Biochar, the byproduct generated by the pyrolysis of biomass, has been proposed as an effective way of C sequestration into soils, as well as an organic amendment which can help to improve the soil fertility by increasing a wide range of soil attributes. Vineyard field plots were amended with an organic input of 5 MgC ha-1 coming from vineshoots biochar (SB plots), cow manure compost (SC plots) and a mixture of C-equivalent fractions of biochar and compost (SBC plots). Soil samples taken two weeks after the amendment were analyzed to quantify different fractions and attributes of soil organic matter (SOM) and incubated at 21ºC and 50% WHC during 100 days to measure the amount of C lost by mineralization. SC plots displayed the largest amounts of labile C, counted as hot water extractable (664.47±97.58 µg g-1 and 353.19±56.00 µg g-1 in SC and SB plots, respectively) and also by the measure of absorbance at 465 nm (0.031±0.004 % and 0.012±0.001 % in SC and SB plots, in that order), while SB plots showed a slightly larger proportion of C resistant to acid hydrolysis (0.52±0.02 and 0.46±0.03 in SB and SC plots, respectively). Concerning soil respiration along the 100-d incubation, SC plots released the maximum amount of Co2-C (925±88 mg CO2-C kg-1) whereas SB did not significantly increase the production of CO2-C (621±39 mg CO2-C kg-1 and 574±19 mg CO2-C kg-1 in the SB and S plots, respectively). When soil respiration has been computed as a fraction of the SOM (estimated as the weight loss at 375ºC due to the difficulty of quantifying the biochar-C with dichrornate), SB plots exhibited the lowest values (13.85±1.50), whereas the results of the S and SC plots were statistically higher (19.60±0.97 and 19.34±1.50, respectively). No differences were detected between the stability of the SOM of the S and SC plots, suggesting a high maturity of the cow manure compost. The arnounts of K2Cr2O7-reactive and hot water extractable C, the proportion of non hydrolyzable C and the aromaticity of the extractable C measured as the E4/E6 ratio appeared as the main factors affecting the C mineralization. From a management point of view, our results suggest that biochar and compost are compounds that perform different functions in the soil and, therefore, the decision of its use has to be necessarily determined by the effects that management has to achieve. Their combined use can not be discarded, as evidenced by the intennediate behavior of the plots amended with the mixture of biochar and compost.