Denitrification is the anaerobic respiration by which mainly NO3- and NO2- are reduced to nitric oxide (NO), nitrous oxide (N2O) and dinitrogen gas (N2). The different steps require different enzymes: nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase, respectively. When nitrate and nitrite are converted to dinitrogen it means that nitrogen is lost from the soil ecosystem to the atmosphere. Denitrification occurs only when oxygen is depleted. Anaerobic conditions trigger the bacteria to use nitrate as an alternative to oxygen as the terminal electron acceptor in their respiration. Denitrification is carried out by a large variety of heterotrophic bacteria, the most common ones thought to be Clostridium, Pseudomonas and Bacillus.
Figure 1. The denitrifiction pathway.