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Urease Activity in a Black Spruce Humus Sterilized by Gamma Radiation¹

ABSTRACT

The activity of urease was studied in a black spruce (Picea mariana (Mill.) BSP.) surface organic horizon from Baie-Comeau, Quebec. Samples were sterilized by ⁶⁰Co radiation, incubated with urea for 8 hours at 20C and analyzed for ammonia. Urease activity decreased with an increase in humus decomposition. Field fertilization with 450 kg urea-N/ha 2 years prior to humus sampling increased the urease activity by 40%. Laboratory preincubation of field-fertilized and unfertilized humus with 3,500 ppm of urea-N resulted in increased urease activity of 90%. Enzyme assays performed with 21,000 and 63,000 ppm of urea-N gave similar relative activities to those performed with 3,500 ppm. Some mineralization of N occurred during and following irradiation in the absence of any added substrate. A direct relationship was found between enzyme activity and total and ureolytic numbers of microorganisms.

¹ A contribution of the Department of Forestry and Rural Development of Canada.

² Research Scientist, Department of Forestry and Rural Development of Canada, Sillery, Quebec, and Associate Professor of Microbiology, Macdonald College of McGill University, Ste. Anne de Bellevue, Quebec, respectively.

Received for publication November 1, 1967. Accepted for publication March 5, 1968.