Flynn, B., Scott, N. and Dong, Z. 2014. Nitrogen fixation, hydrogen production and N₂O emissions. Can. J. Plant Sci. 94: 1037-1041. H₂ is a by-product of the nitrogenase reaction. Exposure to H₂ is linked to increased N₂O production, increased CO₂ fixation and plant growth promotion in soil. The effects of H₂ exposure on soil were observed using controlled H₂ gas treatments and field trials with legumes. In field trials, increased N₂O production was observed in soil adjacent to legume nodules and inoculation of H₂-oxidizing isolates led to increased N₂O emissions in corn fields. Many H₂-oxidizing isolates tested positive for key denitrification genes, indicating a connection between H₂ uptake and N₂O emissions. H₂ treatment significantly increased copy number of the nitrite reductase (nirK) gene suggesting increased denitrification as the source of N₂O. There was also a significant increase in copy number and expression of the RubisCO (cbbL) gene in soil. H₂-oxidizing bacterial isolates (JM63 and JM162a) were found to promote plant growth, increasing tiller number and yield in spring wheat and barley. Combined results of T-RFLP and 16S rDNA clone libraries analysis revealed bacterial community structure changes in response to H₂ treatment, primarily with increases to the Gammaproteobacteria and Betaproteobacteria groups. The results of these studies help provide a better understanding of the soil bacterial community's responses to H₂ exposure and may lead to the development of a commercially viable plant growth promoting inoculant.