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This article in JEQ

  1. Vol. 2 No. 2, p. 247-252
    Received: May 30, 1972

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Isotope Fractionation of 15N and 14N in Microbiological Nitrogen Transformations: a Theoretical Model1

  1. D. D. Focht2



The kinetics for first-order multisequence reactions are derived for microbial fractionation of 14N and 15N isotopes. The isotope effect accounts for unreacted substrate becoming progressively enriched in the heavier isotope due to preferential utilization of the lighter isotope by microorganisms. Consequently, during denitrification nitrate becomes enriched in 15N as its concentration diminishes. This inverse proportional relationship is expressed as a hyperbolic function, y = ax−b. Similar curves are derived for nitrate originating from ammonification and/or nitrification. Regression coefficients for a straight line approximation are better than −0.98, but not as good as the actual coefficients for the derived hyperbolic equations. A negative regression “line” for nitrate occurs for all denitrification reactions during isotope discrimination. Nitrate originating from caesin, ammonium, and nitrate are respectively less enriched in 15N for equal nitrate concentration due to ammonium and nitrate being initially enriched in 14N during ammonification and nitrification, respectively.

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