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Oxygen Consumption and Denitrification Kinetics in Soil¹

ABSTRACT

Oxygen consumption and CO₂, N₂O, and N₂ production from a slurry of a slightly acidic soil incubated in a closed container with several different concentrations of NO^-₃ were investigated using a mass spectrometer. Under unshaken conditions, the rates of O₂ consumption and CO₂ production changed with time. Nitrous oxide and N₂ gases were produced even though there was ample O₂ in the atmosphere above the stagnant water. Under these conditions the rates of O₂ consumption and N₂O, N₂, and CO₂ production were controlled by the kind and rate of supply of electron acceptors to the sites of denitrification. When well shaken, the rate of O₂ consumption was constant. The CO₂ production rate had two distinctive values. The rate of CO₂ production when O₂ was still in the system was greater than the value obtained after O₂ in the system was all depleted. Nitrous oxide started to appear only when O₂ was completely consumed. Under the shaken conditions, the measured rates of O₂ consumption, CO₂, and N₂O production were independent of NO^-₃ concentration; however, the average life and the maximum concentration of N₂O were directly related to initial NO^-₃ concentration.

It is useful to define a parameter, denitrification intensity (DNI), which is proportional to the potential maximum rate of electron production for a soil with constant microbial activity under fixed temperature. Determination of the DNI must be made when the rate of electron acceptor (NO^-₃ or NO^-₂) supply is not limiting the reaction. Under these conditions, the inhibition of conversion of N₂O to N₂ results in the rate of N₂O-N production which is greater than the rate of production of the sum of N₂O-N and N₂-N without inhibition. If the rate of supply of electron acceptors (NO^-₃ or NO^-₂) is smaller than DNI, a parameter, gaseous nitrogen production rate (GNPR), the magnitude of which is unaffected by the inhibition of conversion of N₂O to N₂, characterizes the denitrification process.

¹ Contribution from the Dep. of Soil Science, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada.

² Professor, Dep. of Soil Science, University of Manitoba.

Received for publication August 14, 1981. Accepted for publication April 8, 1982.

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