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A Kinetic Study of Ammonium and Nitrite Oxidation in a Soil Field Plot¹

ABSTRACT

Disappearance of NH₄⁺ from percolating solution in a 40 m² plot is at first attributable to both oxidation and to adsorption by the soil. Once a steady state was established and exchangeable NH₄⁺ was equilibrated with NH₄⁺ in soil solution, oxidation alone accounted for disappearance of NH₄⁺ — N in the top 2.5 cm of the soil. Concentration profiles of NO₂^- and NO₃^- are described by a modified Michaelis-Menten equation. Rate constants for oxidation of NO₂^- NO₃^- and NH₄⁺ NO₂^- are 0.6 x 10^-3 and 2.5 x 10^-3 ppm/hour·cm³ per bacterium, respectively. These rates are in good agreement with the values found under controlled laboratory conditions.

Growth and distribution in soil of Nitrobacter and Nitrosomonas sp. were followed by weekly estimates of their numbers at different depths. The former reached a maximum population of about 10⁶/cm³ of soil but declined to a stable density of about 10⁵ organisms/cm³; the latter approached a steady level of about 10⁴ organisms/cm³. Both organisms showed higher densities near the soil surface where the concentrations of substrates were always highest.

¹ Contribution from the College of Agr. Sci., Univ. of California, Berkeley. Supported by the Kearney Foundation of Soil Science and in part by the National Science Foundation, RANN GI-34733XI.

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

Received for publication August 28, 1973. Accepted for publication November 27, 1973.