Relationships between biochar and soil humic substances

Biochar, or the charred material from the pyrolysis of biomass, has a high resistance to microbial degradation. Soil microorganisms will, however, slowly give rise to oxidations of peripheral components of the biochar structure resulting in a material with operationally defined properties similar to soil humic acids. Conditions are described for the preparation of a 'mature' biochar (composed of fused aromatic structures and lacking incompletely transformed plant residues). The most biologically oxidised humic acids (isolated at pH 7) from the IHSS soil standard (a Mollisol) have the NMR aromatic resonances characteristic of biochars, but the significant carboxylic acid functionalities indicate that peripheral structures in the fused aromatic core were biologically oxidised over time in the soil environment.