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.