NITROGEN ISOTOPE FRACTIONATION DURING AMMONIUM EXCHANGE REACTIONS WITH SOIL CLAY

Abstract

Nitrogen isotopic fractionation during ammonium adsorption by clay colloids was shown to occur in two time-dependent stages. A rapid ¹⁵N enrichment of the NH₄⁺ in solution was followed by a gradual depletion until, after an 8- to 10-h equilibrium period, the δ_a¹⁵N of the NH₄⁺ in solution was approximately 1; this value was significantly lower than the initial value of 2.3 for the NH₄Cl used in these experiments. The isotopic equilibrium constant (k_e) for the ammonium adsorption reaction was greater than unity, and higher for the Ca²⁺- than K⁺-saturated clay colloids.