities of soil. Soil Biol Biochem 2001 33 483 189. 

As the one of the main end products of protein metabolism in living organisms, urea is a primary source of organic nitrogen in soil (from animal urine, fertilizers, etc.). Monitoring the level of urea is important for medicine, as well as for environmental protection. Urease is an enzyme that breaks the carbon-nitrogen bond of amides to form carbon dioxide, ammonia and water. This enzyme is widely used for determination of urea in... 

Gianfreda L, Rao MA, Violante A (1992) Adsorption, activity, and kinetic properties of urease on montmorillonite, aluminum hydroxides, and Al(OH)x-montmorillonite complexes. Soil Biol Biochem 24 51-58... 

The second source of biochemicals is molecules excreted from cells such as extracellular enzymes and other organic matter. A typical example is cellulase, which is excreted by fungi such as Penicillium in order to break down wood and woody material into sugars that can be used by the organisms. Other common extracellular enzymes found in soil are ureases and amylases. Often enzymes are associated with clay particles, and in such associations, their activity may be increased, decreased, unchanged, or completely destroyed [15],... 

Because urease activities are much greater in the soil than in the floodwater, the NH4+ is largely formed in the soil as the urea moves downward by mass flow and diffusion. The NH4+, H+ and other reactants will also move between the floodwater and soil-both upward and downward-with NH3 being lost from the floodwater by volatilization. The recovery of N in the crop therefore depends on the rate of movement of urea and its reaction products through the soil and on the rate at which the roots remove N from the downward moving pool. 

Urease activity in soils has been found to reflect the bacterial count and content of organic matter. The urease isolated from an Australian forest soil (87) was crystallized and found to have a specific activity of 75 Sumner units (S.U.) per mg. The molecular weight species were estimated (sedimentation velocity) to be 42, 131, and 217 X 103. That urease activity persists in soils is shown by the finding that enzymic activities, including urease, could be demonstrated in soil samples over 8000 years old (88). 

B.B. Rodriguez, J.A. Bolbot and I.E. Tothill, Urease-glutamic dehydrogenase biosensor for screening heavy metals in water and soil samples, Anal. Bioanal. Chem., 380 (2004) 284r-292. 

Several other proteins show a low, but significant amino acid identity with atzA (Table 22.4). All of these, urease-alpha subunit (urea amidohydrolase), cytosine deaminase, and imidazolone-5-propionate hydrolase, catalyze hydrolytic reactions with substrates involved in the metabolism of nitrogenous compounds (Sadowsky et al 1998). A Rhizobium strain capable of atrazine dechlorination has been isolated from a soil that was previously treated with atrazine (Bouqard et al., 1997). This bacterium could not mineralize atrazine, and it accumulated hydroxyatrazine as the sole metabolite after long-term incubations. Interestingly, 22 of the 24 identified amino acids at the N-terminus of the atrazine halidohydrolase from Rhizobium were identical with atzA from Pseudomonas strain ADP. 

Imamura, A., Yumoto, T., and Yanai, J. (2006). Urease activity in soil as a factor affecting the succession of ammonia fungi. J. Forest Res. 11, 131-135. 

It has not been possible so far to establish that Cr is an essential element required by plants, however, addition of Cr to soils deficient in the element has been shown to increase growth rates and yields of potatoes, maize, rye, wheat or oats (Scharrer and Schropp, 1935 Huffman and Allaway, 1973 Bertrand and De Wolf, 1986). Nickel appears to be an essential element for plants (Farago and Cole, 1988). Zerner and coworkers (Dixon et al., 1975) demonstrated that urease isolated from jack bean (Canavalia ensiformis) was a nickel enzyme. Eskew et al. (1983) have shown that Ni is an essential micronutrient for legumes. Most plants contain nickel in the range 1 - 6 mg kg-1 (Vanselow, 1966 Hutchinson, 1981). The uptake of Ni is enhanced by low pH values, and available nickel increases at pH less than 6.5 as a consequence of the breakdown of Ni complexes in the soil with Fe and Mn oxides. Uptake of nickel by plants and questions of toxicity and tolerance have been reviewed by Farago and Cole (1988). Nickel toxicity toward plants has been reviewed by Vanselow (1966) and Hutchinson (1981). 

Urea undergoes microbial hydrolysis catalyzed by urease, leading to loss of as much as 30% of its nitrogen from ammonia volatilization. The reduced nitrogen availability in the soils appears particularly when urea is surface broadcast on soils. The factors that influence ammonia volatilization include levels of urease activity, moisture availability, nitrification rate, and soil texture (Bernard et al., 2009). 

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