1. Introduction
In addition to the Haber–Bosch process, which made artificial nitrogen fixation possible in 1913, large amounts of nitrogen compounds have been fixed by artificial activities such as the high consumption of fossil fuels. It was estimated that artificial nitrogen fixation had exceeded natural nitrogen fixation by the end of the 20th century (Galloway & Cowling, 2002, Fowler et al., 2013).
Galloway et al. pointed out that the excess nitrogen was retained in groundwater, the soil, or vegetation (Galloway et al., 2008). As a result, nitrogen is no longer a factor limiting the growth of forest ecosystems, and nitrate is leached into stream water (Brown et al., 1988, Galloway et al., 1995). Aber et al. defined this situation as “nitrogen saturation”, which takes place when more reactive nitrogen is chronically supplied from the atmosphere to the forest ecosystem than is required by forest ecosystems. Aber et al. proposed four stages of nitrogen saturation according to its progress, as shown in Table 1-1 (Aber et al., 1989).
On the other hand, Stoddard applied the differences in nitrate leakage into stream water to Aber’s four stages, as shown in Table 1-2 (Stoddard, 1994).
With regard to nitrogen saturation, many experiments and studies have been done to investigate the response of the ecosystems and nitrate leakage to stream water by adding nitrogen on the forest floor (Molden et al., 1995, Tietema et al., 1997, Wallace et al., 2007, Lundin & Nilsson, 2021, Magill et al., 2004) and on the canopy (Tian et al., 2022), and by cutting vegetation (Likens et al., 1970). Olsson et al. reported the nitrate leaching to stream water when a spruce stand was severely damaged by a windstorm (Olsson et al., 2022). The effects of long-term nitrogen addition on the forest ecosystem and on the nitrate leakage to stream water have been reported recently (Magill et al., 2000, Magill et al., 2004). Not all studies supported the hypotheses advocated by Aber et al. and Stoddard; however, many studies reported phenomena in accordance with the hypotheses (Lovett & Goodale, 2011).
Different from these experimental plots, Kureha Hill, Toyama, Japan seems to have suffered from nitrogen saturation, since many streams on Kureha Hill have high concentrations of nitrate from more than twenty years ago, even though no nitrogen has been added, and no vegetation has been cut (Kawakami et al., 2001). In one of the streams, Hyakumakidani, the nitrate concentration in stream water averaged as high as 160 μmol/L from August 1998 to August 2001 (Honoki et al., 2001), which is higher than that of a Japan-wide stream-monitoring campaign (25.7 μmol/L) (Konohira et al., 2006) and that of the Kanto District of Japan (54 μmol/L) (Nishina et al., 2017). Also, the nitrate concentration in the stream water exceeded the deposition of nitrogen by precipitation (Kawakami et al., 2001). In addition, the nitrate concentration seems to be regulated only by the flow rate of the stream, and uptake by vegetation resulted in no observable impact. Therefore, according to Stoddard’s definition (Stoddard, 1994), the nitrogen saturation in Kureha Hill is considered to be Stage 3.
In addition, the stream water of Hyakumakidani is acidified to a pH level of 5.2 (Honoki et al., 2001), and high concentrations of aluminum are being leached (Honoki et al., 2001).
In this study, the 20-year time trends of nitrate concentration and other water qualities, as well as net nitrogen mineralization and the net nitrification of the soil on the nitrogen-saturated watershed and stream water, are discussed.