Summary

Biochar is a charcoal that is used to improve land rather than as a fuel. Biochar is produced from biomass, usually through the process of pyrolysis. Due to the molecular structure and strength of the chemical bonds, the carbon in biochar is in a stable form and not readily mineralized to CO₂ (as is the fate of most of the carbon in biomass). Because the carbon in biochar derives (via photosynthesis) from atmospheric CO₂, biochar has the potential to be a net negative carbon technology/carbon dioxide removal option. Biochar is not a single homogeneous material. Its composition and properties (including longevity) differ according to feedstock (source biomass), pyrolysis (production) conditions, and its intended application. This variety and heterogeneity have so far eluded an agreed methodology for calculating biochar’s carbon abatement.

Meta-analyses increasingly summarize the effects of biochar in pot and field trials. These results illuminate that biochar may have important agronomic benefits in poorer acidic tropical and subtropical soils, with one study indicating an average 25% yield increase across all trials. In temperate soils the impact is modest to trivial and the same study found no significant impact on crop yield arising from biochar amendment. There is much complexity in matching biochar to suitable soil-crop applications and this challenge has defied development of simple heuristics to enable implementation. Biochar has great potential as a carbon management technology and as a soil amendment. The lack of technically rigorous methodologies for measuring recalcitrant carbon limits development of the technology according to this specific purpose.