Nitrate loss from forested catchments in streamwater has been used as an indicator of ecosystem status and response to disruptions in the N cycle. Elevated streamwater nitrate losses are associated with N saturation. Regional synchronicity in inter-annual variation of streamwater nitrate export in the northeastern US has been attributed to climatic patterns; land-use history often influences the magnitude of N export. However, site factors may also play a significant role in controlling the timing and extent of nitrate export.
Recent advances using a hydropedological approach to characterize soils at the Hubbard Brook Experimental Forest in NH based on topographic position, proximity to bedrock outcrops, and hydrologic regime led to the description and mapping of five distinct soil functional groups called hydropedologic soil units (HPU) at the catchment scale. The hydropedologic approach to soil unit classification highlights the central role that water flowpaths play in soil development. This research found that soil, soil solution, and groundwater chemistry vary systematically by HPU and suggested that transects from bedrock outcrops to deeper soils immediately downslope (E to Bhs podzols) were hotspots for C and N cycling in the catchment.
Results/Conclusions
This study used high-frequency sensor measurements of streamwater nitrate, pH, and surrogates for dissolved organic carbon (DOC; F-DOM and A254) and data from a network of wells including water table height and detailed chemistry. Based on these data, we were able to determine which portions of the landscape were likely to be connected to the stream network and contribute to nitrate export. Preliminary results suggest that when stream flowrates are high, the highest nitrate concentrations are found in the N cycling hotspots (E and Bhs podzol HPUs). In contrast, typical spodosols, which cover the greatest area in the watershed, are associated with lower nitrate concentrations and export. Even the near-stream areas, which generally maintain a water table in ~50% of the solum, have lower nitrate concentrations than the N cycling hotpots further away from the stream. These preliminary sensor data suggest that the majority of the stream nitrate export from the watershed comes from the N cycling hotspots (from bedrock outcrops to deeper soils immediately downslope). This shift in understanding of the sources of streamwater nitrate will be useful in the development of process-based models. Future climatic conditions, such as shifts in precipitation volume and intensity that could affect hydrologic flowpaths may also affect patterns of nitrate concentration and export in streamwater.