Biochar (char) soil amendments are speculated to be able to lower emission of the greenhouse gas N2O, yet the mechanism through which chars might control soil N cycling is not understood. To disentangle the role of N₂O production and reduction pathways, we incubated fallow soils containing either woody charcoal or activated C with single and double ¹⁵N labelled NH₄NO₃. This revealed a systemic oversight in conventional thought on char’s impact on N₂O emissions: Although char additions did influence soil N₂O emissions, this influence came primarily via ammonia oxidation and not from changes in production or reduction of N₂O during denitrification. The direction and magnitude of this impact was char dependent, and based on the 27% increase in N₂O emissions with woody charcoal additions, we conclude that the net emissions factor for chars could potentially be positive.