Soil applications of recalcitrant biochar offer the possibility of mitigating climate change effects through long-term carbon sequestration and potentially also by reducing emissions of the potent greenhouse gas nitrous oxide (N₂O). This laboratory study examined the effect of combining a fast-pyrolysis biochar at small (1% by mass) and large (3%) concentrations with anaerobically digested slurry on soil N₂O and carbon dioxide (CO₂) emissions over a period of 55 days. The results showed that fast-pyrolysis biochar applied on its own increased N₂O emissions from soil. However, when biochar was applied together with slurry, the larger biochar concentration decreased N₂O emissions by 47%, relative to those from the slurry treatment with the smaller biochar concentration. Reduced N₂O emissions coincided with enhanced soil microbial activity and immobilization of nitrogen. A combined application of biochar and anaerobic digested slurry could therefore be beneficial for cropping systems in terms of soil nitrogen retention while concurrently mitigating N₂O fluxes and sequestering carbon in soil.