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.