David Johnson’s excellent talk tackles this very question.
His finding correlates very well with my own readings on soil organic carbon (SOC) reaching a tipping point along with fungal diversity at about 3-4% SOC. It also correlates with biochar inputs and excess dissolved organic carbon (DOC) created in soils with higher than 8% SOC, which produce some of the highest crop yields in studies I’ve read. Higher than 8% SOC creating higher DOC would also mean the carbon can move deeper into the soil profile. So if you were top dressing with biochar then you probably want more than 8% biochar. However the higher DOC also has potential with phosphorus bound to the carbon to cause eutrophication when it reaches rivers and streams, as those elements are the major cause of algal blooms.