Nitrogen Isotope Fractionation During Ammonium Exchange Reactions With Soil Clay

Nitrogen isotopic fractionation during ammonium adsorption by clay colloids was shown to occur in two time-dependent stages. A rapid 15 N enrichment of the NH 4 + in solution was followed by a gradual depletion until, after an 8- to 10-h equilibrium period, the δ a 15 N of the NH 4 + in solution was approximately 1; this value was significantly lower than the initial value of 2.3 for the NH 4 Cl used in these experiments. The isotopic equilibrium constant (k e ) for the ammonium adsorption reaction was greater than unity, and higher for the Ca 2+ - than K + -saturated clay colloids.