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.
Bibliographical noteThis work was funded by the Natural Environment Research Council
(NE/B500666/1 Advanced Research Fellowship awarded to E.M.
Baggs). We thank Janet Woo and Gaynor Barrett for assistance with