Fire-induced formation and biodegradation of humic substances in Andosols of Japan

• Charred plant residues (CPRs) are widely distributed in Japanese Andosols.
• CPRs can produce fulvic acids (FAs), especially black humic acids (HAs).
• The biostabilities of Andosol HAs and FAs are higher than those of non-Andosol.
• Plant charring can be an important soil-forming factor.

The aim of this article was to elucidate the possible mechanisms of the formation and degradation of humic substances in Japanese Andosols. Charred plant residues (CPRs) produced as a result of human activity and wildfires are incorporated into soils and degraded into smaller particles, i.e. charred plant fragments (CPFs). During this process, some CPRs are transformed into fulvic acids and, especially, humic acids with graphite-like structures. CPR-derived CPFs and humic substances are widely distributed in Japanese Andosols and contribute to their high carbon accumulation. The biostabilities of Andosol humic substances are higher than that of non-Andosols. Our results show that plant charring is a new concept related to the formation and storage of humic substances in Japanese Andosols.