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Urea is one of the most commonly used nitrogen fertilizers in agricultural lands. The fate of this fertilizer in soils is greatly related to the soil urease enzyme activity, while this enzyme commonly exists in the adsorbed state on the surface of soil colloids, and especially clay minerals. In order to examine the kinetic properties of free and adsorbed Urease on the surface of sepiolite and vermiculite, an experiment was carried out with eight levels of urea concentration (5, 10, 20, 40, 60, 80, 100 and 120 mM). This experiment was done with enzyme concentration of 1 unit in temperature of 25˚C and pH=7. Adsorption of urease on the clay surfaces affected its kinetic properties compared to the free state. The results showed that urease enzyme obeyed Michaelis-Menten kinetic in both free and adsorbed states on the clay surfaces. Except free urease, the highest and lowest values of Vmax and Km were observed in urease-sepiolite and ureasevermiculite complexes, respectively. Also, the results showed that except free urease, the highest levels of Catalytic Efficiency were related to urease in the presence of vermiculite and sepiolite, respectively. The Effectiveness Factor was more for adsorbed urease on the sepiolite surface than adsorbed urease on the vermiculite surface, and the fact somehow suggests more complete diffusion in the presence of sepiolite.

<p>Temperature and soil moisture strongly affect plant root exudates and enzyme activities. Global warming may stimulate root exudation and enzyme activities while drought can drop releasing of root exudates and inhibit enzyme activities. However, how the interaction of warming and drought regulate these processes in the rhizosphere is poorly known. To clarify these interactions, wheat plants were grown for one month at 20 and 30 <sup>º</sup>C in drought (30% WHC) and optimum (70% WHC) condition. To investigate the pattern of root exudates releasing and enzyme activities, we combined β-glucosidase zymography with glucose imaging and enzyme kinetic.</p><p>Drought significantly decreased hotspots of glucose in compare to optimum condition at both temperatures. Releasing of glucose by wheat at 30 <sup>º</sup>C was 53% lower than at 20 <sup>º</sup>C in optimum condition. Hotspots of β-glucosidase activity in drought was 52% and 37.7% lower than in optimum at 20 and 30 <sup>º</sup>C, respectively. β-glucosidase hotspot at 30 <sup>º</sup>C was 12.2% lower than at 20 <sup>º</sup>C in optimum condition. The results of enzyme kinetic (V<sub>max</sub> and K<sub>m</sub>) showed that drought decreased β-glucosidase activity in compare to optimum condition at both temperatures. β-glucosidase activity at 30 <sup>º</sup>C was 2 times higher that at 20 <sup>º</sup>C in optimum condition. On the contrary, it was 56% lower than at 20 <sup>º</sup>C in drought condition. Drought increased K<sub>m</sub> at 20 <sup>º</sup>C while decreased it at 30 <sup>º</sup>C in compare to optimum condition. The affinity of β-glucosidase for substrates in optimum condition was not affected by temperature. K<sub>m</sub> value at 30 <sup>º</sup>C was lower than at 20 <sup>º</sup>C in drought condition. According to these results, the warming in optimum condition (high labile carbon availability) decreased enzyme production and substrates release and did not change the affinity of enzyme for substrates. While warming in drought condition (low labile carbon availability) produced an enzyme pool with high efficiencies and did not change enzyme production and substrates release.</p>