Contribution of charred plant fragments to soil organic carbon in Japanese volcanic ash soils containing black humic acids

Charred plant fragments were isolated from 24 Japanese volcanic ash soils by specific gravity (s.g.) method, using a sodium polytungstate solution. Microscopic observation indicated that the charred plant fragments, which are black or blackish brown, were the main component in both fractions of less than s.g. 1.6 g cm−3 isolated before and after HCl-HF treatment of the soil samples. Furthermore, it was suggested that most of the fragments in the <1.6 fraction isolated before the treatment originated from the xylem of woody plants. While, the fragments in the <1.6 fraction isolated after the treatment were mainly amorphous. The organic-C content of the former fraction, ranging from 0.34 to 6.1 g per kg whole soil, was much lower than that of the latter fraction, ranging from 0.74 to 42 g per kg whole soil. Total organic-C content of the whole soil was highly correlated with contribution to this carbon from the charred plant fragments. In many soils, the percentage contribution was 9% or more with a maximum value of 33%, suggesting that charred plant fragments should not be overlooked as one of the constituents of organic matter in volcanic ash soils. From the findings in the present and previous studies, it was assumed that charred plant fragments deserve close attention as one of the sources of black humic acids which characterize Japanese volcanic ash soils.