Flushing of subsurface soil nitrate as the primary cause of increased stream nitrate during storm events

The export flux of nitrate from forested catchments via streams typically increases during storm events due to simultaneous increases in the nitrate concentration and flow rate. Potential nitrate sources responsible for this increase include nitrate dissolved in rainwater and nitrate accumulated in soil layers. To identify the dominant source during storm events, we monitored the temporal variations in the concentration and stable isotopic composition (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) of stream nitrate eluted from a forested catchment (IJ1; 298 ha) during three storm events (I, II, and III). The stream nitrate concentration increased significantly, from 29.9 to 78.6 μM, from 14.8 to 51.3 μM, and from 35.4 to 108.9 μM during the storm events I, II, and III, respectively. In contrast, the triple oxygen isotopic composition (Δ¹⁷O) of stream nitrate exhibited a significant linear correlation with the reciprocal of concentration (1/[NO3−]). This relationship indicates that variations in the mixing ratio between stream nitrate during baseflow periods and the source of increased stream nitrate during storm events are responsible for the variations in the stream nitrate concentration and isotopic composition. Although the isotopic composition of nitrate in rainwater and surface soil layers deviated significantly from the composition of the nitrate-enriched endmember estimated from the mixing lines, the composition of subsurface soil nitrate aligned closely with that of this endmember. Accordingly, we concluded that nitrate flushed from subsurface soil layers was the primary source of increased stream nitrate during storm events. Our results demonstrate that monitoring the temporal variations in the isotopic composition of stream nitrate during storm events provides a viable approach for estimating the isotopic composition of spatially integrated soil nitrate.