Rate Of Urea Hydrolysis Research Articles

Evaluation of water potential, fertilizer placement and incubation time on volatilization losses of urea in two northern Idaho soils

Abstract The increased use of urea as a nitrogen (N) fertilizer source in northern Idaho may require precautions to prevent potential losses via volatilization. The purpose of this study was to determine the effects of urea placement, water potential and time on volatilization losses of N in two northern Idaho soils under laboratory conditions maintained at 25°C. A completely randomized block factorial design with repeated measures [2 soils x 2 urea placement treatments x 4 soil water potentials x 3 replications‐measured at 0, 2, 4, 8, and 16 d] was utilized in this study. The two soils, a DeVoignes silt loam (fine silty, mixed nonacid, frigid Histic Humaquept) and a Santa silt loam (coarse silty, mixed, frigid Ochreptic Fragixeralf), were maintained at water potentials of ‐0.8, ‐2.0, ‐3.0 or ‐6.0 bars. Urea was either surface applied or incorporated into the soils at the rate of 200 kg/ha. Data were analyzed by SAS‐GLM and Omega squared (ω2) values were used to identify the impact of each main effect and interaction. The 4‐factor interaction of urea placement (NIT) x soil (SL) x water potential (WP) x time of incubation (IT), 4–3 factor interactions, 6–2 factor interactions and 4 main effects were found to be significant in this study. Due to the number of significant sources of variation, ω2 values were used to assess their relative importance. The urea placement treatment accounted for 31.8% of the variation in this study as N losses after 16 d of incubation from the surface urea applications were 28.35% compared to 6.52% where urea was incorporated into the soil. The incubation time x urea placement interaction was the second most important factor and accounted for 21.98% of the observed variability. Volatilization losses were much greater at all incubation times in the surfaceapplied urea treatment when compared to incorporated urea. The differences in N lost by volatilization increased with time. Soil water potential also affected N volatilization as losses increased with increasing soil moisture levels. When the soil water content was not limiting, substantial volatilization occurred. This may be due to an increased urea hydrolysis rate as the moisture content increases. This study demonstrated that volatilization losses may be significant for surface applications of urea on some Idaho soils. To counter these losses incorporation of urea into the soil should be considered.

Kinetics of urea hydrolysis in wheat residue

When urea granules dissolve on crop residue, the concentration near the point of dissolution may be high enough to slow the hydrolysis rate. Our objective was to determine urease activity in wheat residue at a wide range of urea concentrations (0.007-5 M urea). Urease was assayed in the laboratory at 35°C with l g ground wheat residue (1 mm size), toluene, THAM-buffer (pH = 8.0) and urea solutions. The rate of urea hydrolysis was measured as the rate of ammonium appearance. A Michaelis-Menten model modified for uncompetitive substrate inhibition was fitted to the hydrolysis rate data using a non-linear regression procedure Rate = V max(1 + ( K m/[ U]) + ([ U]/ K i) where Rate is the rate of urea hydrolysis, [ U] = urea concentration (m); V max, K m and K i, are the maximum rate (mgNH 4 +-Nkg residue −1 h −1), the concentration at 1 2 V max (M urea), and inhibition constant [or dissociation constant of enzyme substrate complex (m −1) which does not form product], respectively. The values of V max, K m and K i, for the wheat residue urease were 792, 0.003 and 5.94, respectively. The urea hydrolysis rate increased with increasing concentrations up to 0.05 M urea, remained steady between 0.05 and 0.5 m urea, and then declined above 0.5 m urea. The model adequately described the hydrolysis rate up to 2.0 m urea, but overestimated it at higher concentrations. Thus, the decreased rate of urea hydrolysis above 2.0 M urea could not be attributed to uncompetitive inhibition alone but to a combination of factors. Since high urea concentrations cause denaturation of proteins, the rate declines not explained by the model may be due to denaturation of the urease enzyme.

