[delta]13C VALUES OF ORGANIC CONSTITUENTS AND POSSIBLE SOURCE OF HUMIC SUBSTANCES IN JAPANESE VOLCANIC ASH SOILS.

To gain a better understanding about the [delta]13C values of organic constituents and possible source of humic substances in Japanese volcanic ash soils, we determined the [delta]13C 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 [delta]13C values of the charred plant fragments, humic and fulvic acids, and whole soils studied ranged from -25 to -17[per mille sign], -25 to -17[per mille sign], -23 to -15[per mille sign], and -24 to -17[per mille sign], respectively. The [delta]13C 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 [delta]13C 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 [delta]13C 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.