Influence of natural and synthetic humic substances on the activity of urease

The activity of a purified urease, obtained from Bacillus pasteurii, was inhibited by humic and fulvic acids obtained from an agricultural soil. Enzyme kinetic studies showed that the humic substances affected the affinity of the enzyme for its substrate (K_m) and the maximum velocity of the reaction (V_max). The V_max was inhibited to the same extent by both humic (HA) and fulvic (FA) acids, the precise effect depending on the pH and concentration of humic substance. At pH 4.0, HA concentrations of 25 pg cm^?3 and 10 μg cm^?3 inhibited the V_max by 38.5% and 20% respectively. HA and FA had similar effects on the K_m but in this case the lowering of the affinity of the enzyme for its substrate was not concentration dependent in the range 0–25 μg cm^?3 of humic substance. Typically, the affinity was decreased from a K_M of 50 mM in the control to 67 mM in the presence of HA and FA. The effects were not due primarily to the ash or N contents of the humic substances because de-ashed humic acid and synthetic model humic (made from catechol, guaiacol, pyrogallol, resorcinol and protocatechuic acid) and fulvic acid (made from polymaleic acid), containing virtually no ash or N, were equally as effective. The effect was not related to the phenolic monomers which, before polymerization, had no effect on urease activity.     

Kinetic parameters of dehydrogenase in the assessment of the response of soil to vermicompost and inorganic fertilisers

 Kinetic parameters (V _max and K _m) of dehydrogenase activity were determined in order to assess the metabolic response of a soil about 1 year after organic and mineral treatments. The soil was planted with maize (Zea mays) and treated with the following fertilisers: organic (vermicompost; VC), mineral (ammonium phosphate and urea), and an organo-mineral mixture. V _max, which represents a measurement of the quantity of enzyme, markedly increased in organic and organo-mineral treatments, indicating that the addition of organic matter caused an increase in dehydrogenase in the active microbial biomass. K _m, representing enzyme-substrate affinity and/or different sources of the enzymes, was similar in VC-treated soil and control soil, while it doubled in organo-mineral and mineral treatments. These results suggest that the use of VC did not alter the enzyme-substrate affinity, while mineral fertiliser reduced this affinity or changed the composition and activity of soil microbiota. A positive correlation was found between V _max, the metabolic index (dehydrogenase/water-soluble carbon ratio), and the soil organic matter content. The kinetic constants of dehydrogenase activity and the metabolic index may be considered valid parameters to monitor the evolution of microbiological activity in soil. Received: 4 February 2000     

Ermittlung kinetischer Parameter der sauren Phosphatasen intakter Zuckerrübenwurzeln bei variierter Phosphaternährung

Determination of kinetic parameters of acid phosphatases in intact sugar beet roots of variable phosphorus nutrition Organically bound phosphorus has to be hydrolysed before its P can be taken up by plants. Both microbes and plant roots possess phosphatases, which could be of importance especially in soils with low concentrations of inorganic phosphorus in the soil solution. This could be the reason why nutrient uptake models underestimate the P-uptake by plants when P-mobilization by the phosphatases of roots is not taken into consideration. Therefore the activity of acid phosphatases (P_ase) was determined to answer the following questions: 1) To which extent does the root bound acid phosphatase (P_ase) follow Michaelis-Menten kinetics? 2) By which of the four linear transformations of the Michaelis-Menten equation (Lineweaver/Burk, Hanes, Eadie/Hofstee, Eisenthal/Cornish-Bowden) can plausible values of V_max and K_m be determined? 3) Which effect has the P nutrition of the plant on these kinetic parameters? Sugar beet plants were grown in full nutrient solution containing 1 and 100 μM P respectively. The P_ase activity of the intact roots was measured at pH 5 using p-nitrophenylphosphate (25—15000 μM p-NPP). V_max values were calculated per m root length. Acid phosphatase activity principally followed Michaelis-Menten kinetics. Transformations and calculations of V_max and K_m after Eadie/Hofstee and Eisenthal/Cornish-Bowden suggested the existence of at least two enzyme systems (P_{ase 1}, P_{ase 2}). The following kinetic parameters were found: P_{ase 1}: P deficient plants: V_max: 43—45 nmol m^—1 min^—1, K_m: 31—37 μM NPP; P sufficient plants: V_max: 7 nmol m^—1 min^—1, K_m: 47—53 μM NPP. P_{ase 2}: P deficient plants: V_max: 230—293 nmol m^—1 min^—1, K_m: 1579—3845 μM NPP; P sufficient plants: V_max: 123—171 nmol m^—1 min^—1, K_m: 3027—7000 μM NPP. Thus plants with sufficient P nutrition have a lower affinity to P_org and a lower hydrolysis of P_org. For P nutrition of crops P_{ase 1} might be the most important enzyme.     

