You inquired about nitrogen isotope effects of sorption of NH4+. I do not
know much about this, but there is a short review of this in Hubner's
chapter in "Handbook of Enviromental Isotope Geochemistry volume 2" (Fritz
and Fontes eds.) Page 381.
This chapter suggests that there are nitrogen isotope fractionations of 1
to 7 permil associated with sorption onto various clays (sorbed NH4
enriched in N-15).
I have not looked into this subject any further, so you should check the
Ecosystem Sciences Div. - ESPM
151 Hilgard Hall MC#3110
University of California
Berkeley, CA 94720-3110