A Study of the Environmental Changes inferred from the Spatial Distribution of Soil Humus Characteristics in the Southern Foot Area of Akagi Volcano, Central Japan

Abstract

By a series of studies to clarify the humus characteristics of volcanic ash soils and soil forming environment, the influence of climate variation was recognized in the eastern foot area of Nantai Volcano, Nikko (Watanabe 1985, 1987). The purpose of this study is to obtain useful information for the reconstruction of global scaled environmental changes from spatial distribution of soil humus characteristics. Volcanic ash soils developed in the southern foot area of Akagi Volcano were examined as a case.
The result is summarized as follows.
1. Soils developed over southern foot area of Akagi Volcano show several types of soil profiles regulated by the thickness and rate of tephra deposits and geomorphic surfaces. On mountain slopes formed by pyroclastic flow, brownish volcanic ash bed are to be seen between surface and buried humus layer. Soils distributed on central cone lacustrine surface have thick and dark humus layer called “gleyed Kuroboku”. Soils with light humus layer develop on piedmont surface under the influence of artificial activity. The brownish volcanic ash bed mentioned above is difficult to distinguish from this layer.
2. The chronological sequence of soil formation in the study area was considered on the basis of properties of soil parent materials and radiocarbon dates. Horizon I is the layer which continues to accept humus supply and humification under soil forming environment in the present. Fresh volcanic ash errupted from Asama and Haruna Volcano, about 200-1700 years ago are included in this layer. Horizon II correspondent to the brownish volcanic ash bed has accepted humus substances from the upper layer. Though the source volcano is unknown, this layer is assumed to be deposited about 4, 000 years ago. Horizon III, a buried humus layer intercepted from surface environment, started soil formation about 9, 000 years ago following to the erruption of Asama Itahana Yellowish Pumice (YP).
3. The vertical distribution of humus characteristics (brightness of soil color, carbon content and humus composition) was classified into two types, which reflects the conditions of vegetation and groundwater level regulated by gemorphic surface.
4. The spatial distribution patterns of humus characteristics such as carbon content and RF value of humic acid in each horizon show vertical zonality. The principal areas of humus accumulation in the age of horizon II and III formation were located in higher location than present, about 400m upward. Humic acid classified such as type A+, P+ +, have remarkable absorption spectra near 615, 570, 455nm which are regarded to be the features of soils formed under vegetations such as Fagus, Quercus mongolica and also under cool and wet condition in the study area. The appearance of these humic acid types in each horizon also tend to move its location upward by 300m in the age of horizon III formation.