The sources of nitrate exported from a watershed containing mixed forest, paddy fields, and urban areas in Japan: differences between baseflow conditions and rainfall events

Stable isotope ratios of nitrate (NO₃⁻) can be used as a tool to investigate NO₃⁻ dynamics in watersheds over the world. However, most of the NO₃⁻ source analyses from watersheds using isotopes have been conducted during baseflow conditions, and more information is required on conditions during rainfall, which is a crucial period for nitrogen export to downstream ecosystems. Additionally, there is limited information regarding the sources of stable isotopes of NO₃⁻ in watersheds of Japan. We measured the nitrogen and oxygen isotopes of NO₃⁻ (δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻) in the Yasu River during baseflow and rainfall events and those from major NO₃⁻ sources in the watershed. The δ¹⁵N–NO₃⁻ exported from forests and rice paddies showed small fluctuations, while there were large fluctuations in the δ¹⁵N–NO₃⁻ exported from sewage treatment plants, suggesting the need to obtain data on δ¹⁵N–NO₃⁻ exported from sewage treatment plants. The NO₃⁻ concentrations in the Yasu River during summer baseflow were too low to be explained by the mixing of NO₃⁻ sources within the watershed, suggesting that NO₃⁻ consumption processes within the river and groundwater influence the NO₃⁻ concentrations. Furthermore, the sources of NO₃⁻ exported from the watershed differed during baseflow and rainfall, with a larger contribution of forest-derived NO₃⁻ during rainfall. Monitoring during rainfall is therefore essential to determine the sources of NO₃⁻ exported from the watershed.