δ¹³C values of organic constituents and possible source of humic substances in Japanese volcanic ash soils

To gain a better understanding about the δ¹³C values of organic constituents and possible source of humic substances in Japanese volcanic ash soils, we determined the δ¹³C values of charred plant fragments, humic and fulvic acids, and whole soils, using 10 volcanic ash soil samples. Furthermore, the characteristics of the humic acid obtained from the dil. H₂O₂-treated residues of charred plant materials, which were produced during the burning of a grassland, were compared with those of black (type A) humic acids in volcanic ash soils. The δ¹³C values of the charred plant fragments, humic and fulvic acids, and whole soils studied ranged from -25 to -17‰, -25 to -17‰, -23 to -15‰, and -24 to -17‰, respectively. The δ¹³C values of the whole soils were highly correlated with those of the charred plant fragments (r = 0.968, significant at 0.1% level), humic acids (r = 0.947, significant at 0.1% level) or fulvic acids (r = 0.900, significant at 0.1% level), suggesting that in Japanese volcanic ash soils, the δ¹³C data of whole soils are valuable for discussing and estimating the origin of carbon of charred plant fragments as well as humic and fulvic acids. The δ¹³C values of the charred plant fragments were highly correlated with those of the fulvic acids (r = 0.792, significant at 1% level) and especially humic acids (r = 0.951, significant at 0.1% level). The contribution ratios of C4-plant-derived carbon on the carbon in the charred plant fragments, humic and fulvic acids, and whole soils ranged from 15 to 69%, 16 to 65%, 29 to 86%, and 24 to 66%, respectively. The ¹³C-NMR spectrum and X-ray diffraction pattern of the humic acid obtained from the oxidative degradation products of the charred plant materials were similar to those of type A humic acids reported previously. Based on these findings, it was assumed that both charred C3- and C4-plant materials merit close attention as an important source of humic substances in Japanese volcanic ash soils.