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Mineralization and Immobilization of Nitrogen in Bacterial Cells and in Certain Soil Organic Fractions¹

ABSTRACT

Nitrogen¹⁵-enriched Pseudomonas cells and (NH₄)₂SO₄ were added separately to a Chicot sandy loam and to a black spruce raw humus (F horizon). At intervals from zero to 100 days the soil N was fractionated (Cheng and Kurtz, 1963) into: exchangeable NH₄⁺- + NO₃^--N, fixed NH₄⁺-N, N in acid hydrolyzate (i.e. NH₄⁺ from hydrolysis, amino sugar-N, amino acid-N, acid-soluble alkali-insoluble humin-N), and acid insoluble humin-N.

At zero time 92.5 and 93.0% of the added NH₄⁺-N appeared in the exchangeable fraction of the two soils, respectively; 55.3 and 83.7% of the added cell-N appeared in the amino acid fraction of the two soils, respectively. During incubation in both soils about 40% of the added NH₄⁺-N remained in the exchangeable fraction and 50 to 60% was immobilized in the amino acid fraction. In the raw humus about 13% of the added NH₄⁺-N appeared in the insoluble humin fraction. In both soils 20–30% of the added cell-N was rapidly mineralized but at least 50% remained in the amino acid fraction. There was evidence of a "priming effect" or "N interchange" resulting in increased mineralization of native soil organic N. This effect was particularly pronounced in the raw humus.

¹ Contribution from the Department of Microbiology, Macdonald College of McGill University, Macdonald College P.O., Quebec, Canada. This work was supported by the Quebec Agricultural Research Council Iissued as Macdonald College Journal Series No. 537.

² Graduate Student and Assistant Professor of Microbiology, respectively.

Received for publication August 6, 1965. Accepted for publication October 29, 1965.