peatland in Honshu, Japan, and covers 8 km
2 in area.
High-moor Peat soils composed of Sphagnum dominate, and
have micro-topographical features that are characteristic of
high-moor peatland. These High-moor Peat soils were
formed by volcanic activity of a young active volcano, called
Hiuchigatake, that produced lava and mudflows, forming flat
plains where the climate is subarctic humid due to the high
elevation (Sakaguchi 1974).
Organic soils in the west of Honshu, including the
Kyushu and Shikoku regions, are terrestrialization-type
peatlands formed in the mountains and highlands of volcanoes active in the Quaternary Period. These peatlands
occupy a significantly smaller area compared to the
paludization-type peatlands of Northern Japan and are also
scattered throughout the area.
4.2.3 Characteristics of Organic Soils in Japan
The chemical and physical characteristics of Organic soils,
including Peat soils, are controlled by the plant community
(component plant species) of the peats, as well as the degree
of decomposition and humification (Kondo 1997).
The plant communities of peatlands are dependent on the
type of water and nutritional conditions available. The type
of water that recharges peatlands is categorized into precipitation, surface water, and groundwater. In low-moor
peatlands, the recharge water can belong to all of these water
types, but the contribution of precipitation is low compared
to that of surface water and groundwater. By contrast, in
high-moor peatlands, the recharge water is almost exclusively derived from precipitation. Since surface water and
groundwater supply nutrient salts to peatlands, eutrophic
plant communities of herbaceous plants, including common
reed and large sedges, as well as trees including Japanese
alder and Manchurian ash (Fraxinus mandshurica), predominate. Since precipitation lacks nutrient salts, oligotrophic plant communities such as Sphagnum and Horomui
sedge predominate (Sakaguchi 1974).
In peatlands, many plant species flourish and approximately 200 species are known to grow in the Kushiro
peatland. However, the dominant plant species that comprise
peats number around only 20 species, while the others are
only accompanying plants, which grow in the environment
but are not component plants species of peats (Kondo 1997).
In the peatlands of Hokkaido, the main component plant
species are common reed, Japanese alder, hare’s-tail cottongrass, Japanese moor grass, Myrica gale, Rannoch-rush,
cinnamon fern (Osmundastrum cinnamomeum), small cranberry, Horomui sedge, and Sphagnum (Sakaguchi 1974).
As discussed in Sect. 4.2.2, most of Japan’s peatlands are
paludization-type, which that can be fluvial peatlands
forming in the back swamp of an alluvial plain or seepage
peatlands forming in the marshy ground left after marine
regression. Therefore, recharge waters are mainly surface
and groundwater, which causes a eutrophic plant community
to dominate, forming Low-moor Peat soils with common
reed and Japanese alder as component plant species. Particularly in the paludization-type peatlands of Hokkaido,
common reed is the predominant component plant species of
peats. On the contrary, Transitional-moor Peat soils with
Japanese moor grass and hare’s-tail cottongrass or
High-moor Peat soils with Sphagnum and Horomui sedge
are relatively sparse, except in some parts of the Ishikari
peatland and the Sarobetsu peatland in Hokkaido (Sakaguchi
1974; Kondo 1997).
Along with the component plant species, the degree of
decomposition is another controlling factor determining the
characteristics of peats. The acceleration of decomposition is
caused by: (1) the reduction of plant biomass production by
reaching the limit of peatland formation; (2) eutrophication
by the input of inorganic materials such as volcanic ash and
clays, as well as the neutralization of acidity; (3) eutrophication and improvements of aeration by the input of coarse
particles such as pumice and sand; and (4) improvements of
aeration by the lowering of groundwater (Sakaguchi 1974).
Paludization-type peatlands, the dominant type in Japan,
receive frequent inputs of inorganic materials from flooded
river sediments. Additionally, inputs of volcanic ash and
pumice often occur near young volcanoes and create
favorable environments for the decomposition of peats
through eutrophication. Due to the presence of westerlies,
peatlands on the eastern side of volcanoes receive volcanic
ash, which is subsequently incorporated into the peat,
thereby accelerating the decomposition of the peat above and
below the incorporated inorganic materials (Fig. 4.7b).
As fully explained in the USDA Soil Taxonomy, the
degree of decomposition of peat can be classified based on
the plant fiber content after rubbing and on the PI (Soil
Survey Staff 2014). Fiber content is assessed by washing a
hand-rubbed peat sample over a 100-mesh sieve (0.15 mm
aperture) and measuring the volume fraction that remains.
This technique is also used for classifying soil subgroups in
the Japanese soil classification system (refer to Sect. 4.2.1).
The PI is judged against the Munsell soil color chart using
the value (lightness) and chroma of a filter paper used for
paper chromatography, after the paper is used to absorb an
extract of a peat sample which was saturated in sodium
pyrophosphate overnight. The peat sample is classified as
fibric soil material when the difference between the value
and chroma is large, and it is classified as sapric soil material
when the difference is small (Soil Survey Staff 2014). PI
indicates that more humification took place due to the greater
decomposition of the peat. It is judged by the small difference between the value and chroma indicative of melanized
humic substances. In Japan, the assessment of humification
4 Major Soil Types
79
2 in area.