Microbial community structure and resource availability drive the catalytic efficiency of soil enzymes under land-use change conditions

The turnover of nutrients bound to organic matter is largely mediated by extracellular hydrolytic enzymes (EHEs) produced by soil microorganisms. However, little is known about the environmental drivers (e.g., soil pH, C content, C:N ratio) of the catalytic properties of EHEs and their functional link to the structure of soil microbial communities. We linked catalytic properties, K_m and V_max, determined by Michaelis–Menten kinetics, to a set of environmental and microbial properties in the soils of a land-use sequence ranging from undisturbed natural forest to pastures of different ages and to secondary succession in the Andes of southern Ecuador. The sensitivity of the substrate affinity constant (K_m) and the maximum rate (V_max) of six EHEs (β-cellobiohydrolase (CBH), β-glucosidase (BG), N-acetylglucosaminidase (NAG), α-glucosidase (AG), xylanase (XYL), acid phosphomonoesterase (AP)) to changing environmental conditions was tested by fluorogenic substrates. We used the V_max-to-K_m ratio (K_a) as a proxy for the catalytic efficiency and the signature membrane phospholipid fatty acids as a proxy of microbial community structure.Microbial communities adapted to environmental changes, selected for enzymes with higher substrate affinity (K_m) and catalytic efficiency (K_a) compared with pure cultures. Along the land-use sequence, catalytic efficiency increased from natural forest to young pasture, while it decreased during long-term pasture use and secondary succession. This is consistent with three to five times faster turnover of tested substrates (estimated based on Michaelis–Menten kinetic parameters) at the young pasture compared with the long-term pasture and secondary succession. Environmental drivers of the K_m were enzyme-specific (e.g., the pH for XYL, the C:N ratio for AP, and the C availability for NAG) and differed from those for V_max. A decoupled response of V_max and K_m to land-use changes observed for AG, BG, CBH, XYL, and AP, implies divers consequences for ecosystem processes mediated by these enzymes. A high abundance of Gram(−) bacteria triggered the catalytic properties (K_m and/or K_a) of enzymes decomposing cellulose, hemicellulose, starch, and monophosphoesters. The importance of climatic factors for catalytic properties of EHEs was emphasized by the K_a values extracted from the literature and demonstrated good correspondence of K_a between soils from geographically distinct experimental plots.     

Activity and stability of jack bean urease in the presence of peat humic acids obtained using different extractants

 The effects of two humic acid extractants, 0.1 M Na₄P₂O₇ plus 0.1 M NaOH (NaPP) and 0.1 M NaOH (NaOH), on the activity and stability of a humic-urease complex were studied. The two humic acids isolated (HA_NaPP and HA_NaOH) exhibited different elemental compositions, metal concentrations and structural modifications in the FT-IR spectra. Depending on the pH, HA_NaPP and HA_NaOH influenced both the urease activity and urease kinetic parameters (V _max and K _m) in the same way. They inhibited urease activity between pH 6 and 7 and reduced the V _max of the reaction at pH 6 and 7. The presence of humic acids improved the affinity of the enzyme for the substrate (K _m). The stability of the urease with time, and in the presence of pronase, was improved by HA_NaPP and HA_NaOH with respect to free enzyme. These results confirm the importance of the interaction of urease with humic acids as a fundamental gateway for extracellular urease stabilisation. Since no difference in the extent of urease inhibition and urease stabilisation was observed for the two humic acids, it may be concluded that neither urease activity nor stability are influenced by the humic acid extractant used. Received: 5 November 1999     

Carbon availability regulates soil respiration response to nitrogen and temperature

