Review of the stability of biochar in soils: predictability of O:C molar ratios

TLDR

Biochar is a heterogeneous material whose chemical composition varies with pyrolysis conditions and biomass source, producing diverse black‑carbon forms. The study proposes that the O:C molar ratio could serve as a more reliable indicator of biochar stability than traditional production parameters. The authors find that biochar stability correlates with its O:C ratio, with ratios below 0.2 predicting half‑lives of at least 1000 years.

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 conditioning/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.