Temperature control on biochar decomposition in soil - implications for long-term carbon sequestration

Biochar is an aspirational strategy for long-term carbon sequestration in soil, emergent in guidelines by the Intergovernmental Panel on Climate Change (IPCC). Yet, the rate and pathways of biochar mineralization remain uncertain, and information is scarce on the role of soil temperature. Recent studies predicting the 100-yr stability of biochar in soil use a profile of temperature sensitivity (Q₁₀) for biochar mineralization that deviates markedly from common biochemical temperature relationships, especially at mean annual temperatures of 0-10°C, which prevail in many temperate soils. Here, we compared estimates of biochar stability using (i) empirical Q₁₀ data and (ii) Arrhenius activation energies for biochar mineralization similar to those for other recalcitrant biomolecules. The results indicate that empirical Q₁₀ data used so far overestimate the long-term stability of biochar in soils at 0-10°C, but underestimate the stability at >10°C. The size of these effects increases with higher molar ratio of hydrogen to organic carbon (H/C_org) in the biochar, meaning that predictions for labile biochars are more uncertain. We conclude that care should be taken when normalizing biochar stability data to prevailing soil temperatures and call for further studies to document the temperature sensitivity of biochar mineralization