There is a need for further studies to compare the decomposition of biochar to that of the original feedstock and determine how these amendments affect the cycling of native organic matter (NOM) of different soils to improve our understanding of the resulting net C sequestration potential. A 510-days incubation experiment was conducted (i) to investigate the evolution of CO2 from soils amended with either fresh corn stover (CS) or with biochars produced from fresh CS at either 350 (CS-350) or 550 degrees C (CS-550), and (ii) to evaluate the priming effect of these amendments on NOM decomposition. Two soil types were studied: an Alfisol and an Andisol, with organic C contents of 4% and 10%, respectively. Except for the controls (with no C addition), all treatments received 7.18t Cha(-1). We measured C efflux in short-term intervals and its isotopic signature to distinguish between C evolved from C-4 amendments and C-3-dominated NOM. Emission rates were then integrated for the whole time period to cover total emissions. Total CO2-C evolved from the original C in fresh CS, CS-350 and CS-550 was greater in the Andisol (78%, 13% and 14%) than in the Alfisol (66%, 8% and 7%). For both soils, (i) no significant differences (P>0.05) were observed in the rate of CO2 evolution between controls and biochar treatments; and (ii) total accumulated CO2 evolved from the uncharred amendment was significantly higher (P<0.05) than that from the other treatments. In the Alfisol, a significant (P<0.05) net positive priming effect on NOM decomposition was observed when amended with fresh CS, while the opposite was detected in biochar treatments. In the Andisol, no significant (P>0.05) net priming effect was observed. A C balance indicated that the C lost from both biochar production and decomposition broke even' with that lost from fresh residue decomposition after <35weeks. The break-even' point was reached earlier in the Andisol, in which the fresh CS mineralizes faster. These results provided experimental evidence for the potential of biochar to sequester C and avoid CO2 emissions from original feedstock while protecting native soil organic matter.