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Abstract Disappearance of NH 4 + from percolating solution in a 40 m 2 plot is at first attributable to both oxidation and to adsorption by the soil. Once a steady state was established and exchangeable NH 4 + was equilibrated with NH 4 + in soil solution, oxidation alone accounted for disappearance of NH 4 + — N in the top 2.5 cm of the soil. Concentration profiles of NO 2 ‐ and NO 3 ‐ are described by a modified Michaelis‐Menten equation. Rate constants for oxidation of NO 2 ‐ → NO 3 ‐ and NH 4 + → NO 2 ‐ are 0.6 × 10 ‐3 and 2.5 × 10 ‐3 ppm/hour·cm 3 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 10 6 /cm 3 of soil but declined to a stable density of about 10 5 organisms/cm 3 ; the latter approached a steady level of about 10 4 organisms/cm 3 . Both organisms showed higher densities near the soil surface where the concentrations of substrates were always highest.
Ardakani et al. (Fri,) studied this question.