Nitrate injections were performed during anaerobic digestion of two kinds of wastes in order to better understand the nitrate conversion reactions which can occur in landfill when nitrified leachate is recirculated. Heterotrophic denitrification was the main nitrate conversion reaction observed. When H2S concentration exceeded 0.18 mmol/L in the headspace, denitrification was replaced by DNRA (Dissimilatory Nitrate Reduction to Ammonia). When nitrate injections were performed in reactor containing stabilized waste, autotrophic denitrification was observed. A modification of the waste degradation process was observed with nitrate injections : more dissolved matter and less matter converted as biogas were produced. The three nitrate injections performed at days 3, 17 and 45 delayed the beginning of the methanogenic phase compared to control reactors.