National initiatives to mitigate and adopt to climate change imply the need for enhancing carbon (C) sequestration in soils. Soil process models often render a C baseline hardly representative and its C sequestration potential questionable when initial soil C pools are not accounted. Here we use the soil process model Roth C to simulate site-specific soil C across 4431 sites in Australia. After matching the modelled and measured C pools at initial equilibrium, the model predicted C sequestration potentials in soils, coherent to changes among the pools, in response to soil inputs that represent soil and residue management. We show that the extent of C sequestration is constrained by C composition that is further influenced by climate and land use. Our results highlight the importance of improving and optimising climate change mitigation and adaptation based on soil C composition