Biochar stability assessment methods: A review

Biochar is being developed as a candidate with great potential for climate change mitigation. Sequestering biochar carbon in soil contributes greatly to the reduction of greenhouse gases emissions, and biochar stability is the most decisive factor that determines its carbon sequestration potential. However, methods that can be used universally for direct or indirect assessment of biochar stability are still under investigation. This present review aims to give comprehensive and detailed up-to-date information on the development of biochar stability assessment methods. The method details, advantages and disadvantages, along with the correlations between different methods were reviewed and discussed. Three stability assessment method categories were identified: I) biochar C structure analysis, II) biochar oxidation resistance determination, and III) biochar persistence evaluation by biochar incubation and mineralization rate modelling. Biochar persistence value (e.g., mean residence time, MRT) obtained from incubation and modelling and biochar elemental ratios such as H/C_org and O/C_org are the current most commonly used biochar stability indicators. Incubation and modelling method is too time-consuming while H/C_org and O/C_org ratios are qualitative and conservative, although the effectiveness of these two methods can be further improved. On the other hand, biochar C structures such as aromaticity and degree of aromatic condensation obtained from nuclear magnetic resonance (NMR) analysis and benzene polycarboxylic acids (BPCA) molecular markers and biochar oxidation/degradation recalcitrance obtained from proximate analysis (volatile matter and fixed carbon yields), thermal recalcitrance index (R₅₀), and H₂O₂- and heat-assisted oxidation (Edinburgh stability tool) are being developed as promising proxies to indicate biochar stability.