You will certainly want to look at:
Nitrogen isotope fractionation during ammonium exchange reactions with clay minerals, by Karamanos R.E. and Rennie D.A. (1978) in Canadian Journal of
Soil Science, 58, pp 53-60.
They talk about two stages of Ca (or K) replacement by NH4+, with a final "equilibrium" fractionation of 7 to 11 per mil.
NERC Isotope Geosciences Laboratory
Nottingham NG12 5GG UK
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