Previous studies on foliar δ15N values, in certain bryophytes, have indicated signature similarities to source pollutants. The object of this study was to investigate the effect further, by examining the mechanisms whereby isotopic fractionation occurs in systems such as atmospheric ammonia (NH3), throughfall, vegetation and soil. Measurements taken in and around point emission sources will then be used to characterise the various fractionation effects associated with these N transformations, as well as to demonstrate some of the issues associated with using δ15N values as pollution indicators. The atmospheric dispersion model UK-ADMS has also been used to model atmospheric δ15NH3 emissions, with signatures exhibiting marked negative shifts immediately downwind of an agricultural NH3 source. Similar dispersion patterns were mapped for NH3 concentration data illustrating the link between these two forms of measurement. Copyright © 2006 John Wiley & Sons, Ltd.