Nitrate (NO⁻3) export can vary widely among forested watersheds with similar nitrogen loading, geology, and vegetation, which suggests the importance of understanding differing internal retention mechanisms. Transport should be studied at the hillslope scale because the hillslope is the smallest unit with spatial and temporal resolution to reflect many relevant NO⁻3 retention and transport (flow-generation) processes, and headwater forested watersheds are largely comprised of sections of hillslopes. I conducted two experiments to elucidate subsurface flow dynamics and NO⁻3 transport and retention mechanisms on a constructed experimental hillslope model.

In the first experiment, I tested whether decadal pedogenetic changes in soil properties in the experimental hillslope used by Hewlett and Hibbert (1963) would lead to changes in recession flow. I repeated (twice) their seminal experiment, whose results led to the development of the Variable Source Area paradigm, by also saturating, covering, and allowing the experimental hillslope to drain until it no longer yielded water. In the historical experiment there was fast drainage for 1.5 d, followed by slow drainage for ~140 d, which led the authors to conclude that recession flow in unsaturated soil could sustain baseflow throughout droughts. This long, slow drainage period was not reproduced in my experiments. Shapes of the drainage curves in my experiments were similar to the historical curve, but slow drainage was truncated, ending after 17 and 12 d, due likely to a leak in the boundary conditions, rather than to pedogenetic changes since the historical experiment. Leakage to bedrock, analogous to the leak in the hillslope model, is a commonly observed phenomenon and this study highlights how that can reduce drainage duration and the contribution of moisture from soils to support baseflow.

In the second experiment, I tested whether movement of NO⁻3, which is considered a mobile ion, would be delayed relative to movement of water through a hillslope. I added concentrated pulses of ¹⁵NO⁻3 and a conservative tracer (²H₂O) on the same experimental hillslope, which was devegetated and irrigated at hydrologic steady state. Retention of the ¹⁵NO⁻3 tracer was high in the soil surface (0–10 cm) layer directly where the tracer was added. The portion of the ¹⁵NO⁻3 tracer that passed through this surface layer was further retained/removed in deeper soil. The reduction in the peaks in δ¹⁵N breakthrough was an order of magnitude larger than in δ₂H breakthrough at the outlet 5 m downslope of the tracer addition. The peaks in δ¹⁵N were also delayed relative to the peaks in δ₂H by 1, 6, 9 and 18.5 d for slope distances of 0, 2, 4, and 5 m, respectively, from tracer addition to the outlet. The excess mass of ¹⁵NO⁻3 recovered at the outlet was less than 3% of the original tracer mass injected. Nitrification and denitrification were estimated to be roughly 1:1 and were large fluxes relative to lateral transport into and out of the riparian zone. This tracer experiment shows that bedrock leakage, coupled with multiple retention/removal mechanisms can significantly delay export of added NO⁻3 with implications of additional NO⁻3 sink strength at the watershed scale.