A model for predicting the fate of nitrogen fertilizer in lowland ricefields

SUMMARYA sensitivity analysis of the model is given for conditions in which there is no plant uptake of N and over the range of conditions likely in tropical lowland ricefields, and the mechanisms governing inorganic carbon, nitrogen and acidity dynamics in the floodwater and soil are discussed. Changes in the rate of CO2 production in the soil, floodwater algal activity and floodwater carbonic anhydrase activity altered diurnal changes in floodwater pH but little influenced the daily mean pH, and consequently little influenced cumulative N loss by NH, volatilization. Soil pH buffer power and rate of urea application also little influenced cumulative percentage N loss. Increases in soil ammonium buffer power, floodwater depth and downward water flux, and decreases in floodwater‐ and air‐turbulence, moderately decreased N loss. N loss greatly decreased with decrease in urea hydrolysis rate and with urea incorporation in the soil. Incorporation decreased N loss more in the absence of standing floodwater. With low urease activity, deep floodwater gave consistently lower N loss; with high urease activity, shallow floodwater gave lower N loss when urea was incorporated.

A model for predicting the fate of nitrogen fertilizer in lowland ricefields

SUMMARYEquations are developed for the simultaneous movement and reaction in flooded soils of urea, ammoniacal species, carbonate species, and acids and bases, the simultaneous uptake of N by a developing rice root system, and losses of NH3 and CO2 by volatilization from the floodwater. The equations allow for the initial distribution of urea and other reactants in the soil and floodwater, for movement both by diffusion and convection, and for the rates of urea hydrolysis, floodwater algal activity, organic‐C and ‐N mineralization, and H+ release from roots to balance excess uptake of cations over anions. The equations are combined in a computer model which can predict the fate of N fertilizer in lowland ricefields for any combination of crop and environmental conditions.

Preparation of Alumina Hydrate-Coated SiC Whisker

Alumina hydrate-coated SiC whisker, a starting powder for fabrication of SiC whisker reinforced alumina ceramics, was prepared from SiC whisker and aluminum sulfate by homogeneous precipitation method using urea. Effects of reaction conditions on the coalescence of coated bodies and formation of independent alumina hydrate particles were studied. Observation of coated bodies during reaction showed that coalescence of coated bodies occurs by collision and agglomeration followed by deposition of alumina hydrate on the collided specimens. Coalescence of coated bodies was suppressed by intensive stirring, use of a flow reactor, high [SiC]/[Al2(SO4)3] ratio, low hydrolysis rate of urea, and high urea concentration. Generation of independent alumina hydrate particles was suppressed at a high SiC whisker concentration, a low aluminum sulfate concentration, and a low hydrolysis rate of urea. When the deposition rate of alumina hydrate was very low, coating was not uniform.

Urease activity and kinetics of urea hydrolysis in some soils from semiarid regions of Northwestern India

Abstract Urease activity of the 26 soil samples, collected from semiarid regions of northwestern India ranged from 6.5 to 103.6 μg urea‐N hydrolyzed g‐1 soil h‐1 and was related to organic carbon and clay contents of the soil. Soil urease activity in different soils was represented by Michaelis‐Menten‐type kinetics. Hydrolysis of urea over time followed first‐order kinetics in the three representative soils. The first‐order rate constant k increased with rise in temperature from 8 to 35°C and was greater at field capacity moisture regimes than at the moisture regime on either side of field capacity. Among the three soils, rate of urea hydrolysis was fastest in the soil with the highest urease activity and organic carbon. Thermodynamic properties (energy of activation, free energy, entropy, enthalpy) have also been calculated for the selected soils. Of the two urease inhibitors, N‐(n‐butyl)thiophosphoric triamide (NBPT) was more effective in inhibiting urea hydrolysis than phenyl phosphorodiamidate (PPD).

