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Oxidation of Ammonium to Nitrate in a Soil Column¹

ABSTRACT

A mathematical model based on Michaelis-Menten kinetics for oxidation of ammonium to nitrate during downward flow in a column of soil mixed with sand has been tested. First the column was perfused with nitrite to stimulate the growth of nitrite oxidizing microorganisms in order to decrease the concentration of nitrite at any time in the column during subsequent perfusion with ammonium. The nitrite oxidizers multiplied in a quasilogistic fashion to nearly maximal numbers, exceeding ammonium oxidizers by at least a factor of 10². After a population density of 10³/cm³ was reached for NO₂^- oxidizers, the column was perfused with ammonium solution; in a steady state the ammonium concentration decreased with depth (proportional to time of flow) in accord with Michaelis-Menten kinetics. The rate constants for oxidation of NH₄⁺ were proportional to the numbers of ammonium oxidizing microorganisms extant at any given time of observation; these numbers increased exponentially at first but leveled off after about 3 weeks of continuous perfusion of the column.

As expected, the decline of ammonium concentration with depth during solution flow just equalled the appearance of nitrate with very low concentrations of nitrite in the steady state. From the data the normalized rate constant is 4.5 x 10^-3 ppm cm³/hour per microbe at room temperature, about five times greater than the corresponding figure for nitrite oxidation.

¹ Contribution from the College of Agr. Sci., Univ. of California, Berkeley. Supported by the Kearney Foundation of Soil Science and RANN of the National Science Foundation, Grant no. G134733X.

² Assistant Research Chemist, Graduate Research Assistant, and Professor of Soil Biology, respectively; Dep. of Soils and Plant Nutrition, Univ. of California, Berkeley 94720.

Received for publication June 18, 1973. Accepted for publication September 7, 1973.