Holocene Volcanic Ash soils at Waimangu Road Tephra Section, North Island, New Zealand. : Soil Formation-Vegetation Relationship

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As part of a field survey, opal phytolith analysis and humus analysis were carried out on buried volcanic ash soils (Mamaku soil and Taupo soil) and present volcanic ash soil (Rotomahana soil) in the accumulated tephra profile at Waimangu Road tephra section south of Lake Tarawera, on the North Island of New Zealand. The results are as follows.1) It was found that trees were the main source of phytoliths during the formation period of Mamaku soil between the Mamaku ash (7, 050y. B. P.) and the Taupo Pumice (1, 819y. B. P.). Therefore, the Mamaku soil is thought to have been under the forests. The A horizon of the Mamaku soil (III Ab) looked dark yellowish brown, and the rate of humic acid was low and belonged to P type. BC horizon (III BCb) looked light yellowish brown. These facts show that the Mamaku soil which has tephra as parent material can be situated in Brown Forest soil formed under forests.2) The A horizon of Taupo soil (II Ab) showed black mull, whose humic acid belongs to A type, its upper part (II A11b) had a higher degree of humification than its lower part (II A12b). This phenomenon implies that during the production of this soil the main source of phytolith changed from trees to ferns and graminous plants that is, from forest to grassland type vegetation. This change was due to the destruction of the forests about 1, 000 years ago by Polynesians who were the ancestors of Maolians. Therefore, in the beginning the Taupo soil started as the natural soil under forests; however, the destruction of the forests by humans changed it into soil with grass vegetation, and finally it was situated as polygenetic soils of Maori soil similar to Kuroboku soil (Andosol).3) The A horizon of Rotomahana soil (I(A)) contained a large amount of the phytolith whose source is graminous plants. In spite of that, its humic acid belonged to type Rp. The fact indicates that Kuroboku Soil could not be produced if the period of soil formation were less than 100 years even under the vegetation of grass.4) Thus it was clarified that the produtive form of volcanic ash soils of Waimangu Road tephra section are basically identified with that of Te Ngae Road tephra section and that the destruction of forests by humans about 1, 000 years ago had a decisive influence on its volcanic soil formation.

Holocene Volcanic Ash soils at Waimangu Road Tephra Section, North Island, New Zealand. : Soil Formation-Vegetation Relationship

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