Figure 1.

Simplified diagram (adapted from Spott et al.¹⁶, Weeg-Aerssens et al.¹⁸, Schmidt et al.⁵⁵) showing abiotic denitrification, parallel denitrification, sequential denitrification and codenitrification pathways. During abiotic production an electrophile (e.g. the nitrosonium cation NO⁺ which is formed under acidic soil conditions) replaces the hydrogen atom of a nucleophile with a hybrid N-N bond formed following deprotonation. The parallel pathway results in a non-hybrid N-N bond as the result of two NO₂⁻ or two NO molecules being bound, simultaneously to one enzyme (E), which theoretically excludes the possibility of a nitrosation reaction occurring and the formation of a hybrid N-N bond^55,56. However, a two-step process occurs in the sequential pathway when NO₂⁻ or NO molecules initially bind to an enzyme (E) followed by a free NO₂⁻, or NO molecule, (originating from the original NO₃⁻ pool) reacting with the enzyme complexed N species to form a non-hybrid N-N bond. The two-step sequence also permits the enzyme complexed N species to function as an electrophile which is able to be to be attacked by nucleophiles producing a hybrid N-N bond. Nucleophiles able to partake in codenitrification reactions include amines, ammonium, hydrazine, and ammonia.