4. Conclusion
Nitrate leakage to some of the streams on Kureha Hill, Toyama, Japan
tended to decrease in the 24 years from 1998 to 2022. The reduction in
nitrate leakage could be attributed to the reduction in nitrogen
deposition. However, since the rate of reduction in nitrate leakage
varied across rivers, there must be other factors regulating nitrate
concentrations, such as nitrogen pools in the soil. The Hyakumakidani
stream, one of the streams on Kureha Hill, had an acidic pH of 5.2 due
to the leached nitrate. Despite the recent decline in nitrate
concentration, the pH level has been unchanged. This may be due to
decreased concentrations of sulfate and ANC.
The C/N ratio of the soil in the Hyakumakidani watershed ranged from 12
to 18 from 2000 to 2023, which is lower than the nitrogen saturation
threshold of 25. Net nitrification rate and net mineralization rate
showed no significant changes between 2002 and 2023. The potential to
produce nitrate has been unchanged for 20 years. Similar values in net
nitrification and net nitrogen mineralization rates, which are
characteristic of Stage-3 nitrogen saturation according to Aber’s
prediction, reflect that the Hyakumakidani watershed remains in Stage-3
nitrogen saturation.