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Landscape heterogeneity drives contrasting concentration–discharge relationships in shale headwater catchments

Solute concentrations in stream water vary with discharge in patterns that record complex feedbacks between hydrologic and biogeochemical processes. In a comparison of three shale-underlain headwater catchments located in Pennsylvania, USA (the forested Shale Hills Critical Zone Observatory), and Wales, UK (the peatland-dominated Up- per Hafren and forest-dominated Upper Hore catchments in the Plynlimon forest), dissimilar concentration-discharge (C-Q) behaviors are best explained by contrasting landscape distributions of soil solution chemistry - especially dissolved organic carbon (DOC) - that have been established by pat- terns of vegetation and soil organic matter (SOM). Specif- ically, elements that are concentrated in organic-rich soils due to biotic cycling (Mn, Ca, K) or that form strong com- plexes with DOC (Fe, Al) are spatially heterogeneous in pore waters because organic matter is heterogeneously dis- tributed across the catchments. These solutes exhibit non- chemostatic behavior in the streams, and solute concentra- tions either decrease (Shale Hills) or increase (Plynlimon) with increasing discharge. In contrast, solutes that are con- centrated in soil minerals and form only weak complexes with DOC (Na, Mg, Si) are spatially homogeneous in pore waters across each catchment. These solutes are chemo- static in that their stream concentrations vary little with stream discharge, likely because these solutes are released quickly from exchange sites in the soils during rainfall events. Furthermore, concentration-discharge relationships of non-chemostatic solutes changed following tree harvest in the Upper Hore catchment in Plynlimon, while no changes were observed for chemostatic solutes, underscoring the role of vegetation in regulating the concentrations of certain ele- ments in the stream. These results indicate that differences in the hydrologic connectivity of organic-rich soils to the stream drive differences in concentration behavior between catchments. As such, in catchments where SOM is dom- inantly in lowlands (e.g., Shale Hills), we infer that non- chemostatic elements associated with organic matter are re- leased to the stream early during rainfall events, whereas in catchments where SOM is dominantly in uplands (e.g., Plyn- limon), these non-chemostatic elements are released later during rainfall events. The distribution of SOM across the landscape is thus a key component for predictive models of solute transport in headwater catchments.