Forest Catchment Structure Mediates Shallow Subsurface Flow and Soil Base Cation Fluxes

Abstract

Across forested landscapes with complex topography, hydrologic behavior and soil properties can exhibit high spatial variability. The interaction of groundwater with spatially distinct soils generates and transports solutes across catchments, although spatiotemporal relationships between groundwater dynamics and soil solute fluxes are difficult to directly evaluate. While whole-catchment export of solutes with shallow subsurface flow represents an integration of soil environments and conditions, soil solute fluxes are often compartmentalized as hillslope vs. riparian, deep vs. shallow, or as individual soil horizon contributions. This potentially obscures and underestimates the hillslope variation and magnitude of solute fluxes and soil development across the landscape. This study measured the spatial variation and correlation between shallow soil base cation fluxes associated with weathering reactions (Ca, Mg, and Na), soil elemental depletion, and soil saturation dynamics within a small, forested watershed at the Hubbard Brook Experimental Forest, NH. Base cation fluxes were calculated using a combination of ion-exchange resins placed in shallow groundwater wells (0.3 – 1 m depth) located across a hillslope position gradient (ridges to lower backslopes) and measurements of groundwater levels. Groundwater levels were also used to create metrics of annual soil saturation behavior. Base cation fluxes were positively correlated with soil saturation frequency and were greatest in soil profiles that where primary minerals were most depleted of base cations (highly weathered). Spatial differences in soil saturation across the catchment are interpreted to result from variations in transient groundwater behavior and were strongly related to topographic properties of the upslope draining area. Results from this work suggest that the structure of a catchment defines the spatial architecture of base cation fluxes, likely reflecting the mediation of subsurface stormflow dynamics on soil developmental processes. Furthermore, this work highlights the importance of further compartmentalizing solute fluxes along hillslopes, where certain areas may reflect disproportionate contributions of solutes to the whole catchment. Refining catchment controls on base cation generation and transport could be an important tool for opening the black box of catchment elemental cycling.

Keywords: forest soils, nutrients, mineral weathering, groundwater, Hubbard Brook, Critical Zone Science