High-moor Peat soils composed of Sphagnum dominate, and
have micro-topographical features that are characteristic of
high-moor peatland. These High-moor Peat soils were
formed by volcanic activity of a young active volcano, called
Hiuchigatake, that produced lava and mudflows, forming flat
plains where the climate is subarctic humid due to the high
elevation (Sakaguchi 1974).
Organic soils in the west of Honshu, including the
Kyushu and Shikoku regions, are terrestrialization-type
peatlands formed in the mountains and highlands of volcanoes active in the Quaternary Period. These peatlands
occupy a significantly smaller area compared to the
paludization-type peatlands of Northern Japan and are also
scattered throughout the area.
4.2.3 Characteristics of Organic Soils in Japan
The chemical and physical characteristics of Organic soils,
including Peat soils, are controlled by the plant community
(component plant species) of the peats, as well as the degree
of decomposition and humification (Kondo 1997).
The plant communities of peatlands are dependent on the
type of water and nutritional conditions available. The type
of water that recharges peatlands is categorized into precipitation, surface water, and groundwater. In low-moor
peatlands, the recharge water can belong to all of these water
types, but the contribution of precipitation is low compared
to that of surface water and groundwater. By contrast, in
high-moor peatlands, the recharge water is almost exclusively derived from precipitation. Since surface water and
groundwater supply nutrient salts to peatlands, eutrophic
plant communities of herbaceous plants, including common
reed and large sedges, as well as trees including Japanese
alder and Manchurian ash (Fraxinus mandshurica), predominate. Since precipitation lacks nutrient salts, oligotrophic plant communities such as Sphagnum and Horomui
sedge predominate (Sakaguchi 1974).
In peatlands, many plant species flourish and approximately 200 species are known to grow in the Kushiro
peatland. However, the dominant plant species that comprise
peats number around only 20 species, while the others are
only accompanying plants, which grow in the environment
but are not component plants species of peats (Kondo 1997).
In the peatlands of Hokkaido, the main component plant
species are common reed, Japanese alder, hare’s-tail cottongrass, Japanese moor grass, Myrica gale, Rannoch-rush,
cinnamon fern (Osmundastrum cinnamomeum), small cranberry, Horomui sedge, and Sphagnum (Sakaguchi 1974).
As discussed in Sect. 4.2.2, most of Japan’s peatlands are
paludization-type, which that can be fluvial peatlands
forming in the back swamp of an alluvial plain or seepage
peatlands forming in the marshy ground left after marine
regression. Therefore, recharge waters are mainly surface
and groundwater, which causes a eutrophic plant community
to dominate, forming Low-moor Peat soils with common
reed and Japanese alder as component plant species. Particularly in the paludization-type peatlands of Hokkaido,
common reed is the predominant component plant species of
peats. On the contrary, Transitional-moor Peat soils with
Japanese moor grass and hare’s-tail cottongrass or
High-moor Peat soils with Sphagnum and Horomui sedge
are relatively sparse, except in some parts of the Ishikari
peatland and the Sarobetsu peatland in Hokkaido (Sakaguchi
1974; Kondo 1997).
Along with the component plant species, the degree of
decomposition is another controlling factor determining the
characteristics of peats. The acceleration of decomposition is
caused by: (1) the reduction of plant biomass production by
reaching the limit of peatland formation; (2) eutrophication
by the input of inorganic materials such as volcanic ash and
clays, as well as the neutralization of acidity; (3) eutrophication and improvements of aeration by the input of coarse
particles such as pumice and sand; and (4) improvements of
aeration by the lowering of groundwater (Sakaguchi 1974).
Paludization-type peatlands, the dominant type in Japan,
receive frequent inputs of inorganic materials from flooded
river sediments. Additionally, inputs of volcanic ash and
pumice often occur near young volcanoes and create
favorable environments for the decomposition of peats
through eutrophication. Due to the presence of westerlies,
peatlands on the eastern side of volcanoes receive volcanic
ash, which is subsequently incorporated into the peat,
thereby accelerating the decomposition of the peat above and
below the incorporated inorganic materials (Fig. 4.7b).
As fully explained in the USDA Soil Taxonomy, the
degree of decomposition of peat can be classified based on
the plant fiber content after rubbing and on the PI (Soil
Survey Staff 2014). Fiber content is assessed by washing a
hand-rubbed peat sample over a 100-mesh sieve (0.15 mm
aperture) and measuring the volume fraction that remains.
This technique is also used for classifying soil subgroups in
the Japanese soil classification system (refer to Sect. 4.2.1).
The PI is judged against the Munsell soil color chart using
the value (lightness) and chroma of a filter paper used for
paper chromatography, after the paper is used to absorb an
extract of a peat sample which was saturated in sodium
pyrophosphate overnight. The peat sample is classified as
fibric soil material when the difference between the value
and chroma is large, and it is classified as sapric soil material
when the difference is small (Soil Survey Staff 2014). PI
indicates that more humification took place due to the greater
decomposition of the peat. It is judged by the small difference between the value and chroma indicative of melanized
humic substances. In Japan, the assessment of humification
4 Major Soil Types
79