6.4.2 Nitrogen Budget
Soybeans require a large amount of N due to the large
accumulation of N in their seeds. Although soybeans obtain
some of their total N accumulation from symbiotic N 2 fixation in their root nodules, they also take up significant
amounts of soil N to meet their high N requirements.
Therefore, there is a possibility that the N output from
soybean-cultivated fields can exceed the N input to the field,
and thus the N budget could be negative, indicating N loss
from the field. The N loss from the field could thus cause a
decrease in soil available N in rotated paddy–upland fields.
Although significant N loss from rice paddy fields is not
thought to have occurred, a detailed N budget in a
rice-cultivated rotated paddy field has not yet been well
established. Therefore, in order to maintain soil N fertility in
rotated paddy fields, it is essential to evaluate their N budget
during both the soybean and rice cultivation period.
(1) Nitrogen budget in an upland field with soybean
cultivation
The annual N budget in a soybean-cultivated upland field was
evaluated for 3 years in an experimental lysimeter field filled
with Gray Fluvic soil (Fulvic paddy soil), a soil type which is
typically found in the Sea of Japan side of Northern Japan.
Input (seed, fertilizer, bulk N deposition, and symbiotic N 2
fixation) and output (harvested grain, N leached via drainage
water and nitrous oxide (N 2 O) emission) flow of N were
measured, and the field N budget was estimated by subtracting
the N output from the N input (Fig. 6.20, Takakai et al. 2010,
2017a). The grain yield of soybean for the 3 years ranged
from 291 to 410 g m
−2 , with an average of 341 g m
−2 .
The annual N budgets in an upland field with soybean
cultivation for 3 years were consistently negative (i.e., there
was a net N loss from the field), and ranged from −10.9 to
−7.9 g N m
−2 y
−1 , with an average of −9.6 g N m
−2 y
−1 .
The validity of the estimation was confirmed by its agreement with the amount of N loss estimated based on changes
in soil N storage (Takakai et al. 2017a). The major component of total N input (20.1 g N m
−2 y
−1 ) was symbiotic
N 2 fixation, which accounted for 83% of total input, and the
major components of total N output (29.8 g N m
−2 y
−1 )
were harvested grain and N leaching, which accounted for
74% and 25% of total N output, respectively. Based on the
cultivation guidelines for this region, to avoid the excessive
growth of soybean, chemical fertilizer was not applied for
the first year of soybean cultivation after field conversion.
The amount of chemical fertilizer application for the following 2 years (second and third years) was 2 g N m
−2 ,
which was applied as basal fertilizer. Bulk N deposition
tended to increase in the fallow season, including in winter.
The percentage of soybean N accumulation that was derived
from N 2 fixation remained relatively constant (60–69%) over
the 3 years.
(2) Nitrogen budget in a paddy field with rice
cultivation
Following the 3 years of soybean cultivation under upland
condition, the annual N budget during rice cultivation under
flooded paddy conditions was also evaluated for 3 years.
The input and output N flows were measured and compared
with those measured during soybean cultivation (Table 6.8;
Takakai et al. 2017a). The yield of brown rice for the 3 years
ranged from 422 to 608 g m
−2 , with an average of
519 g m
−2 .
The three-year average of N budget during rice cultivation
was negative (–2.3 g N m
−2 y
−1 ), indicating a net N loss,
although this loss was less than that observed during soybean
cultivation. The N budget indicated a significant loss of N,
unlike previous reports in continuous paddy fields which
indicated no change in N budget or a slight N gain (e.g.,
+1.28 g N m
−2 ; Katayanagi et al. 2013). Additionally, the N
budget measured during rice cultivation varied greatly among
the 3 years (–5.7 to +0.2 g N m
−2 y
−1
) due to the difference
in fertilizer application rates, which accounted for 63% of the
total N input over the 3-year period. In paddy fields converted
from upland fields, the reduction of basal N fertilization is
recommended to avoid excessive growth and lodging due to
the increased nitrogen uptake. Chemical fertilizer was
applied at a rate of 0 or 6 g N m
−2 to all plots as a
basal fertilizer in the first or second and third years, respectively. A total of 2–3 g N m
−2 of chemical fertilizer was also
applied as topdressing in late July. The N loss from the paddy
fields in this study could be significantly higher in the first
year after conversion with low N fertilization. Consequently,
in the paddy–upland rotation system, considerable loss of
N may occur in both the soybean and rice cultivation periods.
The major components of the total N output were harvested grain and N leaching, which accounted for 49 and
29% of total N output, respectively. The annual N loss via
leaching during soybean cultivation was lower than that
during rice cultivation.
In order to maintain the soil N fertility in a rotated paddy
field with soybean cultivation, techniques for the mitigation
of N loss, such as the application of organic matter (e.g.,
green manure such as hairy vetch and manure compost;
Sato T, in this chapter; Nishida M, in this chapter; Nishida
et al. 2013) may be essential. However, because the application of organic matter to rotated paddy fields can
significantly change the N flows, its effects on the N budget
6 Tohoku Region
211
Soybeans require a large amount of N due to the large
accumulation of N in their seeds. Although soybeans obtain
some of their total N accumulation from symbiotic N 2 fixation in their root nodules, they also take up significant
amounts of soil N to meet their high N requirements.
