Different forms of nitrogen: dissolved inorganic-N (or total ammonium-N), suspended organic-N and dissolved organic-N were followed during anaerobic storage of N-15 labelled pig slurry The kinetics of nitrogen transformations were recorded at 0, 1, 3, 7, 14, 28, 63 and 84 days during a laboratory incubation. At day 0, N-15 enriched urea (150 mgN/kg of slurry with a isotopic excess of 50%) was added to the slurry After a rapid total hydrolysis of the urea (within 1 day), a continuous decay of the suspended organic-N fraction was observed concurrently with the increase in the total ammonium-N pool. This mineralization reached 19% of the initial total organic-N fraction, resulting in an increase of the fetal ammoniacal nitrogen content from 2410 to 2780 mgN-NH4+/kg of slurry Labelling techniques allowed measurement of a simultaneous slight immobilization (into the suspended organic-N fraction) of 4.6% of the initial ammoniacal-N.