typically ranges between 5 and 6. The pH change between oxidized and reduced
soils is exemplified by the reaction Fe(OH) 3 (amorphous) ⇄ Fe
2+ (negatively
charged site) + 3OH
À + e
À .
The Eh value of irrigation water (Fig. 5.2a) is high due to the addition of O 2 from
the air. In the oxidizing layer (Fig. 5.2b), the Eh value decreases steeply to around
À0.2 V at the boundary between the oxidizing layer (Fig. 5.2b) and the reducing
layer (Fig. 5.2c), estimated from the presence of dark-colored noncrystalline iron
sulfide (Fig. 5.1b). The lowest Eh value in the reducing layer is typically
À0.2 ~ À0.3 V. The thickness of the oxidizing layer ranges between around 0.5
and 5 cm.
Oxygen diffuses from the air to the rice roots through the aerenchyma, whereas
the bulk soil is reduced under submergence. A redox interface is also formed
between the aerenchyma and the bulk soil. At this redox interface, there are three
or more cell layers, which are the epidermis, exodermis, and sclerenchyma (one or
more layers), forming the outer part of a rice root (Kondo et al. 2000). With aging,
the epidermis layer is sloughed off first, whereas the exodermis and/or sclerenchyma
remain along with deposition of hydrated iron oxide and other materials.
Methane can be formed by reduction of CO 2 , which may form through oxidation
of organic matter by microbes. Carbon dioxide and CH 4 gases can form vesicular or
irregular pores in the submerged and reduced soil. Although the vapor phase ratio of
the puddled plow layer soil is only 1–3%, it increases to 9–12% at around 40 days
after puddling and submergence, possibly due to formation of these gases (Saito and
Kawaguchi 1971a, b). One can easily notice the existence of gases from bubbling
when he steps in the submerged paddy field 1 or 2 months after submergence.
Methane formed in the reduced soil can be released to the air through the aerenchyma of the rice plants (Yagi 1997).
Iron is the most abundant member of the redox-sensitive elements in soil. Ferrous
iron reacts with phosphate, carbonate, and sulfide to produce vivianite (see Sect.
5.3), siderite (see Sect. 5.4), and iron sulfide (see Sects. 5.5.1 and 5.5.2),
respectively.
The occurrence of oxidizing conditions in the reduced paddy field soil depends on
the management of irrigation water, as shown in Fig. 5.4. Oxidizing conditions after
drainage can be detected most clearly from a lack of dipyridyl reaction and the
presence of iron mottles (see Sect. 5.2).
Since the manganese content in soil is typically one-fifth or less of that of iron, it
has limited effects on the morphological properties of Ap horizon soil, although faint
and soft manganese concretions may be found in the paddy field subsoil. Nitrate is
easily reduced to N 2 by denitrifiers, which affects the efficiency of N fertilizers, but
does not affect the morphological properties of the Ap horizon soil.
5.1 Introduction
101
soils is exemplified by the reaction Fe(OH) 3 (amorphous) ⇄ Fe
2+ (negatively
charged site) + 3OH
À + e
À .
The Eh value of irrigation water (Fig. 5.2a) is high due to the addition of O 2 from
the air. In the oxidizing layer (Fig. 5.2b), the Eh value decreases steeply to around
À0.2 V at the boundary between the oxidizing layer (Fig. 5.2b) and the reducing
layer (Fig. 5.2c), estimated from the presence of dark-colored noncrystalline iron
sulfide (Fig. 5.1b). The lowest Eh value in the reducing layer is typically
À0.2 ~ À0.3 V. The thickness of the oxidizing layer ranges between around 0.5
and 5 cm.
Oxygen diffuses from the air to the rice roots through the aerenchyma, whereas
the bulk soil is reduced under submergence. A redox interface is also formed
between the aerenchyma and the bulk soil. At this redox interface, there are three
or more cell layers, which are the epidermis, exodermis, and sclerenchyma (one or
more layers), forming the outer part of a rice root (Kondo et al. 2000). With aging,
the epidermis layer is sloughed off first, whereas the exodermis and/or sclerenchyma
remain along with deposition of hydrated iron oxide and other materials.
Methane can be formed by reduction of CO 2 , which may form through oxidation
of organic matter by microbes. Carbon dioxide and CH 4 gases can form vesicular or
irregular pores in the submerged and reduced soil. Although the vapor phase ratio of
the puddled plow layer soil is only 1–3%, it increases to 9–12% at around 40 days
after puddling and submergence, possibly due to formation of these gases (Saito and
Kawaguchi 1971a, b). One can easily notice the existence of gases from bubbling
when he steps in the submerged paddy field 1 or 2 months after submergence.
Methane formed in the reduced soil can be released to the air through the aerenchyma of the rice plants (Yagi 1997).
Iron is the most abundant member of the redox-sensitive elements in soil. Ferrous
iron reacts with phosphate, carbonate, and sulfide to produce vivianite (see Sect.
5.3), siderite (see Sect. 5.4), and iron sulfide (see Sects. 5.5.1 and 5.5.2),
respectively.
The occurrence of oxidizing conditions in the reduced paddy field soil depends on
the management of irrigation water, as shown in Fig. 5.4. Oxidizing conditions after
drainage can be detected most clearly from a lack of dipyridyl reaction and the
presence of iron mottles (see Sect. 5.2).
Since the manganese content in soil is typically one-fifth or less of that of iron, it
has limited effects on the morphological properties of Ap horizon soil, although faint
and soft manganese concretions may be found in the paddy field subsoil. Nitrate is
easily reduced to N 2 by denitrifiers, which affects the efficiency of N fertilizers, but
does not affect the morphological properties of the Ap horizon soil.
5.1 Introduction
101