of the grass family Poaceae and are C4 plants. Ishizuka et al.
(2014) analyzed stable carbon isotope ratios (d
13 C) of “black
soils” collected throughout Japan and obtained the following
results. The contribution of C4 grass to soil organic carbon
(SOC) in black soils was generally high (> 44.6%) according to mass balance calculations from the d
13 C of SOC. The
melanic index, an index of the black color of humus, was
negatively correlated with d
13 C values. According to these
results, Ishizuka et al. (2014) concluded that C4 grass played
an important role in generating the dark-colored organic
matter in “black soils.”
The “black soils” were distributed throughout Japan, and
the area of these soils accounts for 17% of the country’s
territory (Okamoto 2009). The color of humus in “black
soils” has been attributed to “A type” humic acids (Kumada
1987), which are characterized by a deep black color. Due to
this spectroscopic property, “black soils” are classified as
Melanudands in the U.S. Soil Taxonomy (Soil Survey Staff
2010) and correspond to melanic Andosols in the World
Reference Base for Soil Resources 2014 (WRB) (IUSS
Working Group WRB, 2015). In the Soil Classification
System of Japan (The Fifth Committee for Soil Classification and Nomenclature, 2017), “black soils” are mainly
classified as Humic Non-Allophanic Andosols or Humic
Allophanic Andosols.
In order to maintain grassland vegetation, grassland was
burned in early spring every year. As a result, small grains of
charcoal are often found in black soils (Okamoto 2009).
According to Shindo (2012), the amount of charcoal grains
comprises up to 33% of SOC, and a high correlation
(r = 0.777 where r is correlation coefficient) was observed
between the amount of charcoal grains and total SOC.
Shindo et al. (1986) compared physicochemical and spectroscopic properties of humic acids obtained from the charred residue of susuki grass, and that from the “A horizon” of
black soils, and found that both were quite similar in their
elemental composition and spectroscopic properties. This
evidence suggests that the black color of humus might be
derived from charcoal particles which were formed by the
burning of grass.
The “black soils” have been found to have a very high
humus content: The concentration of SOC in the A horizon
reaches 30% (Shindo et al. 1986), and the average carbon
stock at a depth of 0–30 cm was measured as 13.8 Mg/m
2 ,
which is second only to peaty soils among forest soils
(Morisada et al. 2004). As for Andosols, the large accumulation of organic matter results from a combination of
high detritus input associated with the generally high fertility
and productivity of Andosols and from the effective stabilization of soil organic matter (SOM) against decomposition
(Takahashi and Dahlgren 2016). The stabilization of SOM in
Andosols has been attributed to the incorporation of SOM
into organo-mineral and/or organometallic (Al/Fe–humus)
complexes (Takahashi and Dahlgren 2016).
2.3.2 Agriculture
(1) History and present state of agriculture in Japan
The Köppen climate classification of Japan, except for the
northernmost island of Hokkaido, is mostly Cfa or Dfa,
characterized by warm temperatures in summer and a certain
amount of rainfall throughout the year. This climatic condition is suitable for the production of paddy rice, an
excellent staple food crop with a large yield and good
nutritional quality and taste. In addition, paddy fields can
maintain good productivity after continuous cultivation over
a number of years and are free from growth injury due to
continuous cropping.
Some archaeological studies have suggested that agriculture in Japan started in the Jōmon period, between around
15,000 years ago and the fourth century BC—the era preceding the Yayoi period. However, it is thought that hunting
and gathering was the dominant means of obtaining food in
the Jōmon period. The beginning of full-fledged agriculture
in Japan was in the Yayoi period, between the fourth century
BC and the third century AD, although recent studies using
14 C dating by the accelerator mass spectrometry method
have pointed out that the beginning of the Yayoi period
might be the tenth century BC (Fujio et al. 2005). The Yayoi
period is defined by widespread paddy rice farming that was
introduced from the Asian continent.
In the Yamato period, which started in the fourth century
AD, governors constructed various political schemes concerning the development and possession of paddy fields in
order to manage people’s agricultural activities and collect
taxes. In the sixth century AD (the Nara period), local
governors surveyed their land fertility and compiled Fudoki
in response to an imperial order. For example, “Harima
Fudoki” classified land fertility into 9 grades in each sato
(small village or hamlet) in Hyogo Prefecture of the Kinki
region. Tax payments by rice in kind were the fundamental
basis of the Japanese taxation system until the nineteenth
century. As for upland farming, shifting cultivation such as
slash-and-burn and semipermanent farming in uplands had
been conducted from ancient times. However, the principal
means of food production was paddy rice farming, due to its
advantages as described above, and upland farming was the
alternative in fields for which paddy rice production was
difficult, until the late modern period. After the Meiji
Restoration in 1868, upland farming was developed with the
formation of a market for upland crops under modern capitalism as background (Cho 1986).
