A critical re-analysis of biochar properties prediction from production parameters and elemental analysis

Biochar is the product of intentional pyrolysis of organic feedstocks. It is made under controlled conditions in order to achieve desired physico-chemical characteristics. These characteristics ultimately affect biochar properties as a soil amendment. When biochar is used for carbon storage, an important property is its persistence in soil, often described by the proportion of biochar carbon remaining in soil after a 100 years ($F_{\text{perm}}$). We analyzed published data on 1230 biochars to re-evaluate the effect of pyrolysis parameters on biochar characteristics and the possibility to predict $F_{\text{perm}}$ from the maximum temperature reached during pyrolysis (HTT). We showed that biochar ash and nitrogen (N) contents were mostly affected by feedstock type. The oxygen to carbon (O:C) and hydrogen to carbon (H:C) ratios were mostly affected by the extent of pyrolysis (a combination of HTT and pyrolysis duration), except for non (ligno)cellulosic feedstocks (plastic waste, sewage sludge). The volatile matter (VM) content was affected by both feedstock type and the extent of pyrolysis. We demonstrated that HTT is the main driver of H:C -- an indicator of persistence -- but that it is not measured accurately enough to precisely predict H:C, let alone persistence. We examined the equations to estimate $F_{\text{perm}}$ available in the literature and showed that $F_{\text{perm}}$ calculated from HTT presented little agreement with $F_{\text{perm}}$ calculated from H:C. The sign and magnitude of the bias depended on the equation used to calculate $F_{\text{perm}}$ and the dispersion was usually large. This could lead to improper compensation of carbon emissions and wrong reporting of carbon sinks in national carbon accounting schemes. We recommend not to use HTT as a predictor for persistence and stress the importance to rapidly develop more accurate proxies of biochar C persistence in soil.