3-2. Nitrogen budget
Figure 3-8 shows the nitrogen budget in the Hyakumakidani watershed from
2019 to 2021 in addition to from 1999 to 2002. The average inputs and
outputs for 2019–2021 were 1046 eq/ha/year and 1097 eq/ha/year,
respectively. In contrast, the average inputs and outputs for 1999–2002
were 1598 eq/ha/year and 2317 eq/ha/year, respectively. Compared to
1999–2002, both input and output decreased significantly from 2019 to
2021. Accordingly, the decrease in output could be attributed to the
decrease in input. However, when taking consideration of other streams
than the Hyakumakidani stream (Fig 3-7), the Kanaya stream showed the
same nitrate concentration while the Anyobo stream showed a drastic
decrease. There must be other factors regulating nitrate concentration
such as nitrogen pool in the soils (You et al. 2001). In 1999–2002,
output exceeded input every year. However, output became smaller than
input in 2020 and 2021. The nitrogen saturation is considered to be
mitigated or slowed in its progress.