Formation and mobility of soil organic carbon in a buried humic horizon of a volcanic ash soil

•Hydrophilic fulvic acid fractions had received much biological transformations.•Humic acid fractions had formed at early stage of soil formation with less biological transformation.•Fire event could be a possible formation mechanism of HA fractions.•Mobility of soil organic carbon fractions was limited to <4 mm per century.•The thick humic horizon reflects the successive up-building accumulation of soil particles containing soil organic carbon.