High in the White Mountains of New Hampshire, there’s a small, forested watershed that scientists treat like a living laboratory. Here, at the Hubbard Brook Experimental Forest, researchers have long investigated how water, soil, and forest ecology interact—creating one of the most studied forest ecosystems in the world. In their new paper, “Forest catchment structure mediates shallow subsurface flow and soil base cation fluxes,” the authors delve into an especially intriguing puzzle: why do certain parts of a forested hillside export more nutrients than others.
It’s all about how the “plumbing” of the land is laid out—both above and below the surface.
A Subsurface Symphony
“In this study we aim to characterize the spatial patterns of annual base cation fluxes through variably saturated shallow soil,” the authors explain. They’re particularly interested in base cations—calcium (Ca), magnesium (Mg), and sodium (Na)—all of which are vital for healthy forest soils. Unlike nitrogen or carbon, which you might have heard discussed more commonly, these base cations often get overshadowed. Yet they’re crucial in helping soils buffer acidity and nurture tree growth.
According to the paper, “Base cations (i.e., Ca, Mg, Na, and K) are of interest to pedologists, biogeochemists, ecologists, and hydrologists alike.” Acid rain, which once ravaged the northeastern U.S., depleted many of these essential elements, making their replenishment and movement through the forest a priority for study.
They offer this examination of how these elements move through the landscape. “Soil forming processes critically depend on soil water to transport, translocate, and transform solid and dissolved material,” the authors write. As rain or snowmelt percolates into the ground, it dissolves minerals and takes nutrients downhill. These flows can be fast or slow, shallow or deep, depending on how the underlying soils and bedrock layers are arranged.
Catchment Structure: The Shape of the Land
A “catchment” is simply an area of land where water collects when it rains, funneling downhill into streams or rivers. In mountainous forests with complex slopes, the authors note “high variability” in both soil properties and hydrologic behavior.
Thus, they set out to map where shallow groundwater flows, and how frequently it rushes to the surface. Their catchment of choice—Watershed 3 at Hubbard Brook—offers steep hillslopes, bedrock outcrops, and varied soil depths. According to the paper, “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 development.”
In simpler terms, steep upper slopes where thin soils meet exposed bedrock send quick bursts of water through the soil after a storm. Downhill areas tend to stay saturated longer but don’t see as many abrupt “flushes.” Such differences may not seem dramatic at first. Yet the authors reveal that they matter a great deal for nutrient transport.
Measuring the Mystery: Ion-Exchange Resins
To capture the year-round flux of base cations, the authors used ion-exchange resins. These resin-filled packs were placed in PVC wells about 0.3–1 meter deep in the soil—right where shallow groundwater would pass through. As water flowed around the resin, the cations stuck to the beads, giving scientists a time-integrated way to track how much Ca, Na, and Mg were moving through.
The paper calls these “passive flux meters,” a clever way to sidestep the challenge that “the transient water table may only exist for a few hours or less during a storm event.” Traditional water sampling might miss that short window, but the resin sits there continuously, collecting data.
Over a year of periodic swaps, the authors discovered striking patterns: certain wells in shallow, rocky, upper-slope soils captured far more cations. In the words of the paper, “Base cation fluxes were greatest in soils that were most weathered,” indicating active mineral dissolution in those zones. Put another way, whenever heavy rain or melting snow quickly raised and lowered the water table, it led to more chemical reactions that released cations into the flow.
Storm Events, “Flushing,” and Frequent Saturation
The study repeatedly mentions the importance of saturation frequency—the number of times the water table reached a given depth and then receded. “Base cation fluxes were positively correlated with soil saturation frequency,” the authors write, “and were greatest in soil profiles where primary minerals were most depleted of base cations (i.e., highly weathered).”
Picture a household sponge that’s repeatedly soaked and squeezed. Each time you do so, some solutes (or soap in a real sponge analogy) get flushed out. Forest soils near ridges and bedrock outcrops act similarly during storm events: water rises, saturates the soil, then drains swiftly, carrying cations away. Meanwhile, in lower slope or riparian areas, the sponge (soil) stays wet for a longer period—yet it doesn’t get “squeezed” as often. The authors confirm, “While shallow subsurface flow is a recognized mechanism by which soil water reaches streams, direct quantification…is currently lacking.”
This frequent push of new water not only mobilizes elements but also exposes fresh mineral surfaces to chemical reactions, further speeding the release of cations. “Repeated hydrologic flushing of the soil profile…could promote conditions that favor mineral dissolution reactions,” the authors explain, citing prior research on how strong flows enhance weathering.
Implications for Forests and Streams
Forest health depends on these base cations to balance soil acidity and nourish plants. “Refining catchment controls on base cation generation and transport,” the authors emphasize, “could be an important tool for opening the black box of catchment elemental cycling.”
Moreover, acid rain might have eased somewhat, but the legacy of depleted soils endures. The study’s findings suggest that these ridges and thin-soil areas might be hotspots for regenerating and releasing essential nutrients—as long as we understand how the water flows. Climate change, with its pattern of heavier downpours, could even heighten the “flush factor,” accelerating nutrient losses in certain areas and gains in others.
The Bigger Picture: Landscapes, Hidden Currents, and Future Questions
“Variations in stream solute export as a function of discharge (e.g., concentration-discharge relationships) justify the notion that soil–water contact time and solute sources shift across flow conditions,” the authors state. It’s a reminder that each storm event can redraw a map of which soils are saturated and how minerals dissolve.
In total, the study underscores that “base cation fluxes are neither uniform nor random” but follow a “systematic spatial arrangement determined by the structure of a catchment’s topography and hydrology.” The more precisely we can trace these underground currents, the better we’ll grasp the forest’s capacity to bounce back from historical acidification—and anticipate new shifts as weather patterns change.
For forest managers, pinpointing these nutrient “hotspots” could lead to strategies that protect or restore base cations in the most vulnerable areas. As the authors conclude, “This work highlights the importance of further compartmentalizing solute fluxes along hillslopes, where certain areas may disproportionately contribute solutes to the whole catchment.”
In the grand symphony of a forest’s water cycle, these findings compose a revealing new section—showing us where the orchestra of soil, rock, and flowing water plays its most vigorous notes. And sometimes, it’s the steepest spots that ring out the loudest, flushing precious nutrients all the way down to the stream.
Cited Paper: Pennino, A., Strahm, B. D., McGuire, K. J., Bower, J. A., Bailey, S. W., Schreiber, M. E., Ross, D. S., Duston, S. A., & Benton, J. R. (2024). Forest catchment structure mediates shallow subsurface flow and soil base cation fluxes. Geoderma, 117045. https://doi.org/10.1016/j.geoderma.2024.117045.