How Iron Minerals Enable Long-Term Carbon Storage in Soil

Ludmilla Aristilde, professor of civil and environmental engineering at Northwestern University, has spent years examining how carbon moves through soils and sediments. In a recent study, Aristilde and her research team provide a detailed explanation of why iron oxide minerals are particularly effective at locking away organic carbon for long periods, sometimes lasting decades or even centuries. Their findings help clarify one of the less visible but highly influential mechanisms in the global carbon cycle.

Dai, Y., Chan, H., Vriza, A., Fan, J., Kim, F., Wang, Y., Liu, W., Shan, N., Xu, J., Weires, M., Wu, Y., Cao, Z., Miller, C. S., Divan, R., Gu, X., Zhu, C., Wang, S., & Xu, J. (2025). Adaptive AI decision interface for autonomous electronic material discovery. Nature Chemical Engineering, 2(12), 760–770. https://doi.org/10.1038/s44286-025-00318-3

Soils represent one of the largest reservoirs of carbon on the planet, holding more carbon than the atmosphere and vegetation combined. Despite their importance, the physical and chemical processes that allow soils to retain carbon remain only partly understood. Iron oxide minerals have long been associated with stable carbon storage, but the precise reasons for their effectiveness have remained uncertain. The new study addresses this gap by examining interactions at the molecular scale.

Ludmilla Aristilde, professor of civil and environmental engineering at Northwestern University, stated,

“Iron oxide minerals are important for controlling the long-term preservation of organic carbon in soils and marine sediments. The fate of organic carbon in the environment is tightly linked to the global carbon cycle, including the transformation of organic matter to greenhouse gases. Therefore, it’s important to understand how minerals trap organic matter, but the quantitative evaluation of how iron oxides trap different types of organic matter through different binding mechanisms has been missing.”

The research centers on ferrihydrite, a common iron oxyhydroxide mineral that forms readily in environments rich in organic matter. Ferrihydrite is especially prevalent in soils near plant roots and in sediments influenced by biological activity. Previous studies linked ferrihydrite to carbon sequestration but often relied on simplified assumptions about how organic molecules attach to mineral surfaces.

One of the key findings of the study is that ferrihydrite does not rely on a single binding process. Instead, it uses multiple chemical and physical mechanisms simultaneously. This multi-mechanism approach allows ferrihydrite to interact with a wide range of organic compounds, increasing the likelihood that carbon entering the soil becomes stabilized rather than rapidly decomposed by microbes.

Earlier models typically described ferrihydrite as having an overall positive surface charge. This led to the assumption that negatively charged organic molecules would be the primary contributors to mineral bound carbon. However, Aristilde’s team found that this description overlooks important surface level complexity. Using high resolution molecular modeling and atomic force microscopy, the researchers discovered that ferrihydrite’s surface contains a patchwork of positive and negative charge regions at the nanoscale.

This uneven charge distribution explains why ferrihydrite can bind organic compounds with very different chemical characteristics. Positively charged, negatively charged, and neutral molecules are all able to attach to the mineral surface, depending on local charge conditions and molecular structure. The mineral’s overall positive charge is therefore the result of many small charge regions rather than a uniform surface.

To understand how these surface features translate into carbon retention, the research team conducted controlled laboratory experiments using organic molecules commonly found in soils. These included amino acids, organic acids derived from plants, sugars, and ribonucleotides. Each group of molecules exhibited distinct binding behavior when exposed to ferrihydrite.

Some amino acids bonded through electrostatic attraction, attaching to surface regions with opposite charge. Other compounds were first drawn to the mineral surface by weaker interactions and then formed stronger chemical bonds directly with iron atoms. Sugars, which lack strong electrical charge, were primarily retained through hydrogen bonding. Although these bonds are weaker, they still reduce microbial access and slow decomposition.

The coexistence of these binding pathways helps explain why iron oxides are associated with a substantial fraction of soil carbon worldwide. Organic molecules that bind more strongly to mineral surfaces are less accessible to microorganisms that would otherwise convert them into carbon dioxide. Over time, this protective effect allows carbon to accumulate and remain stored in soils.

Iron oxide minerals are estimated to be linked to more than one third of all organic carbon preserved in soils. This makes them a critical factor in regulating the exchange of carbon between land and atmosphere. The new findings suggest that mineral composition may be just as important as biological inputs when assessing a soil’s capacity to store carbon.

From an engineering perspective, the study provides quantitative insight into mineral organic interactions that can be incorporated into carbon cycle models. Many current models treat soil carbon as a relatively uniform pool, but the results highlight the importance of chemical specificity. Different organic compounds follow different stabilization pathways, leading to varied persistence times.

The research also has implications for land management and climate mitigation strategies. Practices that influence soil mineral composition or iron oxide formation could indirectly affect how much carbon soils are able to retain. While the study does not propose direct engineering interventions, it strengthens the scientific basis for considering mineral chemistry in carbon sequestration discussions.

Aristilde’s work builds on earlier research from her group that examined how clay minerals bind organic matter and how soil microbes preferentially degrade certain compounds. By extending this framework to iron oxides, the study offers a more complete picture of how physical, chemical, and biological factors interact to control carbon fate in soils.

Future investigations will focus on what happens after organic molecules bind to mineral surfaces. Some compounds may undergo chemical transformation that increases their stability, while others may eventually be released and decomposed. Understanding these downstream processes will be essential for predicting how long carbon remains stored under changing environmental conditions.

Although the interactions described in the study occur at the nanoscale, their cumulative impact is global. Iron minerals quietly influence how much carbon remains buried beneath our feet, shaping Earth’s climate balance over timescales far longer than human observation.