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A Model of Competition for Ammonium Among Heterotrophs, Nitrifiers, and Roots¹

ABSTRACT

The purpose of this study was to predict the fate of ammonium nitrogen in soils. Ammonium, either from mineralization of native organic material or from applied fertilizer, is taken up by roots, heterotrophs, and nitrifiers. The density of roots in the soil is assumed to remain constant for time periods which are long relative to microorganism response times. It is assumed that heterotroph populations are generally controlled by some substrate other than N. Ammonium levels in soil are shown to be controlled by nitrifiers except for short periods of time following perturbation of the system. Nitrifier populations are controlled by the flow of NH⁺₄ to the substrate pool. Predicted steady state NH⁺₄ concentrations and nitrifier populations agree well with values which are commonly measured in the field. When the system is perturbed, its return to equilibrium is shown to be described by a simple damped oscillation in NH⁺₄ concentration and nitrifier population. The period and time constant of this damped oscillation appear to agree with published values. Careful measurement of steady-state concentrations and characteristics of the oscillation may allow determination of several important microbe parameters.

¹ Contribution from the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853.

² Asst. Prof., Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853; Prof., Dep. of Agronomy and Soils, Washington State Univ., Pullman, WA 99164; Asst. Prof., School of Physics, Univ. of New South Wales, P.O. Box 1, Kensington, N.S.W., Australia 2033.

Received for publication September 3, 1985.

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