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The Formation of Nitrate from Ammonium Nitrogen in Soils: I. Effect of Temperature¹

ABSTRACT

The quantitative relationship between temperature and nitrification rates was determined in four different soils under laboratory conditions. The formation of nitrates took place at all temperatures studied between 2 and 35° C. when other factors were favorable. The rate of nitrification increased with temperature with the greatest increase between 7 and 15° C.

In Genesee silt loam (pH 7.7) nitrate nitrogen was formed at rates of 2, 10, 45, 60, 90, and 120 ppm. per week at 2, 7, 15.5, 21, 27, and 35° C., respectively, until the 200 ppm. ammonium nitrogen (NH₄ -N) added was oxidized. The maximum rate in the Mellott C horizon (pH 7.8) was nearly equal to that achieved in the Genesee; the rate in the Chalmers silty clay loam (pH 6.2) was intermediate; the slowest rates found occurred in the Clermont silt loam (pH 5.0) which had maximum rates of 0, 0, 25, 30, 30, and 15 ppm. per week at the temperatures from 2 to 35° C. given above. The initial rate in the Mellott C and Clermont was much slower than the maximum rate. After 4 weeks the rate decreased in the Chalmers and Clermont, probably due to increased acidity.

A marked decrease in the rate of nitrification occurred as the pH dropped below neutrality. Change of pH by adding calcium carbonate did not change the temperature range of nitrification in Clermont silt loam. Differences in temperature range existed between the soils as well as differences in the rate of nitrification.

Temperatures fluctuating in a 24-hour cycle generally resulted in an increased rate of nitrification at temperatures below 15.5° C. and a decreased rate above 15.5° C. in Genesee silt loam.

¹ Journal Paper No. 906, Purdue University Agr. Exp. Sta., Lafayette, Ind. Contribution from the Departments of Agronomy, and Botany and Plant Pathology. Presented before Div. III, Soil Science Society of America, St. Paul, Minn., Nov. 10, 1954.

² Assistant Microbiologist. The assistance of Mr. T. E. Barnes, Agronomy Department, in carrying out the analyses is gratefully acknowledged. The author is now associate Professor of Soils, Department of Agronomy, Iowa State College, Ames, Iowa.

Received for publication September 30, 1955.