Formation of hybrid N2O and hybrid N2 due to codenitrification: First review of a barely considered process of microbially mediated N-nitrosation

At the end of the 19th century an experimental study had already reported N gas production during microbial nitrate reduction, which significantly exceeded the amount of nitrate N supplied to the microorganism. The observed excess gas production was suggested to be caused by a reaction of nitrous acid (produced during microbial nitrate reduction) with amino acids contained in the nutrient solution. Since the 1980’s a number of 15N tracer experiments revealed that this biotic excess gas production was based on the formation of hybrid N2O and/or hybrid N2. It was shown that the N–N linkage forms due to a microbially mediated N-nitrosation reaction by which one N atom of nitrite or nitric oxide combines via a nitrosyl intermediate with one N atom of another N species (e.g., amino compound). Because of its cooccurrence with conventional denitrification this process was later on termed “codenitrification”. Although the phenomenon of N2O and N2 formation by codenitrification was recognised over a century ago its impact on global N cycling is still unclear today. Nonetheless, the present literature review reveals codenitrification as a potentially important process of biospheric N cycling since (i) most codenitrifying species are already known as typical denitrifiers (e.g., Pseudomonas sp., Fusarium sp. etc.) and (ii) codenitrification was already reported to occur within the three domains archaea, bacteria, and eukarya (kingdom fungi). Furthermore, the present literature suggests that codenitrification acts not only as an additional source of N gas formation due to a mobilisation of organic N by N-nitrosation, but also acts as an N immobilising process due to a bonding of inorganic N (e.g., from NO3− or NO2−) onto organic compounds due to e.g., N- or even C-nitrosation reactions. From this it can be concluded that N gas formation by codenitrification represents a sub-phenomenon of a variety of possible biotic nitrosation reactions. Moreover, the review reveals that biotic nitrosation also occurs among nitrifying species, even under aerobic conditions. Furthermore, recent studies support the assumption that even anaerobic ammonium oxidation (anammox) appears to be based on biotically mediated N-nitrosation. Therefore, we propose to introduce the term BioNitrosation, which includes all biotically mediated nitrosation reactions resulting either in N gas release or in N immobilisation, independently from the acting microbial species or the environmental conditions.