Environmental Determinants of Stream Nitrate Concentrations During Baseflow Conditions in Undisturbed Mountain Streams of Southern Parts of Kyushu, Japan

This study investigated the environmental determinants of nitrate (NO₃ ^-) concentrations from undisturbed mountain streams in the southern part of Kyushu Main Island, Japan. Four hundred twenty-seven water samples were collected between April 2021 and December 2022 under the baseflow condition. Random Forest (RF) regression model was used to identify the important environmental factors affecting stream NO₃ ^- concentrations. The RF result revealed that annual precipitation is the most significant determinant, negatively correlation with NO₃ ^- concentrations. This suggests that higher precipitation enhances NO₃ ^- flushing from forest soils, reducing NO₃ ^- availability during subsequent baseflow periods. In contrast, temperature was positively correlated, indicating that higher temperature may increase nitrogen mineralization and nitrification rates, leading to more NO₃ ^- leaching. Atmospheric nitrogen (N) depositions derived from agricultural emissions and fuel combustions are also significantly influenced stream NO₃ ^-. Stream NO₃ ^- concentrations tended to increase when total annual N deposition exceeded approximately 12 kg N ha^-1 year^-1. These findings highlight the importance of climatic variables, particularly precipitation and temperature, and N depositions in determining stream NO₃ ^- concentrations in the wider area, providing a valuable framework for predicting and mitigating N pollution in similar ecosystems globally.