The addition of exogenous ammoniacal nitrogen to slurry can be used to study the influence of ammoniacal nitrogen concentration or the presence of different ions on ammonia emissions. This preliminary study aimed to test the validity of such a direct incorporation of ammonium salts to slurries in relation to ammonia volatilization. It was conducted using cylindrical glass volatilization chambers and N-15-labelling techniques. Labelled pig slurry was prepared by direct addition of N-15-(NH4)2SO4 (N-15-AS). The fate of total ammoniacal nitrogen was followed for 48 h from surface application to soil. The analysis of isotopic excess (E%) of volatilized ammonia 0.25, 2, 6, 12, 24 and 48 h after labelled slurry application suggested that the added N4+ behaved as the endogenous NH4+ initially present in the slurry. An average TAN-N-15 (total ammoniacal nitrogen) recovery of 97.5% (CV 2%) was obtained. During this study, no immobilization or nitrification of slurry-derived nitrogen was noticed.