Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N(2)) gas. The greenhouse gas nitrous oxide (N(2)O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N(2)O. However, N(2) can be formed from two distinct N sources (known as hybrid N(2)) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N(2) produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N(2)O was consumed during N(2) formation. Experiments with gas chromatography indicated N(2) was formed in the presence of live and dead fungi and in the absence of fungi, while N(2)O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N(2) under both anoxic and 20% O(2) atmosphere conditions. Our findings question the assumptions that (1) N(2)O is an intermediate required for N(2) formation, (2) production of N(2) and N(2)O requires anaerobiosis, and (3) hybrid N(2) is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N(2).