Microscopic observation of charred plant fragments isolated from several Ando soil samples by the specific gravity method

Charred plant fragments were isolated from 5 Ando soil samples, containing Type A humic acids, by the specific gravity (s.g.) method using a sodium polytungstate solution as heavy solution. Microscopic observation indicated that the charred plant fragments, which are black or blackish brown, were the main components in both fractions of less than s.g. 1.6 Mg m−3 that had been isolated before and after HCl-HF treatment of the soil samples. Furthermore, it was suggested that most of the fragments in the < 1.6 fractions isolated before the treatment originated from woody plants. On the other hand, the fragments in the < 1.6 fractions isolated after the treatment were mainly amorphous, although the shape of several fragments was similar to that of vascular tissues of woody and herbaceous plants. The sum of the organic-C contents of both < 1.6 fractions ranged from 4.08 to 47.8 g kg− whole soil, and 3.8–32.7% of total organic-C of the whole soil originated from these fractions. No constant relationship was observed between the organic-C contents of both fractions and the morphological characteristics of the charred plant fragments.