| Summary: | Biochar soil amendments, along with non-tillage agriculture, are often proposed as a strategy for carbon sequestration. It is still questionable how the quality of biochar might influence the priming effect on soil organic matter and whether the addition of unprocessed organic amendments will affect biochar stability. In the study, six different biochars and three exogenous organic matter sources were added to two distinct arable soils. CO<sub>2</sub> emission was monitored for 100 days of incubation and CO<sub>2</sub> flux was estimated. Results showed that biochar increased soil CO<sub>2</sub> fluxes. The highest peaks, up to 162 µg C-CO<sub>2</sub> h<sup>−1</sup> 100 g<sup>−1</sup>, were recorded in treatments with food waste biochars, suggesting that they serve as a source of easily available carbon to soil microbes. Co-application of raw organic materials (manure and fresh clover biomass) enhanced CO<sub>2</sub> emission and carbon losses, especially in sandy soil, where 0.85–1.1% of total carbon was lost in the short-term experiment. Biochar properties and content of labile C can stimulate CO<sub>2</sub> emission; however, in a long-term period, this contribution is negligible. The findings of our study showed that more attention should be paid to priming effects caused by the addition of exogenous organic matter when applied to biochar-amended soils.
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