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Ureolysis, Immobilization, and Nitrification in Black Spruce (Picea mariana Mill.) Humus^{1, 2,}

ABSTRACT

The transformations of urea in black spruce humus from an untreated plot and from a plot fertilized with 400 lb urea-N/acre 2 years before sampling were studied using incubation tests. Urea-N (3,500 ppm) added in the laboratory disappeared within 2 days. During the first 3 days, exchangeable ammonium-N equivalent to about 82% of the added urea-N appeared and the pH increased by 1.5 units, but no nitrates were formed. Thereafter, and up to 42 days, there was a very small immobilization of the ammonium-N and a decrease in pH of approximately 0.5 unit. Nitrification occurred only after 28 days, and then only in the field-treated humus when urea was again added in the laboratory. The addition of (NH₄)₂CO₃-N (3,500 ppm) had very similar effects except that a slightly greater rise in pH was accompained by somewhat more immobilization of the ammonium-N than occurred following the addition of urea.

It is concluded that ureolysis was very rapid; that, in spite of the high C/N ratio, immobilization was small; and that, in most treatments, nitrification was negligible, and therefore leaching of nitrates following the fertilization of acid forest soils with 400 lb urea-N/acre is not likely to be a problem.

¹ Presented before the Canadian Society of Microbiologists at Laval University, Quebec, in June 1965. Issued as Macdonald College Journal Series no. 538.

² Study in cooperation with the Pulp and Paper Research Institute of Canada.

³ Research Officer, Canada Department of Forestry.

⁴ Scientific Consultant, Pulp and Paper Research Institute of Canada, and Assistant Professor of Microbiology, Macdonald College of McGill University, Macdonald College P. O., Quebec, Can.

Received for publication August 23, 1965. Accepted for publication November 29, 1965.