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Increased Denitrification in Soils by Additions of Sulfur as an Energy Source¹

ABSTRACT

Denitrification rates were studied in four large soil columns using Hanford sandy loam and Moreno silty clay loam soils. One column of each soil was amended with sulfur to serve as an energy source for the bacterium Thiobacillus denitrificans. Limestone was also added as a pH buffer. The other column of each soil was left untreated to serve as a control. A solution of Ca(NO₃)₂ containing 425 ppm NO₃-N was perfused continuously through the columns. The columns were monitored periodically at depths of 10, 30, 50, 70, and 90 cm for nitrate, nitrite, redox potential (E_h) and microbial numbers. Highly anaerobic conditions developed in all columns as was evidenced by low E_h values at each depth. All of the nitrate was reduced in each column, and nitrates penetrated to lower depths in the untreated columns. Nitrite concentrations were found to be negligible. Denitrification rate constants were established as 0.174, 0.520, 0.186, and 1.426 days^–1, for the Hanford-untreated, Hanford-treated, Morneo-untreated, and Moreno-treated columns, respectively. Sulfur additions to field soils which are low in microbial energy sources could be an effective method of reducing the nitrate level in waters percolating through the profile.

Key Words: redox potential • denitrification rate constants • Thiobacillus denitrificans • autotrophic denitrification • ground water

¹ Contribution from the Department of Soil Science and Agricultural Engineering, Univ. of California, Riverside 92402. Supported in part by Kearney Foundation Grant.

² Research Assistant, Assistant Professor of Soil Science, Staff Research Associate, and Professor of Soil Physics, respectively, Univ. of California, Riverside.

Received for publication December 17, 1971.