Development of a gained stability index for describing biochar stability: Relation of high recalcitrance index (R50) with accelerated ageing tests

•Inorganic elements modify the thermogravimetric combustion profile influencing R50 index.•GS index (R50) provides insight into structural changes describing biochar stability.•GS index (R50) and ageing tests link structural changes and permanency in soils.

Carbon stability is an important parameter to measure prior to the application of biochar to soil and in the past few years, several methods have been developed to measure it. The carbon sequestration potential (CO2 reduction) for biochar can be estimated by the recalcitrance index (R50 index). This study investigates further refinement of the R50 index as a measure of carbon stability. The following key findings were observed: 1) prior washing of the biochar in water before measurement of the R50 index influences the measure of char stability determined, particularly for biochar containing high levels of water leachable inorganic materials; 2) a new subscale is proposed to interpret the understanding of the biochar stability changes as a function of pyrolysis temperature defined as “gained stability” and 3) a strong correlation (R = 0.97) was observed between the R50 index and the accelerated ageing test when their function is linearised, providing a better understanding of the carbon stability evaluation.

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