The response of soil CO₂ fluxes (R_soil) to interactions between carbon (C) and nitrogen (N) availability or C and temperature conditions is not well understood, but may increasingly affect future C storage under the combined anthropogenic impacts of N deposition and climate change. Here we addressed this uncertainty through a series of laboratory incubation experiments using soils from three contrasting ecosystems to investigate how changes in C, N, and temperature regulate R_soil through changes to Michaelis–Menten parameters (i.e. V_max and K_m). Results of this study demonstrate that R_soil response to N enrichment and changes in temperature are dependent on the C availability of soil substrates. N addition influenced R_soil through both the maximum rate (V_max) and the half saturation constant (K_m). The increase in K_m corresponded to a decrease in R_soil when C was limited. Alternatively, when C was abundant, N enrichment increased R_soil, which corresponded to an increase in V_max. Regulation of temperature sensitivity through V_max and K_m was also dependent on C availability. Both V_max and K_m demonstrated positive temperature responses, supporting the hypothesis of a canceling effect at low C concentrations. While temperature sensitivity was influenced by both C quantity and C complexity, our results suggested that C quantity is a stronger predictor. Despite strong differences in climate, vegetation, and management of our soils, C–N and C-temperature interactions were markedly similar between sites, highlighting the importance of C availability in the regulation of R_soil and justifying the use of Michaelis–Menten kinetics in biogeochemical modeling.     

Hydrolases extracted from soil: Kinetic parameters of several enzymes catalysing the same reaction

The kinetic parameters (K_m and V_max) of ureases, phosphatases, casein and benzoylargininae-mide-hydrolysing proteases extracted from two different soils were determined. The Eadie-Scatchard plot, which is the most sensitive graphical technique for detecting deviations from Michaelis-Menten kinetics, was used. In the case of phosphatases extracted from two soils and of the benzoylarginineamide hydrolysing protease extracted from one soil the relationship between V/S and V produced two straight lines. The presence of at least two enzymes (or two forms of the same enzyme) catalysing the same reaction and characterized by markedly different K_m and V_max values, was indicated. A procedure to determine the true constants was reported. The conclusion was that the presence in soil of high and low-affinity enzymes permits an efficient reaction at either low or high substrate concentration.     

Kinetics of soil microbial uptake of free amino acids

 Amino acids and proteins typically form the biggest input of organic-N into most soils and provide a readily available source of C and N for soil microorganisms. Amino acids can also be taken up directly by plant roots, providing an alternative source of available soil N. However, the degree to which plants can compete against the soil microbial population for amino acids in soil solution remains poorly understood. The aim of this study was to measure the rate of microbial uptake of three contrastingly charged ¹⁴C-labelled amino acids (glutamate^1–, glycine⁰, lysine^0.9+) over a wide concentration range (0.1–5 mM) and in two contrastingly managed soils varying in their degree of erosion, organic-C content and microbial biomass. Amino acid uptake was concentration dependent and conformed to a single Michaelis-Menten equation. The mean maximum amino acid uptake rate (V _max) for the non-eroded (control) soil (high organic-C, high biomass) was 0.13±0.02 mmol kg^–1 h^–1, while half maximal uptake occurred at a concentration (K _m) of 2.63±0.07 mM. Typically, V _max was fourfold lower and K _m twofold lower in the eroded soil (low available organic-C, low biomass) compared to the non-eroded (control) soil. Amino acid substrate concentration had little effect on the proportion of amino acid utilized in catabolic versus anabolic metabolism and was similar for both. While the results obtained here represent the summation of kinetics for a mixed soil population, they indicate that amino acid uptake is saturated at concentrations within the millimolar range. Because the affinity constants also were similar to those described for plant roots, we hypothesized that competition for amino acids between plants and microbes will be strong in soil but highly dependent upon the spatial distribution of roots and microbes in soil. Received: 2 March 2000     

亚热带稻田土壤反硝化动力学参数估算

反硝化作用是土壤氮素损失的重要途径,对反硝化潜势的准确估算是农业精准施肥的必然要求。以亚热带典型红壤稻田土作为研究对象,足量添加外源氮进行室内淹水厌氧培养获取反硝化作用动态,并分别用米氏方程和一级动力学方程对其拟合,最后利用土壤基本理化性质对反硝化动力学参数进行估算。结果表明:米氏方程更适合反硝化动力学拟合,最佳的米氏常数(Km)为35mg/kg;米氏最大速率常数(vmax)与一级动力学速率常数(K)具有显著的相关性(r=0.96, P0.05)。土壤总氮,砂粒和粉粒以及土壤容重对vmax影响最大。利用总氮和粉粒含量作为输入参数估算了vmax,准确度达66%。所构建的参数方程既充分挖掘了土壤基础数据潜能,又能快速地获取土壤反硝化动力学曲线,省时省力。     