Urea Transformations in Flooded Soil Columns: I. Experimental Results

AbstractThe main form of fertilizer applied to lowland rice (Oryza sativa L.) is urea, but very little is known about its movement and transformations under flooded soil conditions. Laboratory incubation studies were conducted to measure adsorption, movement, and transformations of urea and hydrolyzed ammoniacal N in flooded soil columns. Urea was added to the floodwater (750 mg N L−1, equivalent to 150 kg N ha−1) of sterilized and nonsterilized Crowley silt loam soil (fine, montmorillonitic, thermic Typic Albaqualf) columns and urea diffusion, urea hydrolysis, and subsequent NH+4‐N diffusion were measured during a period of 30 d. Urea adsorption by the soil increased with increasing concentration of added urea‐N and adsorption coefficients ranged from 0.037 to 0.064 but modeling found adsorption to be too small to be an important factor in urea movement and hydrolysis. Urea hydrolysis rates in the flooded soil columns increased with time and followed first‐order reaction kinetics. Rate constants measured in the soil varied from 0.036 to 0.288 h−1. The diffusion coefficient for both N forms (urea and NH+4) was estimated to be 3.5 × 10−10 m2 s−1 in the flooded soil columns. The basic urea data generated in this study should help provide a data base to model urea‐N behavior in submerged soils.

Assay of urease enzyme activity in an ammonium‐fixing soil

Abstract Rates of substrate disappearance and product formation were compared as measures of urease enzyme activity in an NH4‐fixing and in a non‐fixing soil under tris‐, borate‐ or non‐buffered assay conditions over 4h at 37°C. Tris‐buffered urease activity of the NH4‐fixing soil was 119 μg urea‐N hydrol./g/h or 116 μg (KCl‐extractable) NH4‐N/g/h indicating prevention of NH4 fixation by the buffer; without tris, NH4 production rates amounted to only 35% of coresponding urea hydrolysis rates. Equal rates of urea disappear‐ ance and NH4 formation occurred in the non‐fixing soil irrespective of buffer amendment. Tris‐inhibition of NH4 fixation during 4h incubation at 37°C, however, depended on NH4 Cl rate and buffer strength. 0.025–0.10 M tris (pH 9.0) reduced NH4 fixation to negligible amounts at < 0.03 M NH4C1 whereas, at 0.06–0.24 M NH4Cl, substantial NH4 fixation occurred in the presence of 0.05 M tris; NH4 fixation in unbuffered soil, however, always exceeded that in tris‐buffered soil. Borate buffer (...

Kinetic study of urease‐catalysed urea hydrolysis

AbstractThe kinetics of the urease‐catalysed hydrolysis of urea in phosphate and citrate buffers have been studied. The effects of urease concentration, substrate concentration, pH and temperature of solutions, and particularly the concentrations of buffers and product on the reaction rate of urea hydrolysis have been examined. The activation energies in two buffer systems indicate that the reaction mechanism catalysed by urease may be the same in each case. A substrate inhibition model has been developed that describes the effect of substrate concentration ranging from 0.001 to 4 mol dm−3. The experimental data indicated that the inhibition of urease by phosphate is of partially mixed type, whilst that by citrate is uncompetitive. In contrast ammonium ions exhibited no significant influence on the rate of urea hydrolysis by urease.

Urea hydrolysis in soil: Effects of urea concentration and soil pH

When urea fertilizers are applied in bands, the concentration of urea in soil solution may range from very low at the edge of the bands to very high at the center of the bands. Thus, in order to understand the behavior of banded urea in soil, information is needed regarding the rate of urea hydrolysis under a wide range of urea concentrations. This work was conducted to study the kinetics of urea hydrolysis in samples of Kahola and Reading soils exposed to concentrations ranging from 0.01 to 10 m urea-N, and adjusted to pH values of 5.5, 6.5, 7.5, 8.5 and 9.5. The results were best described by a kinetic model involving two enzymatic reactions, both following simple Michaelis-Menten kinetics but having different affinities for urea. The V max for the high affinity reaction showed a relatively small peak at pH 6.5, followed by a decline and then a sharp increase as the pH increased from 6.5 to 9.5. The V max for the low affinity reaction and the K m values for both reactions showed maximum values at pH 6.5. The urea-N concentration at which both reactions contributed equally to the overall urease activity varied between 0.5 and 1.3 m depending on soil type and pH.

