FACTORS AFFECTING UREASE ACTIVITY IN A BLACK SPRUCE HUMUS STERILIZED BY GAMMA RADIATION

Factors affecting the activity of urease were studied in a black spruce (Picea mariana (Mill.) B S P.) humus sampled at Baie-Comeau, Quebec, and sterilized with a Co-60 source. The enzyme was almost completely inactivated by 4–5,000 kr (about four times the sterilization dose). For a given dose, the measured activity was somewhat lower when a lower radiation flux was used with a longer exposure time. A substrate concentration of 50–100 mg/g urea-N was required for maximum rate of hydrolysis. Even at a much lower concentration (3.5 mg/g), hydrolysis was linear during the first 12 hr of reaction at 20 °C. Urease activity was approximately proportional to temperature in the range 10–40 °C. Activity showed a slight decrease as water content was increased from 60 to 140% of maximum holding capacity. In humus buffered at different pH's, the activity was at a maximum at about pH 7.0. In the absence of buffer, however, activity was higher than when buffered at either the initial or the final pH value of the unbuffered system. Activity did not change as buffer concentration in the samples was increased from 0.25 to 1.00 M.