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

ABSTRACT

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.

Key Words: isotope effect • ¹⁵N • isotope discrimination

¹ Contribution from the Dep. of Soil Sci. and Agr. Eng., Univ. of California, Riverside. Supported in part by Kearney Foundation grant.

² Assistant Professor of Soil Microbiology, Univ. of California, Riverside.

Received for publication May 30, 1972.