cadmium loads were larger, because the cadmium uptakes of
these plants were lower and only the edible parts of the
plants were removed from the field. Thus, there was a difference in the cadmium removal depending on crop type:
Soil cadmium levels tended to decrease for tomato and to
accumulate for cabbage and Japanese radish. The magnitude
of the decrease and the accumulation in a single year were
slight compared to the amount of cadmium accumulation in
the soil, but if similar cultivation is continued for many years
the cadmium in the soil is likely to change significantly.
5.7.3 Soil Management Strategies
(1) Paddy fields
(1) Atmospheric environment
Paddy fields are the major agricultural source of methane in
Hokkaido. One anthropogenic factor that affects the generation of methane is the soil type. Other anthropogenic factors
affecting methane generation are field management, organic
matter treatment, and cultivation, and the amount of methane
generated is influenced by the combination of these factors.
Methane emission increases in proportion to the amount of
residual rice straw left in the field, with about 50% of the
carbon content in the rice straw residue being emitted as
methane (Naser et al. 2007).
The plowing of rice straw in autumn reduces methane
emissions by 55–70% compared to plowing in spring (Goto
et al. 2004). Furthermore, by adding fertilizer and decomposition materials in autumn before rice straw plowing, an
additional 10% reduction in methane emissions is achieved
(Goto et al. 2004).
By performing midsummer drainage in late June,
methane emissions are reduced by about 50% compared to
conditions with continuous flooding. Performing intermittent
irrigation after late June (after the heading stage) reduces
methane emissions by about 40% compared to continuous
flooding (Goto et al. 2004).
Soil type is a factor affecting methane emissions. In
Hokkaido, methane emissions are the lowest in Brown
Fluvic soils, intermediate in gray fluvic soils, and the highest
in gley fluvic soils. Furthermore, when a field is managed
without tillage or without puddling, methane emissions are
reduced as a result of the suppression of soil reduction which
accompanies the improvement in drainage.
(2) Aquatic environment
When paddy fields are flooded, the supply of oxygen to the
soil ceases and denitrification by microorganisms occurs.
Due to this phenomenon, paddy fields have the function of
purifying nitric acid. However, wastewater from paddy fields
contains excess fertilizer components such as nitrogen and
phosphorus, and there is concern about the adverse effects of
these components on the surrounding aquatic environment.
Since the nitrate concentration and water volume of rivers
become markedly higher in the middle of May, the catchment area mainly composed of rice paddies in Hokkaido has
the highest nitric acid outflow at this time. Nitrate nitrogen
tends to flow out from paddy fields to rivers until the middle
of June; however, this discharge is suppressed after the
middle of June. In order to prevent water pollution, it is
important to take measures against wastewater at the time of
puddling.
At the time of transplant from puddling, wastewater
containing a large amount of clay causes river water pollution. Comparing the nitrogen concentration of wastewater
from paddy fields, the concentration of organic nitrogen in
the surface drainage water before transplantation is
remarkably high. This organic nitrogen is derived from
Table 5.18 Cadmium load and
removal in vegetable soils
Soils
Compost
Cd load
a
(g ha
−1
)
Cd removal
b
(g ha
−1
)
Difference
c
(g ha
−1
)
Cd in plowed soil
d
(g ha
−1
)
Tomato
Â
2.57
36.4
−33.8
1584
○
e
6.76
33.6
−26.8
1584
Cabbage
Â
2.41
0.47
1.94
1334
○
4.50
0.63
3.87
1334
Japanese
radish
Â
1.33
0.92
0.41
1334
○
3.45
0.95
2.50
1334
a Applied in compost and fertilizers
b
Cd uptake by edible and residual parts from the tomato soil and by edible part from the cabbage and
Japanese radish soils
c Load–Removal
d
Up to 20 cm deep
e Applied 40t ha
−1 for tomato soil and 20t ha
−1 for cabbage and Japanese radish soils
5 Hokkaido Region
165
these plants were lower and only the edible parts of the
plants were removed from the field. Thus, there was a difference in the cadmium removal depending on crop type:
Soil cadmium levels tended to decrease for tomato and to
accumulate for cabbage and Japanese radish. The magnitude
of the decrease and the accumulation in a single year were
slight compared to the amount of cadmium accumulation in
the soil, but if similar cultivation is continued for many years
the cadmium in the soil is likely to change significantly.
