Site description
Kureha Hill is located in Toyama Prefecture, Japan (36°41’ N, 137°09’
E). Figure 1-1 shows the location of Kureha Hill.
Kureha Hill is a small mountain formed by a fault, and it stretches for
2 km from north to south. Its highest altitude is 145 m. It overlays
alternating layers of sand and mud deposited by Quaternary Lake (Fujii
& Yamamoto, 1979). Eighty percent of the hill is covered by deciduous
trees, primarily 60-year-old Konara (Quercus serrata ). The rest
of Kureha Hill is covered with bamboo (Phyllostachys pubescens )
and afforested 70-year-old coniferous Sugi (Cryptomeria
japonica ). The average annual precipitation at Toyama City’s weather
station, located 1 km from Kureha Hill, is 2388 mm (Japanese
Meteorological Agency). No agricultural areas exist within these
watersheds. There are many small streams, and the characteristics of the
stream water include low pH, low acid-neutralizing capacity (ANC), and
low concentrations of base cations. The nitrate concentrations are high
throughout the year (Honoki et al., 2001). The Hyakumakidani stream is
one of the streams on Kureha Hill whose water quality has been
intensively investigated for more than 20 years.
In the Hyakumakidani watershed, nitrate that exceeds the nitrogen supply
from the atmosphere by rainfall leaches into the stream water. Figure
1-2 shows the nitrogen budget in the Hyakumakidani watershed from 1999
to 2002 (Honoki et al., 2001). The entire area of Kureha Hill is
considered to be nitrogen saturated, since many streams contain high
concentrations of nitrate. However, a recent report indicated that the
concentration of nitrate was not as high as before (Takahashi &
Kawakami, 2023).