Interpretive Summary: This review examines existing research in determining the stability of biochar in the soil system. All biochars are not the same and possess different physical and/or chemical properties. There are several conflicting conclusions for the residency times of various biochars in soils, ranging from decades to millennia (geologic) time scales. There has been over a 100 year history of examining the stability of charcoals in soils, with limited success of developing an accurate predictor on the stability of the charcoal in the soil environment. Not only do biochars properties influence the rate of decomposition, the soil type and make-up of the soil microorganism consortia will also influence the observed rate of degradation. From evaluating the current biochar degradation studies, there is the suggestion of an overall relationship in stability as a function of the atomic ratio of oxygen to carbon (O:C) in the resulting biochar. The O:C molar ratio is a characteristic property of black carbons that can be used to classify the material and correspondingly the stability of the carbonized material. Biochars with an O:C molar ratio of less than 0.3, typically are the most stable biochars possessing a half-life of >1000 years; biochars with an O:C ratio of 0.3 to 0.6 have intermediate half-lives (100-1000 yrs), and finally biochars with an O:C ratio of greater than 0.6 possess a half-life on the order of <100 years. This finding could provide additional insight and direction in the focus of the benefits of biochar additions on plant and soil microbial communities. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochar properties to minimize greenhouse gas implications and improved soil carbon management.

Technical Abstract:
Biochar is not a structured homogeneous material; rather it possesses a range of chemical structures and a heterogeneous elemental composition. This variability is based on the conditions of pyrolysis and the biomass parent material, with biochar spanning the range of various forms of black carbon. Thereby, this
variability induces a broad spectrum in the observed rates of reactivity and, correspondingly, the overall chemical and microbial stability. From evaluating the current biochar and black carbon degradation studies, there is the suggestion of an overall relationship in biochar stability as a function of the molar ratio of oxygen to carbon (O:C) in the resulting black carbon. In general, a molar ratio of O:C lower than 0.2 appears to provide, at minimum, a 1000-year biochar half-life. The O:C ratio is a function of production temperature, but also accounts for other impacts (e.g., parent material and post-production onditioning/oxidation) that are not captured solely with production temperature. Therefore, the O:C ratio could provide a more robust indicator of biochar stability than production parameters (e.g., pyrolysis temperature and biomass type) or
volatile matter determinations.