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 15 N/ 14 N 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 15 N/ 14 N 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.
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Shearer et al. (1974) studied this question.