Properties of Pg Absorption Strength of Humic Acids and the Factors Regulating its Distribution in Volcanic Ash Soils

Abstract

The authors have an interest in establishing a valuable method for obtaining reliable information on environmental changes in the past by examining humus properties of volcanic ash soils. In this study we investigated Pg absorption, the spectral characteristics of humic acids. Remarkable absorbances in alkaline humic acids at wavelength of 615, 570 and 450nm were named Pg absorption by Kumada et al. (1967). Kumada (1981) considered that Pg is a kind of green pigments derived from sclerotia and that Pg contains compounds similar to DHPQ the derivative of 4 : 9-hydroxyperylene-3 : 10-quinone. Though Pg is widely found in world soils and is suggested to be a reliable indicator to infer vegetational and climatic environments of the past, the mechanisms of Pg formation is not yet known well. The purpose of this study is to clarify the environmental factors regulating the distribution of Pg absorption strength in the field and to evaluate Pg absorption strength as an environmental indicator through the investigation of properites of Pg absorption strength in volcanic ash soils in Japan.
In this study we investigated 21 sites in six volcanic areas, the Towada, Akagi, Nantai (Nikko) of Northeastern Japan and Ashitaka of Central Japan and the Daisen and Kirishima of Western Japan. Each soil were sampled from surface layers of uncultivated lands. Besides soil humus analyses, which are of organic carbon content, humus composition and spectral properties of humic acids, phytolith composition was investigated to examine the relationship between Pg absorption strength and vegetational environment at present. Warmth Index and Humidity Index, which are climatic indices proposed by Kira (1945), were also calculated for each sample site using climatic grid data compiled in the Digital Information on Japanese Land.
There was no relation between Pg absorption strength and carbon content. On the other hand, PQ (percentage of humic acid to total humus) and RF (color intensity of humic acid) both show negative correlation with Pg absorption. These facts confirm that Pg absorption strength is regulated not by the amount of humus but by the quantity and quality of humic acid. Referring to the report of Sato (1974), we consider that brown fractions in humic acids called Pb fractions mask Pg absorption appeared in spectral curves of humic acids.
As for climatic environment, Pg absorption strength has a negative correlation with Warmth Index (n=20, r=-0.55*). The relationship between Pg absorption and Humidity Index is not clear enough to remark. Furthermore, we estimated the yield of grasses from the density of grass phytolith in soils. We found a strong positive relation between Pg absorption strength and the yield of Bambusoideae Sasa (n=21, r= 0.81***), while a negative correlation with that of Miscanthus (n=21, r=-0.67***). From this study, we confirmed that the strength of Pg absorption is strongly regulated by the vegetation. Particularly Miscanthus acts to control down the strength of Pg absorption.
We must notice that most of the investigated sites in this study range in cool temperate forests. The role of Pg absorption strength as an environmental indicator will be clearer at the time when we make further investigation in soils formed under warm temperate forests.