2 Soil-Forming Factors
45
(2014) analyzed stable carbon isotope ratios (d
13 C) of “black
soils” collected throughout Japan and obtained the following
results. The contribution of C4 grass to soil organic carbon
(SOC) in black soils was generally high (> 44.6%) according to mass balance calculations from the d
13 C of SOC. The
melanic index, an index of the black color of humus, was
negatively correlated with d
13 C values. According to these
results, Ishizuka et al. (2014) concluded that C4 grass played
an important role in generating the dark-colored organic
matter in “black soils.”
The “black soils” were distributed throughout Japan, and
the area of these soils accounts for 17% of the country’s
territory (Okamoto 2009). The color of humus in “black
soils” has been attributed to “A type” humic acids (Kumada
1987), which are characterized by a deep black color. Due to
this spectroscopic property, “black soils” are classified as
Melanudands in the U.S. Soil Taxonomy (Soil Survey Staff
2010) and correspond to melanic Andosols in the World
Reference Base for Soil Resources 2014 (WRB) (IUSS
Working Group WRB, 2015). In the Soil Classification
System of Japan (The Fifth Committee for Soil Classification and Nomenclature, 2017), “black soils” are mainly
classified as Humic Non-Allophanic Andosols or Humic
Allophanic Andosols.
In order to maintain grassland vegetation, grassland was
burned in early spring every year. As a result, small grains of
charcoal are often found in black soils (Okamoto 2009).
According to Shindo (2012), the amount of charcoal grains
comprises up to 33% of SOC, and a high correlation
(r = 0.777 where r is correlation coefficient) was observed
between the amount of charcoal grains and total SOC.
Shindo et al. (1986) compared physicochemical and spectroscopic properties of humic acids obtained from the charred residue of susuki grass, and that from the “A horizon” of
black soils, and found that both were quite similar in their
elemental composition and spectroscopic properties. This
evidence suggests that the black color of humus might be
derived from charcoal particles which were formed by the
burning of grass.
The “black soils” have been found to have a very high
humus content: The concentration of SOC in the A horizon
reaches 30% (Shindo et al. 1986), and the average carbon
stock at a depth of 0–30 cm was measured as 13.8 Mg/m
2 ,
which is second only to peaty soils among forest soils
(Morisada et al. 2004). As for Andosols, the large accumulation of organic matter results from a combination of
high detritus input associated with the generally high fertility
and productivity of Andosols and from the effective stabilization of soil organic matter (SOM) against decomposition
(Takahashi and Dahlgren 2016). The stabilization of SOM in
Andosols has been attributed to the incorporation of SOM
into organo-mineral and/or organometallic (Al/Fe–humus)
complexes (Takahashi and Dahlgren 2016).
2.3.2 Agriculture
(1) History and present state of agriculture in Japan
The Köppen climate classification of Japan, except for the
northernmost island of Hokkaido, is mostly Cfa or Dfa,
characterized by warm temperatures in summer and a certain
amount of rainfall throughout the year. This climatic condition is suitable for the production of paddy rice, an
excellent staple food crop with a large yield and good
nutritional quality and taste. In addition, paddy fields can
maintain good productivity after continuous cultivation over
a number of years and are free from growth injury due to
continuous cropping.
Some archaeological studies have suggested that agriculture in Japan started in the Jōmon period, between around
15,000 years ago and the fourth century BC—the era preceding the Yayoi period. However, it is thought that hunting
and gathering was the dominant means of obtaining food in
the Jōmon period. The beginning of full-fledged agriculture
in Japan was in the Yayoi period, between the fourth century
BC and the third century AD, although recent studies using
14 C dating by the accelerator mass spectrometry method
have pointed out that the beginning of the Yayoi period
might be the tenth century BC (Fujio et al. 2005). The Yayoi
period is defined by widespread paddy rice farming that was
introduced from the Asian continent.
In the Yamato period, which started in the fourth century
AD, governors constructed various political schemes concerning the development and possession of paddy fields in
order to manage people’s agricultural activities and collect
taxes. In the sixth century AD (the Nara period), local
governors surveyed their land fertility and compiled Fudoki
in response to an imperial order. For example, “Harima
Fudoki” classified land fertility into 9 grades in each sato
(small village or hamlet) in Hyogo Prefecture of the Kinki
region. Tax payments by rice in kind were the fundamental
basis of the Japanese taxation system until the nineteenth
century. As for upland farming, shifting cultivation such as
slash-and-burn and semipermanent farming in uplands had
been conducted from ancient times. However, the principal
means of food production was paddy rice farming, due to its
advantages as described above, and upland farming was the
alternative in fields for which paddy rice production was
difficult, until the late modern period. After the Meiji
Restoration in 1868, upland farming was developed with the
formation of a market for upland crops under modern capitalism as background (Cho 1986).
2 Soil-Forming Factors
45