Causal Relationships between Stream NO3− Concentrations at Baseflow Conditions and Key Regulating Factors in the Kinki Region of Japan, Including the Japan-sea Side Area and Encompassing a Climatic Gradient

Large nitrate (NO3−) leaching from forests is undesirable. Along the Japan-sea side, stream NO3− concentrations are low level in Japan despite the high level of nitrogen (N) deposition. Here, the causal relationships between stream NO3− 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 NO3− 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 NO3− 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 NO3− leaching. Heavy rain over the entire KIN and snow along JSK lowered NO3− concentration in soil water and NO3− 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 NO3− leaching using SEM.