The early days of soil remediation studies at the Hubbard Brook Experimental Forest in New Hampshire were driven by a simple yet alarming question: how do air pollutants and acid rain reshape our forests from the ground up? By the mid-twentieth century, Hubbard Brook had become famous for measuring acid precipitation and cataloging its effects on soil composition, water quality, and forest health. Gradually, researchers realized that soils are not just inert backdrops but dynamic, living matrices, brimming with organic matter and metals that bind or release nutrients—and sometimes pollutants. Over the decades, as global interest in carbon management and ecosystem restoration ballooned, Hubbard Brook’s painstaking records helped show that the soil’s capacity to cleanse itself depends, in part, on subtle chemical relationships. The story that began with acid rain would soon expand to include studies on short-range-ordered (SRO) minerals, the complex forms of aluminum (Al) and iron (Fe) often extracted by mild acids, like oxalic acid, to gauge how soils protect or release carbon.
Against this backdrop, a recent open-access paper, “Moisture and soil depth govern relationships between soil organic carbon and oxalate-extractable metals at the global scale,” by Sophie F. von Fromm, Hermann F. Jungkunst, Bright Amenkhienan, Steven J. Hall, Katerina Georgiou, Caitlin Hicks Pries, Fernando Montaño-López, Carlos Alberto Quesada, Craig Rasmussen, Marion Schrumpf, Balwant Singh, Aaron Thompson, Rota Wagai & Sabine Fiedler, advances our understanding of the very processes Hubbard Brook researchers have studied for decades. In analyzing more than 37,000 soil samples from around the world, the authors focus on how oxalate-extractable metals—essentially forms of aluminum and iron that can be measured using oxalic acid—promote or limit carbon storage. As the authors put it, “Our results underline the importance of oxalate-extractable metals as predictors for organo-mineral interactions at the global scale.”
The historical endeavors at Hubbard Brook paved the way for such a study. Back in the 1960s and 70s, scientists found that forest soils, even under stress, had remarkable ways of binding contaminants. Often, the presence of aluminum complexes—especially in acidic soils—proved a pivotal factor in either storing metals or releasing them into the watershed. Now, the same chemical processes that once helped handle acid inputs are being scrutinized for their role in sequestering carbon. This new paper stands firmly on the shoulders of decades of soil-chemistry sleuthing, connecting local-scale lessons from places like Hubbard Brook to a planetary-scale analysis.
What stands out immediately is the emphasis on climate. “Interestingly, moisture was a more important driver of the observed patterns between SOC and oxalate-extractable metals than temperature,” the authors write. They learned that wetter zones, akin to the glaciated valleys at Hubbard Brook, enhance the binding power of oxalate-extractable metals. The result is more stable soil organic carbon (SOC), locked away beyond the reach of rapid decomposition. In drier climates, these same metal phases lose some of their ability to bind carbon, rendering soils less resilient against disturbances.
A major point of the paper is that soil depth matters. “Oxalate-extractable metals are usually most important between 20 and 100 cm under wet conditions,” the authors note. This finding resonates with decades of Hubbard Brook measurements, which have often surprised scientists by revealing that what happens in the deeper horizons can be just as critical as the surface layer for nutrient and carbon dynamics. By separating the soil into distinct layers, the authors capture the nuances of how metals, water, and organic matter form an elaborate tapestry of chemical interactions.
Crucially, for those concerned about forest recovery and carbon budgets, the paper outlines the predictive power of these metals. “Based on linear mixed-effects models, we found a positive relationship between Mox and SOC across regions and depths, accounting for 49% of the SOC variation,” the authors write. Put plainly: these soil-metal measures, shaped so long ago by acid-rain research and subsequent remedial efforts, now offer a key to predicting global carbon storage potential. And it all circles back to that fundamental insight gleaned at Hubbard Brook—that soils are not a passive sponge. They are active, evolving systems that respond to small chemical cues.
As the authors state, “Our analysis suggests oxalate-extractable metals are good proxies for mineral-induced SOC protection at the global scale,” pointing to a broader future. In the race to remediate soils—whether suffering from industrial pollution or deforestation—land managers may now look toward measuring Mox to gauge the soil’s built-in capacity for resilience. That concept, rooted in decades of localized fieldwork, is evolving into a planetary principle: if you want to store carbon or mitigate toxins in soils, be sure to check the metals that hold the keys.
After all, the story of Hubbard Brook suggests that we should never underestimate the power of meticulous measurement—and an ample dose of oxalic acid—to shed light on how the world beneath our feet really works.
von Fromm, S. F., Jungkunst, H. F., Amenkhienan, B., Hall, S. J., Georgiou, K., Hicks Pries, C., Montaño-López, F., Quesada, C. A., Rasmussen, C., Schrumpf, M., Singh, B., Thompson, A., Wagai, R., & Fiedler, S. (2025). Moisture and soil depth govern relationships between soil organic carbon and oxalate-extractable metals at the global scale. Biogeochemistry, 168, Article 20.