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A Steady-State Model of Isotopic Fractionation Accompanying Nitrogen Transformations in Soil¹

ABSTRACT

A steady-state mathematical model of isotopic fractionation accompanying certain nitrogen transformations in soils is developed. The model takes into account ammonification, nitrification, and immobilization; and predicts that the ¹⁵N/¹⁴N ratios of nitrate and ammonium ion depend in part upon the ratio of the rates of immobilization and ammonification. The ratio of the rates of immobilization to nitrification under field conditions is unlikely to be the same as under the conditions of laboratory incubation experiments. Therefore, this prediction provides a possible explanation for differences observed between the ¹⁵N/¹⁴N ratio of nitrate extracted from soil cores and that of nitrate released during laboratory incubation of the same soils.

The model described is ammenable to experimental test. If verified, an expanded version of the model may be a useful aid in the study of nitrogen transformations in the field.

¹ Contribution from the Center for the Biology of Natural Systems, Washington Univ., St. Louis, Missouri 63130. This research was supported by the National Science Foundation under the Project no. GI-29925 XI. Portions of this paper were presented at the Amer. Soc. of Agronomy 1973 Annual Meeting in Las Vegas, Nevada on 14 Nov. 1973.

² Research Associate, Center for the Biology of Natural Systems; Graduate Research Assistant, Control Systems Science and Engineering Dep. and Center for the Biology of Natural Systems; Associate Professor of Biology, Washington Univ. and Senior Fellow, Center for the Biology of Natural Systems; and Director, Center for the Biology of Natural Systems, Washington Univ., St. Louis, Missouri, respectively.

Received for publication July 25, 1973. Accepted for publication October 24, 1973.