Identifying Controls on Nitrate Sources and Flowpaths in a Forested Catchment Using a Hydropedological Framework

Catchment-scale assessments of nitrogen retention and loss rarely account for soil and landscape heterogeneity and are, thus, unable to account for the suite of nitrogen cycling processes that ultimately affect the export of nitrate via stream water. Long-term study at the Hubbard Brook Experimental Forest, NH has generated a unique data set that facilitates spatially explicit examination of interactions among hydrology, soil development, and nitrogen cycling processes. Using high-frequency streamwater chemistry data with intensive subsurface hydrology and solute monitoring, we tracked areas of the catchment that are hydrologically active under different flow conditions to identify the source area of streamwater nitrate. We hypothesize that as the drainage network expands, increasing hydrologic connection to bedrock outcrop-associated soils, streamwater nitrate concentration, and flux at the catchment outlet increase. Most nitrate export (>80%) occurred during high flows when high nitrate, bedrock-controlled areas of the catchment were most connected hydrologically to the drainage network (similar to 15% of the time). End-member mixing analysis demonstrated that the bedrock-controlled upper part of the catchment influences nitrate concentration at the outlet and contributes most to catchment nitrate export compared to the near-stream soil units and seeps. Most of the time, nitrate at the catchment outlet comes from seeps and the near-stream zone; under high flow conditions, bedrock-controlled hotspots for nitrate production contribute more to export of nitrate. This analysis demonstrates how the source area of streamwater nitrate varies under different flow conditions, suggesting that long-term nitrate dynamics may be driven primarily by a relatively small part of the catchment.