The effect of the urease inhibitor N-(n-butyl) thiophosphoric triamide on the efficiency of urea application in a flooded rice field trial in Thailand

The compound N-(n-butyl) thiophosphoric triamide (nBTPT) was tested for its ability to reduce the rate of urea hydrolysis in applications of urea at 10 d after transplanting flooded rice. The rates of urea hydrolysis were relatively slow, and nBTPT caused a 1-d reduction in the rate of disappearance of urea from the floodwater. Despite this, the vapor pressures of ammonia in the floodwater were significantly lower in the plots with nBTPT than without for the first 5 d following the N application. The vapor pressures of ammonia measured in the afternoons indicate that ammonia volatilization losses were considerable from the treatments without nBTPT and low from the treatments with nBTPT. There was no nitrogen response in this wet-season crop, apparently because of the high availability of N in the soil. N conserved from ammonia volatilization losses by use of the inhibitor was apparently susceptible to denitrification loss, and 50% of the fertilizer was lost in the 37 d following the application of15N-labeled urea both with and without the inhibitor.

Effects of Urea‐Potassium Chloride and Nitrogen Transformations on Ammonia Volatilization from Urea

AbstractAmmonia volatilization following fertilization with urea can be substantial. Losses via NH3 volatilization may be reduced with the concurrent application of KCl, which temporarily reduces soil pH. The objective of this study was to examine the influence of various urea‐KCl mixtures on NH3 volatilization and N transformations in two soils. Urea, granular urea plus KCl, urea plus KCl in solution, and urea coated with powdered KCl were added to a Matapeake silt loam (a fine‐loamy, mixed, mesic Typic Hapludult) and a Hagerstown silty clay loam (a fine, mixed, mesic Typic Hapludalf) at rates equivalent to 260 kg urea ha−1 (120 kg N ha−1) and 260 kg KCl ha−1 (135 kg K ha−1). During the first 13 d of incubation in the Matapeake soil, 26% of the N applied as urea alone was lost via NH3 volatilization. From the KCl‐coated urea, only 13% of the applied N was lost via volatilization. Ammonia losses from the Hagerstown soil were less than from the Matapeake soil but were still significantly reduced by the KCl coating on urea. Differences in NH3 volatilization between the soils were primarily attributed to the rate of urea hydrolysis and subsequent N transformations. These results indicate that coating urea with KCl may offer a significant advantage over uncoated urea, especially in K‐deficient soils.

The Fate of Urea in a Sphagnum Peat Medium as Affected by Lime Source and Rate

Sphagnum peat was blended with CaCO3 or Ca(OH)2 and incubated for 3 weeks at 20C to achieve a pH of ≈ 4.4, 5.4, 6.2, or 7.3. An unlimed control had an initial pH of 3.5. Urea was added to medium treatments at the rate of 125 μg urea-N/cm3. Samples were incubated at 20 ± 1.0C. Medium pH, urea-N, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{2}^{-}-\mathrm{N}\) \end{document}, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{N}\) \end{document} were measured immediately before urea addition (day 0) and 1, 2, 3, 4, 7, 14, 21, and 28 days after urea addition. Medium pH increased when urea was applied for all lime treatments. Medium pH remained at an elevated level, except for the two highest rates of C&amp;CO3, in which pH increased Initially, then decreased. The rate of urea hydrolysis increased as lime rate increased. For both lime sources, urea was completely hydrolyzed within 4 days for the two highest lime rates, except for the highest rate of CaCO3. Nitrite accumulation was evident in the highest lime rate for both lime sources. Nitrate formation was greater with CaCO3 than with Ca(OH)2.

