Environmental factors regulating stream nitrate concentrations at baseflow condition in a large region encompassing a climatic gradient

Abstract

AbstractThough high rates of nitrate (NO3−) leaching from forests are undesirable, the factors significantly regulating stream NO3− 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−1 year−1 of nitrogen (N) deposition. However, NO3− 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 NO3− 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 NO3− concentration at baseflow condition. Random forest showed the significant correlation between the factors related to ecosystem N retention and stream NO3− 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 NO3− concentration in baseflows. These things lowered stream NO3− 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 NO3− leaching from forests into streams.