The importance of denitrification performed by nitrogen-fixing bacteria used as inoculants in South America

Background Replacing synthetic fertilizers by biological nitrogen fixation (BNF) is regarded as an environmentally sound practice, but some diazotrophic bacteria are capable of emitting N2O by denitrification. The ability to use nitrate represents an ecological advantage for the survival of some microorganisms under O-2-limiting conditions, but may contribute to increased N2O emissions. Scope The importance of denitrification performed by N-2-fixing bacteria used as inoculants in South America is discussed, especially the possibility of these bacteria act as N2O source or sink. Conclusions There is no doubt of the importance of BNF as a sustainable N source for plants. Through genome investigation, we demonstrated that some strains widely used as inoculants for BNF harbor the entire denitrification pathway to reduce nitrate to N-2. Others contain none, or only some of the denitrification genes, resulting in complete absence of denitrification or production of intermediates such as NO2-, NO or N2O. Evidence of differential effects of bacterial strains on soil N2O were reported, but more studies are still needed to affirm crop inoculation can be a driver for source or sink of this gas. Finally, considerations were made about BNF as an indispensable resource to indirectly mitigate greenhouse gas emissions in agroecosystems.