Nitrified leachate recirculation represents a promising strategy for a more sustainable landfill management. The objective of this research was to determine reactions involved in nitrate reduction in laboratory scale batch tests. Three control reactors without nitrate addition showed a classical and good reproducible behavior with acidogenic and methanogenic phases during the 195 days of experiment. In two test reactors, nitrate was added (250 mg N-NO3-/L) at various moments (Day 3, 17, 45 and 143) of the waste degradation process. We concluded that : (1) H2S concentration seems to control the nitrate reduction pathway: above a certain threshold of H2S, Dissimilatory Nitrate Reduction to Ammonium (DNRA) replaced denitrification. (2) N2O/N2 ratio varied with the organic carbon concentration : the lower the easily biodegradable carbon concentration, the higher the N2O/N2 ratio. (3) N2 was consumed after denitrification. The possibility of a nitrogen fixation reaction in the presence of NH4+ is discussed. Nitrified leachate recirculation during acidogenesis should be avoided because of higher H2S production which could induce DNRA.