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Laboratory and Greenhouse Investigations of Nitrogen Immobilization¹

ABSTRACT

Immobilization of tracer nitrogen added as ammonium sulfate or as nitrate was followed by analysis of inorganic nitrogen in Moreno sandy loam and Sacramento clay during the course of incubation experiments. In Sacramento clay receiving ammonium sulfate plus straw the nitrifying bacteria were able to compete effectively with the immobilizing flora for nitrogen, whereas in the Moreno soil this was not the case. In both soils immobilization attained a maximum in 6 to 10 days. There was some increase in inorganic nitrogen thereafter, but none of this resulted from remineralization of the tracer nitrogen added initially. In greenhouse experiments competition for added tracer nitrogen between a growing crop and the immobilizing flora emphasized preferential utilization of ammonium nitrogen by soil microorganisms and of nitrate nitrogen by the crop. In Moreno sandy loam, overall recovery of the added tracer was higher under treatments favoring immobilization than in pots receiving only fertilizer.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California, Davis, and the Department of Vegetable Crops, University of California, Riverside. Presented before Div. III, Soil Science Society of America, Nov. 30, 1961, at St. Louis, Mo.

² Professor of Soil Microbiology and Assistant Olericulturist, respectively.

Received for publication December 19, 1961. Accepted for publication March 1, 1962.