Although pyrolysis could be considered as the “burning” of biomass, the absence of oxygen means that most of the carbon in the biomass is retained as carbon. This carbon was extracted from the atmosphere (as CO2) as the plant grew.
Thus Biochar is atmospheric CO2 “made solid”. Scientific consensus is that Biochar will remain stable in soils in the long term (> 1000 years). During the first year that Biochar is incorporated into soil there is some initial carbon loss. However, most of the Biochar will remain in the soil for up to 2000 years. The on-going, long-term and large-scale Terra Preta "experiment" is a further validation of this.