Fig. 1. Denitrification
Complete denitrification reduces nitrate to dinitrogen gas. The nitrate reductase gene (nar ) is widely distributed in non-denitrifying organisms; nitrite reductase (Nir) is considered the canonical first enzyme of denitrification (Graf, Jones and Hallin, 2014), followed by nitric oxide reductase (NOR) and nitrous oxide reductase (Nos).