Transient competitive complexation in biological kinetic isotope fractionation explains nonsteady isotopic effects: Theory and application to denitrification in soils

The theoretical formulation of biological kinetic isotope fractionation often assumes first‐order or Michaelis‐Menten kinetics, the latter solved under the quasi‐steady state assumption. Both formulations lead to a constant isotope fractionation factor, therefore they may return incorrect estimations of isotopic effects and misleading interpretations of isotopic signatures when fractionation is not a steady process. We have analyzed the isotopic signature of denitrification in biogeochemical soil systems by Menyailo and Hungate (2006) in which high and variable 15 N‐N 2 O enrichment during N 2 O production and inverse isotope fractionation during N 2 O consumption could not be explained with first‐order kinetics and the Rayleigh equation, or with Michaelis‐Menten kinetics. When Michaelis‐Menten kinetics were coupled to Monod kinetics to describe biomass and enzyme dynamics, and the quasi‐steady state assumption was relaxed, transient Michaelis‐Menten‐Monod kinetics accurately reproduced the observed concentrations, and variable and inverse isotope fractionations. These results imply a substantial revision in modeling isotopic effects, suggesting that steady state kinetics such as first‐order, Rayleigh, and classic Michaelis‐Menten kinetics should be superseded by transient kinetics in conjunction with biomass and enzyme dynamics.