Therefore, there is a possibility that the N output from
soybean-cultivated fields can exceed the N input to the field,
and thus the N budget could be negative, indicating N loss
from the field. The N loss from the field could thus cause a
decrease in soil available N in rotated paddy–upland fields.
Although significant N loss from rice paddy fields is not
thought to have occurred, a detailed N budget in a
rice-cultivated rotated paddy field has not yet been well
established. Therefore, in order to maintain soil N fertility in
rotated paddy fields, it is essential to evaluate their N budget
during both the soybean and rice cultivation period.
(1) Nitrogen budget in an upland field with soybean
cultivation
The annual N budget in a soybean-cultivated upland field was
evaluated for 3 years in an experimental lysimeter field filled
with Gray Fluvic soil (Fulvic paddy soil), a soil type which is
typically found in the Sea of Japan side of Northern Japan.
Input (seed, fertilizer, bulk N deposition, and symbiotic N 2
fixation) and output (harvested grain, N leached via drainage
water and nitrous oxide (N 2 O) emission) flow of N were
measured, and the field N budget was estimated by subtracting
the N output from the N input (Fig. 6.20, Takakai et al. 2010,
2017a). The grain yield of soybean for the 3 years ranged
from 291 to 410 g m
−2 , with an average of 341 g m
−2 .
The annual N budgets in an upland field with soybean
cultivation for 3 years were consistently negative (i.e., there
was a net N loss from the field), and ranged from −10.9 to
−7.9 g N m
−2 y
−1 , with an average of −9.6 g N m
−2 y
−1 .
The validity of the estimation was confirmed by its agreement with the amount of N loss estimated based on changes
in soil N storage (Takakai et al. 2017a). The major component of total N input (20.1 g N m
−2 y
−1 ) was symbiotic
N 2 fixation, which accounted for 83% of total input, and the
major components of total N output (29.8 g N m
−2 y
−1 )
were harvested grain and N leaching, which accounted for
74% and 25% of total N output, respectively. Based on the
cultivation guidelines for this region, to avoid the excessive
growth of soybean, chemical fertilizer was not applied for
the first year of soybean cultivation after field conversion.
The amount of chemical fertilizer application for the following 2 years (second and third years) was 2 g N m
−2 ,
which was applied as basal fertilizer. Bulk N deposition
tended to increase in the fallow season, including in winter.
The percentage of soybean N accumulation that was derived
from N 2 fixation remained relatively constant (60–69%) over
the 3 years.
(2) Nitrogen budget in a paddy field with rice
cultivation
Following the 3 years of soybean cultivation under upland
condition, the annual N budget during rice cultivation under
flooded paddy conditions was also evaluated for 3 years.
The input and output N flows were measured and compared
with those measured during soybean cultivation (Table 6.8;
Takakai et al. 2017a). The yield of brown rice for the 3 years
ranged from 422 to 608 g m
−2 , with an average of
519 g m
−2 .
The three-year average of N budget during rice cultivation
was negative (–2.3 g N m
−2 y
−1 ), indicating a net N loss,
although this loss was less than that observed during soybean
cultivation. The N budget indicated a significant loss of N,
unlike previous reports in continuous paddy fields which
indicated no change in N budget or a slight N gain (e.g.,
+1.28 g N m
−2 ; Katayanagi et al. 2013). Additionally, the N
budget measured during rice cultivation varied greatly among
the 3 years (–5.7 to +0.2 g N m
−2 y
−1
) due to the difference
in fertilizer application rates, which accounted for 63% of the
total N input over the 3-year period. In paddy fields converted
from upland fields, the reduction of basal N fertilization is
recommended to avoid excessive growth and lodging due to
the increased nitrogen uptake. Chemical fertilizer was
applied at a rate of 0 or 6 g N m
−2 to all plots as a
basal fertilizer in the first or second and third years, respectively. A total of 2–3 g N m
−2 of chemical fertilizer was also
applied as topdressing in late July. The N loss from the paddy
fields in this study could be significantly higher in the first
year after conversion with low N fertilization. Consequently,
in the paddy–upland rotation system, considerable loss of
N may occur in both the soybean and rice cultivation periods.
The major components of the total N output were harvested grain and N leaching, which accounted for 49 and
29% of total N output, respectively. The annual N loss via
leaching during soybean cultivation was lower than that
during rice cultivation.
In order to maintain the soil N fertility in a rotated paddy
field with soybean cultivation, techniques for the mitigation
of N loss, such as the application of organic matter (e.g.,
green manure such as hairy vetch and manure compost;
Sato T, in this chapter; Nishida M, in this chapter; Nishida
et al. 2013) may be essential. However, because the application of organic matter to rotated paddy fields can
significantly change the N flows, its effects on the N budget
6 Tohoku Region
211