Soil urease activity and kinetics

The activity and kinetic properties of urease in several Malaysian soils were examined. The values for K_m and V_max of the soils computed according to the Hanes equation were in general agreement with other reports as far as magnitudes were concerned. A significant correlation between K_m and V_max was also obtained. The urease activity of the soils was variable, and it was noted that expression of the activity as the time required to hydrolyze half of the applied urea has limited use in soils of low activity. In all soils studied, inhibition of urease activity was effectively achieved using Ag⁺, while Cu²⁺ was only effective in two soils, and marginally effective in the other two soils. Urease inhibitors have potential applications in reducing volatilization losses of ammonia derived from urea applied to soils.     

Kinetic properties and stability of potato acid phosphatase immobilized on Ca-polygalacturonate

 Acid phosphatase from potato was adsorbed and immobilized on a pre-formed network of Ca-polygalacturonate, a substrate which has a composition and morphology similar to the mucigel present at the root-soil interface. The influence of different types of organic buffers on enzyme adsorption and activity was investigated. The highest enzyme activity, for free and adsorbed enzyme, was obtained with Na-maleate buffer at pH 6.0, which was used for all subsequent experiments. The Michaelis-Menten kinetic parameters, V_max and K_m, were determined for free and adsorbed phosphatase. V_max showed a 60% decrease upon adsorption (2.09±0.30 U/mg, for the soluble form and 0.84±0.15 U/mg, for the adsorbed enzyme), whereas K_m increased from 0.49±0.15 mM for the free enzyme to 0.99±0.20 mM for adsorbed phosphatase. Phosphatase adsorption decreased as the concentration of NaCl increased, indicating that the enzyme is bound to the carrier gel through coulombic interactions. Adsorption increased stability of the enzyme as compared with the free enzyme (t _1/2 of the activity was 9.4 days and 5.8 days, respectively), but increased thermal and proteolytic inactivation. The pH/activity profile revealed no change in terms of shape or optimum pH (4.5) upon adsorption of the enzyme. These results indicate that adsorption of acid phosphatase on Ca-polygalacturonate induces changes in the kinetic properties and stability of the enzyme, and the same type of response can be extrapolated from these results for acid phosphatases of the rhizosphere. Received: 1 July 1997     

Effect of different treatments with crop residues on soil phosphatase activity

Summary Phosphomonoesterase (both acid and alkaline) and phosphodiesterase activity was either activated or inhibited in a soil treated with different crop residues. Phosphotriesterase activity remained unaffected. The kinetic parameters (V _max and K _m) of treated soil samples were modified in the same way: Increases or decreases in the V _max values corresponded to increases or decreases in the K _m values. The V _max values, rather than the K _m values, were found to have a predominant effect on phosphatase activity, thus indicating a fundamental role for the enzyme concentration. A positive and generally significant correlation was found between the activity of each phosphatase, which suggests an unspecific source of these enzymes. The values of the determination coefficients (R ² × 100) show that a low percentage of the variability may be ascribed to interactions among phosphatase activities.     

旱地长期培肥土壤脲酶和碱性磷酸酶动力学及热力学特征研究

研究了连续25年长期培肥试验条件下土壤脲酶和碱性磷酸酶酶促反应的动力学和热力学特征,从酶学角度揭示长期培肥的效应。结果表明,长期培肥增加了脲酶和碱性磷酸酶酶促反应的Vmax、Vmax/Km和k值;降低了Ea、△G、△H和△S值,说明培肥能提高酶促反应速度、减小活化自由能、加快土壤中物质的生物循环过程。酶促反应动力学参数和热力学参数与土壤性质相关分析表明,酶促反应动力学参数大多依赖于土壤化学性状,基于动力学参数的土壤肥力指标体系可评价土壤肥力水平,且U-Vmax、P-Km、P-Vmax可作为土壤肥力的重要指标。     

