While understanding of biochar persistence in soil has improved in recent years, the ways in which biochar interacts with non-biochar (native) soil organic matter (SOM) are only beginning to be understood. Research within the last decade has revealed that biochar additions to soil can affect the mineralization of soil organic carbon (SOC). Conversely, the mineralization of biochar in the environment has also been shown to vary with soil type, likely due to the effect of native SOM and other soil properties on the mineralization of biochar. It is important to recognize and study biochar-SOM interactions for a number of reasons. First, these interactions must be understood and quantifi ed so that biochar users or land managers can accurately predict future soil C stocks following biochar amendments, which is particularly important in scenarios where the C impact of biochar application must be properly determined for climate change mitigation purposes. Second,

many important properties of soil depend upon the quantity of organic matter (OM) present, including fertility, water holding capacity and aggregation. Third, any changes in SOC cycling following the addition of biochar will have effects on the cycling of other nutrients associated with SOM, including nitrogen (N) and phosphorus (P), possibly altering nutrient availability to soil microbes or plants, nutrient leaching or gaseous emission rates. Thus, the interactions between biochar and SOM could have a critical effect on most or all of the very benefi ts one hopes to gain through the application of biochar to soil. Research on biochar-SOM interactions is still growing (e.g., this chapter and its subject are a new addition to the fi rst, 2009, edition of this book). This chapter reviews what is known about biochar-SOM interactions, theorizes possible mechanisms for these interactions and suggests future research directions and approaches.