Stoichiometric ratio of dissolved organic carbon to nitrate regulates nitrous oxide emission from the biochar-amended soils.

Biochar has the potential to mitigate nitrous oxide (N2O) emissions from soils. However, the mechanisms responsible for N2O emission in biochar-amended soils are yet to be elucidated. In this study, an incubation experiment was carried out to investigate the effects of seven biochars (eucalyptus, softwood mixture, mallee, jarrah, peanut shell, green waste and radiata pine) on the stoichiometric shifts of dissolved organic carbon (DOC), nitrate (NO3--N) and N2O emission in two contrasting soils (Ferrosol with 5.3% total C, 0.46% total N; Tenosol with 0.4% total C, 0.01% total N). All biochar treatments were found to significantly reduce N2O emission in Tenosol by 61-72%. However, in Ferrosol, biochars' impacts on N2O emission were variable, with only peanut shell, green waste and radiata pine bicohars significantly reducing N2O emission by 17-23%. A decrease in NO3- availability in most biochar-amended treatments also was observed in both soils compared with the control. The N2O fluxes in Ferrosol were mainly regulated by the shifts in the availability and stoichiometry of DOC and NO3- induced by the biochar amendments. The DOC derived from biochars increased DOC:NO3- ratio in Ferrosol at the beginning of the experiment, but these effects disappeared 7days after incubation. Overall, the N2O fluxes were C-limited due to the presence of high concentrations of NO3- in Ferrosol. However, in Tenosol, the relationship between stoichiometry of DOC:NO3- and N2O fluxes was much weaker than Ferrosol and N2O fluxes mainly limited by the concentration of NO3-. This study demonstrated that the mechanisms responsible for biochar effects on soil N2O fluxes are considered to be soil and biochar specific.