Carbon source of humic substances in some Japanese volcanic ash soils determined by carbon stable isotopic ratio, δ¹³C

Volcanic ash soils (including Andisols) have been known to accumulate extremely large amounts of humic acids in their thick surface horizons, and their accumulation mechanism is of interest. Two mechanisms have been proposed: (1) active metals like Al and Fe, supplied from weathered volcanic materials, stabilize humic acids through complexation reactions; (2) cultivated Japanese pampas grasses (Miscanthus sinensis A.) and its charred materials are the major carbon source of humic acids. In the present study, contribution ratio of the pampas grass (C4-plant) on the carbons of the humic acids was determined by measuring their stable isotopic ratio of carbon (δ¹³C). In Japanese volcanic ash soils, humic acids were originated from both C3- and C4-plants, and the contribution ratio of C4-plants (mostly pampas grass) ranged from 18% to 52%. Highly humified (dark-colored) humic acids tended to show higher contribution ratio of C4-plants among volcanic ash soils, although the major part of the carbon had originated from C3-plants. It was also clarified that the δ¹³C values of crude soil samples correlated well with those of humic and fulvic acids. Therefore, reported δ¹³C values of crude soil samples in the literature would be useful for estimating the carbon source of soil humic substances. Literature survey of the δ¹³C values of crude soil samples also indicated that a large part of the carbon in humic substances has originated from C3-plants rather than C4-plants, implying the importance of the active metals (such as Al and Fe) on the formation and accumulation of the humic acids in volcanic ash soils.