Contribution of charred and buried plant fragments to humic and fulvic acids in Japanese volcanic ash soils

Abstract

To gain a better understanding of the role of charred plant materials, which were produced during the burning of vegetation by human activity and wildfires, in the formation of humic and fluvic acids in Japanese volcanic ash soils, the quantitative contribution of charred and buried plant fragments to their acids in whole soils were investigated using three volcanic ash soil samples. Charred fragments were the main components in the fraction of less than specific gravity 1.6 g cm−3 (< 1.6 fraction), which was isolated after HCl–HF treatment of the soil samples. The percentage contribution of organic C content in the < 1.6 fractions to that of the whole soils ranged from 13.9 to 32.0%. All humic acids obtained from the < 1.6 fractions and whole soils were classified into Type A, which is characterized by a high degree of darkening and the presence of a graphite-like structure. However, the color coefficient (ΔlogK) and relative color intensity (RF) values of the humic acids in the < 1.6 fractions were different from those in whole soils. In all soils studied, the amounts of NaOH-extractable humic (OH-HA) and fulvic acids (OH-FA) were much greater than those of Na4P2O7-extractable humic (SPP-HA) and fulvic acids (SPP-FA), respectively, and the amounts of humic acids were substantially greater than those of fulvic acids. The proportion of the quantitative contribution of humic and fulvic acids in the < 1.6 fractions to those in the whole soils ranged from 12.0 to 43.8% for OH-HA, from 3.80 to 9.56% for OH-FA, and from 2.92 to 22.3% for SPP-HA, respectively. The proportion was very small for SPP-FA. It was assumed that in Japanese volcanic ash soils, parts of charred plant materials are subjected to oxidative degradation over a long period of time after burning, and are converted to fulvic acids and, particularly, humic acids.