Effects of soil organic matter on pH-dependent phosphate sorption by soils

Effects of soil organic matter (80M) on P sorption of soils still remain to be clarified because contradictory results have been reported in the literature. In the present study, pH-dependent P sorption on an allophanic Andisol and an alluvial soil was compared with that on hydrogen peroxide (H₂0₂)-treated, acid-oxalate (OX)-treated, and dithionite-citrate- bicarbonate (DCB)-treated soils. Removal of 80M increased or decreased P sorption depending on the equilibrium pH values and soil types. In the H₂O₂ OX-, and DCB-treated soils, P sorption was pH-dependent, but this trend was not conspicuous in the untreated soils. It is likely that 80M affects P sorption of soils through three factors, competitive sorption, inhibition of polymerization and crystallization of metals such as AI and Fe, and flexible structure of metal-80M complexes. As a result, the number of available sites for P sorption would remain relatively constant in the wide range of equilibrium pH values in the presence of 80M. The P sorption characteristics were analyzed at constant equilibrium pH values (4.0 to 7.0) using the Langmuir equation as a local isotherm. The maximum number of available sites for P sorption (Q _max) was pH-dependent in the H₂0₂-, OX-, and DCBtreated soils, while this trend was not conspicuous in the untreated soils. Affinity constants related to binding strength (K) were less affected by the equilibrium pH values, soil types, and soil treatments, and were almost constant (log K ≈ 4.5). These findings support the hypothesis that 80M plays a role in keeping the number of available sites for P sorption relatively constant but does not affect the P sorption affinity. By estimating the Q _max and K values as a function of equilibrium pH values, pH-dependent P sorption was well simulated with four or two adjustable parameters. This empirical model could be useful and convenient for a rough estimation of the pH-dependent P sorption of soils.     

Decomposition of atmospheric hydrogen by soil microorganisms and soil enzymes

The decomposition of atmospheric hydrogen in different types of soil was measured. The decomposition of H₂ was apparently a first-order reaction. H₂ decomposition activity was proportional to the amount of soil with maximum activities at soil water contents of approx. 6–11% (w/w). The activity was lower under anaerobic conditions, but was constant between 1–20% O₂. It was destroyed by autoclaving and was partially inactivated by fumigation with NH₃, CHC1₃ or acetone, by u.v. irradiation and by treatment with NaCN or NaN₃, indicating that biological processes in the soil were responsible for the observed H₂ decomposition. Treatment of soil with toluene or CHCl₃ caused only a partial inactivation. Incubation of soil in the presence of streptomycin or actidione reduced H₂ decomposition by less than 50%, whereas CO consumption was abolished. The H₂ decomposition rates showed H₂ saturation curves with apparent Michaelis-Menten kinetics. Cooperative effects were not observed. V_max was reached at approx. 200 μl1^?1. The K_m values for H₂ were in the range of 30μl 1^?1, but increased to higher values, when the soil had been pretreated with high H₂ mixing ratios. Apparently, the observed H₂ decomposition by soil is not only due to the activity of viable microorganisms, but soil enzymes as well.     

Iron and zinc absorption characteristics and copper inhibitions in cucurbitaceae

Iron and Zn absorption, interactions, and Cu inhibitions were characterized in cucumber (Cucumis sativus L.), watermelon (Citrullus lanatus Thunb.), and pumpkin (Cucurbita moschata Poir.) by kinetic parameters V_max and K_m. Influx and V_max values for Fe and Zn absorption decreased in each species as plant age increased. For the Michaelis constant, K_m, Fe values in cucumber and watermelon and Zn values in watermelon and pumpkin were relatively unchanged with increased plant age. K_m values for Zn absorption in cucumber and Fe absorption in pumpkin decreased as plant age increased. Among species, watermelon appeared to have a particularly effective uptake mechanism for Zn at low solution concentrations. Non‐competitive inhibition of Zn absorption by Fe (20, 50 uM) was indicated in each species. Iron uptake in pumpkin was inhibited non‐competitively by Zn (5, 10 uM), however no significant effects of Zn on Fe absorption were evident in either watermelon or cucumber. Copper (0.5, 1, 5 uM) inhibited uptake of Fe non‐competitively and Zn competitively in each species.     

