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

摘要

Solute concentrations in stream water vary with discharge in patterns thatrecord complex feedbacks between hydrologic and biogeochemical processes. Ina comparison of three shale-underlain headwater catchments located inPennsylvania, USA (the forested Shale Hills Critical Zone Observatory), andWales, UK (the peatland-dominated Upper Hafren and forest-dominated UpperHore catchments in the Plynlimon forest), dissimilar concentration–discharge (C–Q) behaviorsare best explained by contrasting landscape distributions of soil solutionchemistry – especially dissolved organic carbon (DOC) – that have beenestablished by patterns of vegetation and soil organic matter (SOM).Specifically, elements that are concentrated in organic-rich soils due tobiotic cycling (Mn, Ca, K) or that form strong complexes with DOC (Fe, Al)are spatially heterogeneous in pore waters because organic matter isheterogeneously distributed across the catchments. These solutes exhibitnon-chemostatic behavior in the streams, and solute concentrations eitherdecrease (Shale Hills) or increase (Plynlimon) with increasing discharge. Incontrast, solutes that are concentrated in soil minerals and form only weakcomplexes with DOC (Na, Mg, Si) are spatially homogeneous in pore watersacross each catchment. These solutes are chemostatic in that their streamconcentrations vary little with stream discharge, likely because thesesolutes are released quickly from exchange sites in the soils duringrainfall events. Furthermore, concentration–discharge relationships ofnon-chemostatic solutes changed following tree harvest in the Upper Hore catchment in Plynlimon, while no changes were observed for chemostaticsolutes, underscoring the role of vegetation in regulating theconcentrations of certain elements in the stream. These results indicatethat differences in the hydrologic connectivity of organic-rich soils to thestream drive differences in concentration behavior between catchments. Assuch, in catchments where SOM is dominantly in lowlands (e.g., Shale Hills),we infer that non-chemostatic elements associated with organic matter arereleased to the stream early during rainfall events, whereas in catchmentswhere SOM is dominantly in uplands (e.g., Plynlimon), these non-chemostaticelements are released later during rainfall events. The distribution of SOMacross the landscape is thus a key component for predictive models of solutetransport in headwater catchments.