Methods for quantification of biochar in soils: A critical review

Biochar is derived from the oxygen-restricted pyrolysis of biomass and is characterized as being biologically stable and having high porosity. Adding biochar to soil is a widely recognized strategy to increase soil carbon (C) stocks and improve soil chemical, physical, and biological properties. Accurate quantification of stable fractions of biochar from soil is vital to assessing its long-term C storage potential for accounting purposes. However, methods specifically developed for quantifying biochar in soils are limited with most current methods adapted from techniques for measuring black carbon (BC). This results in numerous drawbacks for the quantification of biochar. Drawing on existing techniques for determining BC, the potential biochar quantification methods are identified and ordered into four categories: thermal oxidation methods, chemical oxidation methods, benzene poly-carboxylic acids (BPCAs) molecular marker methods, and optical methods. Within this framework, this review then considers factors that might skew measurements, such as loss of biochar and interferences. Measurement principles, technical characteristics, and advantages and disadvantages associated with each of the methodologies are also examined. Ultimately, our objective is to provide researchers with a comprehensive understanding, enabling them to select the most appropriate biochar quantification methods.