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Influence of Low pO₂ on Denitrification Processes and Products¹

ABSTRACT

Soil samples were incubated in a laboratory apparatus constructed to maintain constant partial pressures of oxygen. The effect of oxygen on "aerobic denitrification" was studied using N¹⁵-tagged nitrate as a tracer material. Periodic analyses of the gaseous phase for oxygen and volatile nitrogen products were made on a mass spectrometer.

A marked effect of oxygen on the rate of denitrification was exhibited. Losses after a given period of time with 7 to 8.5% oxygen by volume were approximately 20% of those at 4 to 5.7% and 4% of the losses at 1.0% to 1.6%. Both atmospheric oxygen and combined oxygen (nitrate) were utilized but the decreased gaseous loss of nitrogen at the higher levels of free oxygen disclosed a preferential use of the former. The amount of denitrification was closely associated with the level of the biological pressure or need of oxygen by the microbial population. Studies showed that only under conditions of high levels of carbonaceous material, an available nitrate supply, and an oxygen level of < 7% by volume would an appreciable amount of nitrate be reduced to gaseous forms.

Nitrous oxide (N₂O) was produced in the aerobic systems. Processes leading to gaseous losses of nitrogen in the presence of oxygen are discussed.

¹ Contribution from the Soils Department, North Carolina State College and the Soil and Water Conservation Research Division, ARS, USDA. Published with the approval of the Director of the North Carolina Agr. Exp. Sta. as paper No. 1177 of the Journal Series.

² Formerly Soil Scientist, Eastern Soil and Water Management Research Branch, ARS, USDA, now Assistant Professor of Statistics, Iowa State University; and Professor of Soils, North Carolina State College, respectively.

Received for publication September 30, 1960. Accepted for publication April 4, 1961.