| Summary: | Biochar has been proposed as an amendment that can improve soil conditions, increase harvest yield, and reduce N losses through NO<sub>3</sub><sup>−</sup> leaching and N<sub>2</sub>O emissions. We conducted an experiment to test the hay biochar mitigation effect on N<sub>2</sub>O emissions depending on its production temperature. The pot experiment consisted of the soil amendment with three different production temperature biochars (300 °C, 550 °C, 850 °C) alone and in combination with three different organic fertilizers (cattle slurry, slurry digestate, vinasse), in growth chamber conditions. The effects of biochar and fertilizer were both significant, but the interaction biochar:fertilizer was not. The amendment with the three fertilizer types and with the highest production temperature biochar resulted in significantly higher cumulative N<sub>2</sub>O fluxes. Biochar did not show a mitigation effect on N<sub>2</sub>O emissions when applied with organic fertilizer. Cumulative emissions were higher with biochar addition, with increasing emissions for increasing biochar production temperature. Our results support the idea that biochar cannot be considered as a universal tool for the reduction of N<sub>2</sub>O emissions.
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