Though high rates of nitrate (NO 3 À ) leaching from forests are undesirable, the factors significantly regulating stream NO 3 À concentration is not clarified yet. In Japan, not only near metropolitan areas but also the Japan Sea-side area with heavy snowfall is well known for receiving more than 10 kg-N ha À1 year À1 of nitrogen (N) deposition.However, NO 3 À concentration in stream water is relatively low in the Japan Sea-side area compared with its concentration in other areas. We examined important environmental factors regulating stream NO 3 À concentrations at baseflow condition in a large region of Japan, the Kinki region (KIN) including a part of Japan Sea-side (JSK)using Random Forest regression. The amounts of N deposition and precipitation were common regulating factors for stream NO 3 À concentration at baseflow condition. Random forest showed the significant correlation between the factors related to ecosystem N retention and stream NO 3 À concentration at baseflow condition, and it suggests that large N deposited during the growing season was incorporated into the ecosystem in the entire KIN. Heavy rain and snow flush N and wash out N accumulated in the surface soil, causing small N accumulation in forests. Also, large precipitation dilute NO 3 À concentration in baseflows. These things lowered stream NO 3 À concentration at baseflow condition. Especially in JSK, most of N deposed with the heavy snow flushed out during the snowmelt period. We provided the first statistical confirmation using Random Forest regression that N accumulation and cycling in forest ecosystems were related to NO 3 À leaching from forests into streams.

times that from IM. Therefore, a previous study indicated that the forest ecosystem at the KR site was N-saturated. To elucidate the mechanism underlying N-saturation in KR using a kinetic Nsaturation model, we examined the N sink components of the ecosystem, i.e., N retention in plants and soil and N leaching into hillside streams. The soil N pools did not differ significantly between the two sites, but the net N mineralization and nitrification rates of surface soils depth:cm were significantly higher in the KR. Foliar N concentrations and litter N cycling rates from plants to soil were higher on the KR hillside, indicating that plant N retention in this site exceeded retention levels in IM. Stream nitrate concentrations were higher in water samples collected in KR, indicating that outward N leaching rates were higher in this site than in IM. High N retention levels in plants in the KR likely increase the rate of litter N cycling, which may in turn accelerate rates of surface soil N mineralization, nitrification, and N leaching. These mechanisms may explain the kinetic N-saturation levels at the KR.

Long-term nitrogen ( N ) loading on forest ecosystems can lead to " N-saturated forests " . Such ecosystems are likely to have high stream nitrate( NO − )concentrations. Recent increases in atmospheric emissions from livestock and agriculture activities and combustion of fossil fuels have changed the N cycle in terrestrial ecosystems. We investigated stream NO − concentrations from forest catchments free from human disturbances within Miyazaki Prefecture and assessed the relationships with N depositions and catchment properties( e.g. , climate, topography, vegetation, geology, and soil ) . Stream NO − concentration ranging from ． -． ㎎ N L − indicating that factors controlling stream NO − concentration were variable. High NO − concentrations were more common for sites with low annual precipitation, high ammonia emission-based N depositions from livestock and agriculture, and high annual N deposition. Temperature, elevation, and radiation are also important factors. Our study is the first to show the influence of livestock and agriculture via N deposition on mountain stream chemistry in Japan.

Large nitrate (NO 3 − ) leaching from forests is undesirable. Along the Japan-sea side, stream NO 3 − concentrations are low level in Japan despite the high level of nitrogen (N) deposition. Here, the causal relationships between stream NO 3 − concentrations and key regulating factors were investigated using structural equation modeling (SEM) for the Kinki region (KIN) of Japan, which consists of seven prefectures and includes the Japan-sea side of KIN (JSK), with the objectives of identifying the specific mechanism of NO 3 − leaching for the JSK and comparing it with the one for the entire KIN. In the previous study, stream water was collected from 1,691 watersheds without human-created features in KIN (405 from JSK) between 1997 and 2012, and key factors regulating NO 3 − leaching were clarified using Random Forest regression. SEM mostly supported the previously suggested mechanisms. Deposited N was incorporated into the ecosystem, enhanced N mineralization and nitrification in the soil, and increased NO 3 − leaching. Heavy rain over the entire KIN and snow along JSK lowered NO 3 − concentration in soil water and NO 3 − leaching. Contrary to the expectation, precipitation during the growing season along JSK contributed to N input into the ecosystems. We provided the first statistical mechanism of NO 3 − leaching using SEM.