The interaction of biochar-C (BC) with other forms of soil organic carbon (SOC) has produced both SOC priming and stabilization. But the mechanism is sketchy, with both the pre-existing SOC status and the C-chemistry of BC being shown to play a role in the stability and stabilization potential of BC. BC is thought to be a stabile SOC pool because of its inherent chemical recalcitrance. However, it may also promote aggregate formation through physical stabilization mechanisms, as well as associate to minerals. We will present results from a long-term laboratory incubation, where oak-derived BC (delta13C = -27‰) was added at 0, 1, 5, 10, and 20% rate by weight to four soils varying for texture and SOC content, but all characterized by low delta13C values (-12 to -21‰). The isotopic mixing model approach is used in combination with soil organic matter (SOM) physical fractionation to quantify the contribution of BC vs SOC to CO2 losses and soil C stocks in different SOM size-fractions.