The paper aimed to study the influence of the aeration rate on nitrogen dynamics during the composting of Blackwater sludge with sawdust. The increased aeration was responsible for an increase in ammonia emissions and for the decrease in nitrogen losses through leaching. At high aeration rates, losses of nitrogen in leachates and as ammonia in exhaust gas accounted for 90.99% of the nitrogen removed from the material. At low aeration rates, those accounted for 47.85% of the nitrogen removed from the material. The nitrification rate was found to be proportional to the mean content of NH4+/NH3 in the material, i.e., initial NH4+/NH3 Plus NH4+/NH3 released by ammonification minus the losses in leachates and in exhaust gas. The aeration rate was shown to be a main parameter affecting nitrogen dynamics during composting, since it controlled the ammonification, the ammonia emission and the nitrification processes.