5.7.3 Soil Management Strategies
(1) Paddy fields
(1) Atmospheric environment
Paddy fields are the major agricultural source of methane in
Hokkaido. One anthropogenic factor that affects the generation of methane is the soil type. Other anthropogenic factors
affecting methane generation are field management, organic
matter treatment, and cultivation, and the amount of methane
generated is influenced by the combination of these factors.
Methane emission increases in proportion to the amount of
residual rice straw left in the field, with about 50% of the
carbon content in the rice straw residue being emitted as
methane (Naser et al. 2007).
The plowing of rice straw in autumn reduces methane
emissions by 55–70% compared to plowing in spring (Goto
et al. 2004). Furthermore, by adding fertilizer and decomposition materials in autumn before rice straw plowing, an
additional 10% reduction in methane emissions is achieved
(Goto et al. 2004).
By performing midsummer drainage in late June,
methane emissions are reduced by about 50% compared to
conditions with continuous flooding. Performing intermittent
irrigation after late June (after the heading stage) reduces
methane emissions by about 40% compared to continuous
flooding (Goto et al. 2004).
Soil type is a factor affecting methane emissions. In
Hokkaido, methane emissions are the lowest in Brown
Fluvic soils, intermediate in gray fluvic soils, and the highest
in gley fluvic soils. Furthermore, when a field is managed
without tillage or without puddling, methane emissions are
reduced as a result of the suppression of soil reduction which
accompanies the improvement in drainage.
(2) Aquatic environment
When paddy fields are flooded, the supply of oxygen to the
soil ceases and denitrification by microorganisms occurs.
Due to this phenomenon, paddy fields have the function of
purifying nitric acid. However, wastewater from paddy fields
contains excess fertilizer components such as nitrogen and
phosphorus, and there is concern about the adverse effects of
these components on the surrounding aquatic environment.
Since the nitrate concentration and water volume of rivers
become markedly higher in the middle of May, the catchment area mainly composed of rice paddies in Hokkaido has
the highest nitric acid outflow at this time. Nitrate nitrogen
tends to flow out from paddy fields to rivers until the middle
of June; however, this discharge is suppressed after the
middle of June. In order to prevent water pollution, it is
important to take measures against wastewater at the time of
puddling.
At the time of transplant from puddling, wastewater
containing a large amount of clay causes river water pollution. Comparing the nitrogen concentration of wastewater
from paddy fields, the concentration of organic nitrogen in
the surface drainage water before transplantation is
remarkably high. This organic nitrogen is derived from
Table 5.18 Cadmium load and
removal in vegetable soils
Soils
Compost
Cd load
a
(g ha
−1
)
Cd removal
b
(g ha
−1
)
Difference
c
(g ha
−1
)
Cd in plowed soil
d
(g ha
−1
)
Tomato
Â
2.57
36.4
−33.8
1584
○
e
6.76
33.6
−26.8
1584
Cabbage
Â
2.41
0.47
1.94
1334
○
4.50
0.63
3.87
1334
Japanese
radish
Â
1.33
0.92
0.41
1334
○
3.45
0.95
2.50
1334
a Applied in compost and fertilizers
b
Cd uptake by edible and residual parts from the tomato soil and by edible part from the cabbage and
Japanese radish soils
c Load–Removal
d
Up to 20 cm deep
e Applied 40t ha
−1 for tomato soil and 20t ha
−1 for cabbage and Japanese radish soils
5 Hokkaido Region
165