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH4+-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from 15N balance measurements using forced air exchange (airflow rate: 20 L min-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least 15N loss and vice-versa (r=0.99**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83**). The association between potential and actual N loss was closer for high-CEC soils (≥ 20 cmol [+] kg-1 soil, r=0.91**) than for low-CEC soils (<20 cmol [+] kg-1 soil, r=0.85**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.

Soil Urease Activity in Winter Wheat Residue Management Systems

AbstractSurface residue accumulation under reduced and no‐till cropping systems has been shown to cause changes in the activities of several soil enzymes. The addition of crop residues containing urease increases the rate of urea hydrolysis which can increase the potential for ammonia volatilization. Our objective was to characterize the distribution of urease activity in the surface of two Oklahoma soils (Grant silt loam, fine, silty, mixed, thermic Udic Argiustoll and Norge clay loam, fine, silty, mixed, thermic Udic Paleustoll) which had been in conventional, reduced, and no‐till wheat (Triticum aestivum L.) residue management for four consecutive years. Soil cores 60‐mm deep were taken from residue management plots that had been previously fertilized with 168 kg urea‐N ha−1 yr−1 and separated into 10‐mm increments. Urease activity was determined in the soil increments and in mature undecomposed wheat straw. Urease activity was uniformly distributed in soil to a depth of 60 mm under conventional tillage. Significant increases in urease activity were measured in the surface 10 mm of both soils where accumulation of residue had occurred. Average urease activities in the surface 10 mm of soil under conventional, reduced, and no‐till residue management systems were, 12.1, 41.4, and 45.2 µg NH4‐N g−1 h−1 for Norge clay loam and 9.7, 12.4, and 35.0 µg NH4‐N g−1 h−1 for Grant silt loam, respectively. Activity of urease in mature undecomposed wheat straw grown at the site was 376 µg NH4‐N g−1 h−1 which was 28 × larger than the average activity found in soil. Wheat straw was shown to be a major source of the observed increase in enzymatic activity. Other sources for increased urease activity include increased microbial colonization during decomposition of residue and increased urea concentration in microsites due to fertilization, particularly in combination with residue.

EFFECT OF REDOX POTENTIAL ON UREA HYDROLYSIS AND NITRIFICATION IN SOIL SUSPENSIONS

We conducted laboratory experiments to determine urea-N transformations in four rice soils maintained under oxidized and reduced conditions. Urea hydrolysis rates differed significantly between oxidized and reduced soil suspensions. Urease activity showed considerable variation with flooding, and for three soil suspensions hydrolysis rates were faster under aerobic conditions. Urea hydrolysis followed first-order kinetics, and rate constants in oxidized acid soil suspensions ranged from 0.054 to 0.094 h−1 (t1/2 = 7.4 to 13.8 h) and under reducing conditions from 0.028 to 0.062 h−1 (t1/2 = 11.2 to 24.8 h) after urea application of 50 μg N g−1 soil. For the calcareous soil, disappearance of urea was faster under a reducing environment (0.403 h−1), compared with the oxidized rate (0.369 h−1). Ammonium production under reducing conditions ranged from 0.50 to 4.74 μg NH4+-N g−1 h−1, and nitrification in oxidized soil suspensions showed a significant linear relationship over initial sampling periods, and rates ranged from 0.16 to 0.72 μg NO3−-N g−1 h−1.

