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Biochar is the porous, carbonaceous material produced by thermochemical treatment of organic materials in an oxygen-limited environment. In general, most biochar can be considered resistant to chemical and biological decomposition, and therefore suitable for carbon (C) sequestration. However, to assess the C sequestration potential of different types of biochar, a reliable determination of their stability is needed. Several techniques for assessing biochar stability have been proposed, e.g. proximate analysis, oxygen (O): C ratio and hydrogen (H): C ratio; however, none of them are yet widely recognized nor validated for this purpose. Biochar produced from three feedstocks (Pine, Rice husk and Wheat straw) at four temperatures (350, 450, 550 and 650°C) and two heating rates (5 and 100°C min À1 ) was analysed using three methods of stability determination: proximate analysis, ultimate analysis and a new analytical tool developed at the UK Biochar Research Centre known as the Edinburgh accelerated ageing tool (Edinburgh stability tool). As expected, increased pyrolysis temperatures resulted in higher fractions of stable C and total C due to an increased release of volatiles. Data from the Edinburgh stability tool were compared with those obtained by the other methods, i.e. fixed C, volatile matter, O : C and H : C ratios, to investigate potential relationships between them. Results of this comparison showed that there was a strong correlation (R > 0.79) between the stable C determined by the Edinburgh stability tool and fixed C, volatile matter and O : C, however, H : C showed a weaker correlation (R = 0.65). An understanding of the influence of feedstock and production conditions on the long-term stability of biochar is pivotal for its function as a C mitigation measure, as production and use of unstable biochar would result in a relatively rapid return of C into the atmosphere, thus potentially intensifying climate change rather than alleviating it.

Biochar, a solid product of biomass pyrolysis, is a promising concept for climate change mitigation and adaptation, as it can sequester atmospheric CO 2 while improving quality of soil where it is stored. However, for this potential to be realised, it is necessary for biochar to have high environmental stability, i.e., resist various decomposition processes over long time. The main objective of this work has been to relate biochar production conditions to the yield, and properties of biochar, particularly its long-term stability. We used our lab-scale pyrolysis facilities to produce biochar at three temperatures between 350 and 550 °C, from selected feedstock (pine, mixed larch and spruce chips, hardwood pellets). We measured the yield of biochar and then used an accelerated ageing assay to obtain information on the stability of biochar. Such information is very important for the assessment of the climate change mitigation potential of biochar, as it has not yet been clearly defined what proportion of biochar actually remains "permanently" sequestered and how much is released back to the atmosphere in the short to medium term. The results of this work showed that despite increase in the stability of biochar with increasing pyrolysis temperature, the yield of stable biochar fraction is nearly independent of the temperature. These findings are essential for the optimisation of pyrolysis conditions for production of biochar with selected properties, as well as for modelling biochar systems and their climate change mitigation potential as compared to other uses of biomass, such as bioenergy, biofuels and/or chemicals.

Biochar is being actively explored as a tool for long-term soil carbon sequestration. However, in order for this to be effective the long-term environmental stability of biochar must be assured. Here, we define and test an accelerated ageing method that seeks to reflect the oxidative nature of biochar degradation in soil. The method was applied to a systematic set of biochar samples produced from sugarcane bagasse, and a set of biochar samples produced from four different biomass sources. The stability of carbon in these samples was found to range between 41.6% and 76.1%, loosely correlating with biochar O : C ratio (r = 0.73). Increasing intensity of oxidative treatment eliminated more carbon. It also increased surface O : C ratio in a manner reported for naturally aged charcoal samples. The method effectively discriminated biochar produced under contrasting pyrolysis conditions and could be used as a proxy for environmental ageing of approximately 100 years under temperate conditions.

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