Flushing of Subsurface Soil Nitrate as the Primary Cause of Increased Stream Nitrate During Storm Events

Abstract

The export flux of nitrate from forested catchments via streams typically increases during storms due to simultaneous increases in the nitrate concentration and flow rate. Potential nitrate sources responsible for this increase include rainwater and soil nitrate. To identify the dominant source during storms, we monitored the temporal variations in the concentration and stable isotopic composition (δ ¹⁵ N, δ ¹⁸ O, and Δ ¹⁷ O) of stream nitrate eluted from a forested catchment (298 ha) during three storm events. 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 Δ ¹⁷ O of stream nitrate exhibited a significant linear correlation with the reciprocal of concentration. This relationship indicates that variations in the mixing ratio between stream nitrate during baseflow periods and the source of increased stream nitrate during storms 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 storms. Our results suggest that monitoring the temporal variations in the isotopic composition of stream nitrate during storms offers a promising approach for constraining the isotopic composition of spatially integrated soil nitrate.