Temperature effects on soil urease activity

Accurate estimates of the activation energy for soil urease are needed to model how changes in soil temperature quantitatively change the hydrolysis rate of urea applied to soil. Activation energy values reported in the literature for soil urease range from 16.8 to 95.1 kJ mol −1. The wide variation in these values could be due to confounding of temperature effects by other rate-controlling factors in some of these studies. Therefore, we evaluated the effects of temperature on urea hydrolysis rates under laboratory conditions with other rate-controlling factors held constant. Temperatures of 5, 15, 25, 35 and 45°C were used with soil samples from areas mapped as Kahola and Smolan. The soils were limed to pH 8.2 and THAM buffer was adjusted to the same pH to ensure high and stable pH during experiments. A solution to soil ratio of 2:1 was used to maintain a nonlimiting water content. To ensure a reaction rate independent of substrate concentration, a urea concentration of 1000 mg N kg −1 soil was used, and not more than 10% of it was allowed to hydrolyze at each temperature. Increasing temperature from 5 to 45°C increased the urea hydrolysis rate. The mean activation, energies for urease were 49.4 and 53.6 kJ mol 1̄ for Kahola and Smolan soils, respectively. These values correspond to a Q 10 of approx. 2 in the temperature range studied. They differed from some reported values, probably because of differences in assay procedures.

Influence of Fertigation Systems on the Development of Urea Hydrolytic Activity in Soilless Potting Media

Abstract Ureolytic activity was evaluated in uncropped soilless potting media and in media that had been cropped for 4 to 12 weeks with subirrigation or drip fertigation systems in two sequential experiments. Space heating and root-zone heating treatments were compared in one of these experiments. Higher rates of urea hydrolysis tended to develop with drip fertigation than with subirrigation. Increased media temperatures during cropping were associated with increased urea hydrolysis rates. Ureolytic activity was not consistently affected by length of cropping. Interactions of media, fertigation system, and length of cropping indicate the complexity of the development of ureolytic activity in soilless potting media.

Rapid Determination of Urea Hydrolysis and Nitrification in Potting Media

Abstract Rapid assays for urea hydrolysis and for the oxidation of NH4+ to NO2– and NO3– are required for extensive studies of these processes in potting media. Methods originally developed for use with field soils were evaluated and modified for use with soilless media. Effects of urea concentration, buffer pH, and incubation temperature on rates of urea hydrolysis were tested with two media. Response to these variable was less pronounced in soilless potting media than in field soils. Urea hydrolysis can be conveniently assayed in soilless media under the following conditions: 5 mol urea/m3 medium, Tris buffer pH 8.0, 37°C. Rates of NH4+ and NO2– oxidation were estimated from the accumulation of NO2– in samples incubated for 0 to 24 hr with 4 mol NH4+/m3 medium as substrate in 1.0 mM phosphate buffer, pH 7.0, with or without 20 mol NaClO3/m3 medium. The rate of NO2– accumulation in the presence of ClO3–, an inhibitor of NO2– oxidation, estimates NH4+ oxidation, and the slope of the difference in NO2– between samples with and without ClO3– estimates the rate of NO2– oxidation. Although ClO3– does not completely inhibit NO2– oxidation in one of the two media tested, the method is useful for comparison of nitrification activity in potting media.

ANALYSIS OF ENZYMATIC HYDROLYSIS OF UREA IN A SINGLE PARTICLE: EFFECTS OF pH-DEPENDENT KINETICS, IONIC EQUILIBRIA, PRODUCT INHIBITION, AND NERNST-PLANCK DIFFUSION

Abstract A mathematical model is developed to analyze urea hydrolysis in immobilized urease particles. The reaction rate is described by a modified Michaelis-Menten form which takes into account inhibition by ammonium ions and pH-dependent kinetics. The products of urea hydrolysis may exist in several ionic states, which are assumed to be at local equilibrium. A Nernst-Planck diffusion flux expression is used to describe the transport of the charged species, whose diffusivities differ by an order of magnitude. The coupled nonlinear differential equations are solved numerically using orthogonal collocation. Our simulation results indicate that at a bulk urea concentration less than KM (Michaelis-Menten constant), the effectiveness factor approaches that for product-inhibited Michaelis-Menten kinetics. At higher urea concentrations, the reaction rate of urea hydrolysis within a single particle is dominated by the pH dependence of the kinetics. Ionic equilibria of the product species cause the solution pH to...