Soil holds more carbon than the atmosphere and all living vegetation combined, yet not all of that carbon is equally stable. Researchers distinguish broadly between mineral-associated organic matter, abbreviated MAOM, which clings to the reactive surfaces of clay and silt particles and can persist for decades to millennia, and particulate organic matter, or POM, which consists of recognizable fragments of plant and microbial debris. POM can be further divided into free POM, which sits loosely between soil aggregates and is readily decomposed, and occluded POM, which has become physically trapped inside aggregates where it enjoys a degree of shelter from microbes and their enzymes. Which of these pools a soil accumulates, and in what proportions, determines whether that soil behaves as a long-term carbon vault or a short-term holding pen.

The research team hypothesized that the two fractions respond to different mineral controls. For mineral-associated organic matter, the key variable should be the total reactive surface area offered by the dominant clay mineral. Clays vary enormously in this respect: kaolinite, a low-activity 1:1 clay, presents relatively little surface, illite offers an intermediate area, and montmorillonite, a swelling 2:1 clay with expansive interlayer spaces, provides by far the most reactive real estate. For particulate organic matter, by contrast, the hypothesis centered on pore architecture rather than surface chemistry. The researchers predicted that substrates dominated by illite, which maintain comparatively larger pores, would promote the storage of POM, particularly the occluded fraction protected within those pore spaces, whereas the fine-textured montmorillonite and kaolinite substrates would offer less such physical refuge.

To test these ideas, the team grew two plant species with contrasting traits, the grass Festuca rubra and the leguminous herb Lotus corniculatus, in three soil-forming substrates dominated respectively by kaolinite, illite and montmorillonite. Crucially, the design separated the influence of plant roots from direct contact with clay by employing two exposure modes. In one, plants grew in pots filled entirely with a single clay substrate, so roots permeated the mineral matrix directly. In the other, plants grew in larger pots of sand into which the clay substrates were buried in mesh bags, allowing roots and their exudates to reach the clay by growing through the mesh without the substrate dispersing. This elegant manipulation meant the researchers could ask whether clay effects depend on intimate root-mineral contact or operate regardless of physical arrangement.

The results were strikingly consistent. Across both exposure modes and both plant species, the largest carbon storage occurred in the montmorillonite-dominated substrates, followed by illite and then kaolinite, exactly the ranking predicted by increasing mineral surface area. The accumulation of mineral-associated organic matter followed the same pattern, confirming that the reactive surface area of clay minerals is a decisive control on this slow-cycling, chemically protected carbon pool. Whether carbon arrived as root litter, rhizodeposition or microbial necromass, the abundant surfaces of montmorillonite simply offered more sites on which organic molecules could bind and be withdrawn from circulation by decomposers.

The story changed, however, when the researchers turned to particulate fractions. Free POM, the most labile pool, showed no significant response to clay mineral identity at all, suggesting that once carbon escapes mineral surfaces it decomposes at a rate governed by factors other than the dominant clay type. Occluded POM, on the other hand, peaked in the illite-dominated substrates, significantly exceeding levels in both kaolinite and montmorillonite. This outcome matched the pore-size hypothesis: the coarser pore network of illite-rich material appears to provide physical niches large enough to encase organic particles within stable aggregates, shielding them from decomposition, while the very fine or very dense fabrics of the other two minerals offer fewer such refuges.

Together these findings sharpen a conceptual model that soil scientists have been assembling for decades, in which mineral-associated and particulate carbon are governed by distinct protective mechanisms. Surface area controls adsorption and hence MAOM accumulation; pore size distribution controls physical exclusion and hence occluded POM accumulation. Neither mechanism substitutes for the other, and a soil can be rich in one fraction while poor in the other depending on its mineralogy and structure. This decoupling matters because the two pools respond differently to disturbance: POM tends to be lost quickly when soils are tilled or aggregate structures collapse, whereas MAOM persists until surfaces saturate or chemistry shifts. Global carbon models that lump all soil organic matter into a single pool may therefore misjudge how different soils will respond to land-use change and warming.

The rhizosphere focus of the study adds further weight to its conclusions. Roots are the principal conduit through which fresh carbon enters soil, and the two plant species used here differ in litter chemistry and root traits, yet the mineral-driven patterns held for both. That generality suggests clay mineral properties impose a first-order constraint on rhizosphere carbon storage that overrides moderate differences in vegetation, at least over the timescales of a pot experiment. It also implies that restoring or managing soils for carbon sequestration may benefit from attention to texture and mineralogy: amending sandy, kaolinite-poor soils with high-surface-area clays could raise their ceiling for mineral carbon protection, while preserving aggregate structure in illite-bearing soils could safeguard the physically occluded fraction.

As climate policy increasingly looks to soils as a natural climate solution, studies like this one provide the mechanistic ground truth on which realistic sequestration targets must rest. By demonstrating, in a controlled setting, that surface area and pore size exert opposing and fraction-specific influences on organic matter accumulation, the Prague-led team has given modelers a clearer rulebook and given land managers a more discriminating lens. The carbon beneath our feet, it turns out, is not stored in one great reservoir but in compartments with different locks, and the keys are written in the geometry of clay.

Subject of Research: The contrasting effects of clay mineral surface area and pore size on the accumulation of mineral-associated and particulate soil organic matter fractions in the rhizosphere.

Article Title: Clay mineral surface area and pore size have contrasting effect on accumulation of soil organic matter fractions in the rhizosphere

Article References: Irshad, S., Soukup, K., Setničková, K., & Frouz, J. (2026). Clay mineral surface area and pore size have contrasting effect on accumulation of soil organic matter fractions in the rhizosphere. Biogeochemistry. https://doi.org/10.1007/s10533-026-01370-8

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01370-8

Keywords: clay minerals, soil organic matter, mineral-associated organic matter, particulate organic matter, rhizosphere, pore size, surface area, carbon storage, kaolinite, illite, montmorillonite, soil biogeochemistry