Hydrological and Biogeochemical Controls of the Seasonality of Particulate and Dissolved Nitrogen Exports During Rainfall Events From a Forested Watershed in Monsoon Asia, Japan

Few studies have investigated the effects of hydrological conditions such as rainfall magnitude and soil nitrogen dynamics on nitrogen export from forests during rainfall events. In this study, the seasonal export of particulate and dissolved nitrogen during 24 rainfall events (range: 3.0–417 mm) in a temperate monsoon forest watershed in Japan was measured along with the seasonality of the soil nitrate (NO3− ${{\text{NO}}_{3}}^{-}$) pool size and the nitrification rate. The increase in nitrogen export with increasing streamflow was larger for particulate nitrogen than for NO3− ${{\text{NO}}_{3}}^{-}$ and dissolved organic nitrogen, but NO3− ${{\text{NO}}_{3}}^{-}$ was always the most abundant nitrogen component exported, even during extreme rainfall. For all rainfall events in summer, measured NO3− ${{\text{NO}}_{3}}^{-}$ exports were higher than the averaged NO3− ${{\text{NO}}_{3}}^{-}$ exports, regardless of rainfall magnitude or preceding rainfall. Nitrification activity in the soil was high in summer because of the high temperature and wet conditions of the soil. Nitrification also occurred in the upper slope in summer, whereas during other seasons it is hindered by dry conditions. These results indicate that nitrogen dynamics in the soil also affect NO3− ${{\text{NO}}_{3}}^{-}$ export during rainfall. Comparisons with previous studies suggested that the effects of soil nitrogen dynamics on NO3− ${{\text{NO}}_{3}}^{-}$ concentrations in stream water may be stronger during rainfall than during base flow conditions. They also showed that regions with wet summers due to rainfall are more sensitive to NO3− ${{\text{NO}}_{3}}^{-}$ export, compared with other regions, and that increased summer rainfall can have a particularly large impact on nitrogen export from forest watersheds. Key Points NO3− ${{\text{NO}}_{3}}^{-}$ was the most abundant nitrogen component exported, even during extreme rainfall events For summer rainfall events, measured NO3− ${{\text{NO}}_{3}}^{-}$ exports were higher than averaged ones The relationship between NO3− ${{\text{NO}}_{3}}^{-}$ production potential in soil and nitrogen export applies to rainfall events as well as baseflow