Abiotic factors regulating nitrification in a Swedish arable soil

Summary The effects of temperature, water potential and ammonium concentrations were studied in field and laboratory experiments on arable soil. The two field experiments used different sampling intervals, one at daily (short-term) and the other at monthly (long-term) intervals. In the short-term field experiment, the numbers and activities of nitrifiers were assessed before and after natural rain or irrigation. The nitrifiers were apparently outcompeted by heterotrophs during the first days after wetting the soil. Potential nitrification was affected only slightly by changes in water potential, whereas the numbers of ammonium and nitrite oxidizers appeared more sensitive to these changes. The numbers of ammonium and nitrite oxidizers correlated strongly during the daily samplings. The potential nitrite-oxidation rates correlated with water potentials whereas the potential ammonium oxidation rates did not. Extractable ammonium decreased in proportion to increasing nitrate concentrations in both the rain-fed and the irrigated plots. In the long-term field experiments, the numbers of ammonium oxidizers correlated with water potentials but not with in situ temperature or with ammonium concentrations. The potential ammonium-oxidation rates correlated with water potentials and with ammonium-oxidizer numbers. The potential nitrite-oxidation rates correlated strongly with the potential ammonium-oxidation rates. The field experiments implied that nitrite oxidizers obtained substrate from ammonium oxidizers but also from nitrate reduction. In laboratory experiments nitrate accumulated at a Q ₁₀ of about 2 and the V _max for nitrification was observed at a water potential of –0.11 MPa (65% of water-holding capacity). The K _m for ammonium oxidation at pH 8.2 was 1.72 mg l^–1 soil water.     

Biodegradation kinetics of monosaccharides and their contribution to basal respiration in tropical forest soils

Low molecular weight (LMW) organic compounds in soil solution are easily biodegradable and could fuel respiration by soil microorganisms. Our main aim was to study the mineralization kinetics of monosaccharides using ¹⁴C-radiolabelled glucose. Based on these data and the soil solution concentrations of monosaccharides, we evaluated the contribution of monosaccharides to basal respiration for a variety of tropical forest soils. Further, the factors controlling the mineralization kinetics of monosaccharides were examined by comparing tropical and temperate forest soils. Monosaccharides comprised on average 5.2 to 47.7% of dissolved organic carbon in soil solution. Their kinetic parameters (V _max and K_M ), which were described by a single Michaelis-Menten equation, varied widely from 11 to 152?nmol?g^?1?h^?1 and 198 to 1294?µmol?L^?1 for tropical soils, and from 182 to 400?nmol?g^?1?h^?1 and 1277 to 3150?µmol?L^?1 for temperate soils, respectively. The values of V _max increased with increasing microbial biomass-C in tropical and temperate soils, while the K_M values had no correlations with soil biological or physicochemical properties. The positive correlation between V _max values and microbial biomass-C indicates that microbial biomass-C is an essential factor to regulate the V _max values in tropical and temperate forest soils. The biodegradation kinetics of monosaccharides indicate that the microbial capacity of monosaccharide mineralization far exceeds its rate at soil solution concentration. Monosaccharides in soil solution are rapidly mineralized, and their mean residence times in this study were very short (0.4–1.9?h) in tropical forests. The rates of monosaccharide mineralization at actual soil solution concentrations made up 22–118% of basal respiration. Probably because of the rapid and continuous production and consumption of monosaccharides, monosaccharide mineralization is shown to be a dominant fraction of basal respiration in tropical forest soils, as well as in temperate and boreal forest soils.     

Contparison of the influence of Cu,Zn, and Cd on the activity and kinetics of free and intntobilized acid phosphatase

The influence of Cu, Zn, and Cd on the activity and kinetics of acid phosphatase immobilized by two soil clays, kaolin or goethite was investigated. The ability of Cu to inhibit the enzyme activity was higher than that of Zn in all the enzyme complexes examined. The ability of Cd was negligible. The inhibitory effects of Cu and Zn on the two soil clay- and kaolin-enzyme complexes were much stronger than those on the goethite-enzyme complex. The V _max and K _m values of the enzyme complexes indicated that both Cu and Zn decreased the maximum reaction velocity of the enzymes, but increased the affinity of the enzymes for the substrate. The degree of the decrease and increase was higher in the Cu systems than in the Zn systems.     

Effect of soil on urease inhibition by substituted urea herbicides

The effects of some substituted urea herbicides, fenuron, monuron, diuron and linuron, on soil urease were investigated. All herbicides are soil urease mixed inhibitors and the same inhibition mechanism is presumed. A kinetic relationship, which takes into account herbicide adsorption, is developed in order to calculate the inhibition constants of soil urease from adsorption constants. A linear relationship between Hammett sigma values and log K_i for fenuron, monuron and diuron is obtained, from which the formation of a complex between herbicides and enzyme is proposed By comparing kinetic constants for soil urease with those obtained for jack bean, in the presence of the same herbicides, a possible effect of the soil matrix on the enzyme-herbicide complex is also suggested.