Additions of biochar to soil can result in net increases or net decreases in soil organic matter (SOM) stocks. A small change in SOM mineralization could significantly affect SOM stocks and, hence, atmospheric CO2 and other ecosystem services conferred by SOM. Thus, understanding the effects of biochar additions on SOM and – particularly, soil carbon – is critical. Here we review the trends and mechanisms for increases and decreases in soil organic carbon mineralization, as well as the methods and approaches used to study these phenomena and future directions for this area of research.