The research team, led by Wojciech Jerzak of AGH University of Krakow, pyrolyzed both materials at 700, 800 and 900 degrees Celsius in a fixed-bed reactor flushed with high-purity nitrogen. Each 1.5-gram sample was held at the target temperature for one hour, after which the resulting chars were ground, sieved and subjected to a battery of analytical techniques including Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and nitrogen sorption analysis. Carbon dioxide uptake was then measured at 20 degrees Celsius across pressures ranging from 0.5 to 110 kilopascals.

The starting materials could hardly have been more different. Chicken manure, collected from a poultry farm in southern Poland, contained just 8 percent fixed carbon but a hefty 26.4 percent ash, along with 4.1 percent nitrogen. Onion peels from a processing facility in the Kuyavian-Pomeranian Voivodeship were richer in combustible material, with 56.4 percent volatile matter, 14.9 percent fixed carbon and a lignocellulosic structure containing substantial cellulose and hemicellulose. X-ray fluorescence showed that calcium oxide dominated the ash of both feedstocks, at 48.8 percent for the manure and 40.8 percent for the peels, but the manure also carried elevated potassium, phosphorus, magnesium and sulphur, while the peels were unusually rich in silica at 33.8 percent.

As pyrolysis temperature climbed, both feedstocks lost mass, moisture and volatile components while their ash fractions concentrated in the solid residue. Manure biochar ash content soared from 26.4 percent in the raw material to 65.2 percent at 900 degrees Celsius, while onion peel biochar ash rose from 14.9 to roughly 41.8 percent. Hydrogen contents in both biochars fell below 1 percent, and nitrogen in the manure chars dropped from 4.1 to between 1.1 and 1.8 percent, reflecting the thermal cracking of volatile compounds and the release of small nitrogenous molecules. Onion peel biochars, by contrast, concentrated carbon to around 50 to 52 percent, more than the manure chars managed, thanks to their lower initial mineral burden.

Spectroscopy traced the chemical evolution in fine detail. Infrared spectra showed the progressive disappearance of hydroxyl, aliphatic and carbonyl bands with rising temperature, signalling dehydration, decarboxylation and the growth of a condensed aromatic carbon matrix. Yet phosphate and carbonate signals persisted in the manure-derived chars, underscoring the thermal stability of their inorganic constituents. X-ray diffraction revealed dramatic mineral reorganisation: at 700 degrees Celsius the manure char held quartz, portlandite and rutile, but by 800 and 900 degrees Celsius the ash had transformed into calcium oxide, whitlockite-type calcium-magnesium phosphates, magnesite, oldhamite and eventually sulfoapatite. The onion peel chars followed a different path, developing graphitic carbon alongside quartz, kalsilite, sodium carbonate and, at the highest temperature, the stable alkali aluminosilicate sanidine.

Electron microscopy captured the morphological consequences. Manure biochars remained dense, heterogeneous aggregates with no well-defined pore network, their high mineral content apparently filling or masking the developing carbon porosity. The onion peel chars told a more elegant story: at 700 degrees Celsius they retained elongated, layered fragments of the original plant tissue, and at 800 degrees Celsius they developed a striking channel-like structure the authors liken to a fish skeleton, with aligned, interconnected spaces carved out by devolatilisation. At 900 degrees Celsius, however, that ordered architecture began to fragment and partially collapse, a warning that excessive heat can destroy the very structure adsorbent designers hope to build.

Porosity measurements quantified these visual impressions. The onion peel biochar produced at 900 degrees Celsius achieved the highest specific surface area of the entire study, 109.8 square metres per gram—more than three times that of its 800-degree counterpart and nearly 6.5 times that of the 700-degree sample—with micropores contributing up to 60 percent of the internal surface in the intermediate-temperature samples. Manure biochars reached more modest areas between 14.0 and 36.2 square metres per gram, but their average pore diameter also shrank steadily with temperature, indicating progressive micropore refinement.

The carbon dioxide adsorption results upended the expectation that bigger surface area always means better capture. Among the onion peel chars, the 700-degree sample—despite its modest 17 square metres per gram—delivered the highest uptake, 26.6 cubic centimetres per gram at 100 kilopascals, while the 900-degree sample with the largest surface area actually adsorbed less. The researchers attribute this to pore widening and a shrinking population of narrow ultramicropores, those cavities below 0.7 nanometres whose dimensions closely match the kinetic diameter of CO2 and which nitrogen adsorption at cryogenic temperatures cannot fully resolve. Residual oxygen-containing surface groups may also enhance CO2-surface interactions in the cooler char.

The manure biochars behaved in almost mirror-image fashion, with CO2 uptake rising steadily as pyrolysis temperature increased, tracking the growth of microporosity and surface area. But texture alone could not explain the performance of the 900-degree manure char, which captured substantial CO2 despite a smaller surface area than its onion peel rivals. X-ray diffraction and fluorescence pointed to the missing piece: calcium oxide and calcium-rich phosphate phases that furnish basic adsorption sites with a strong affinity for acidic CO2 molecules, potentially even fixing some of the gas chemically as carbonate or bicarbonate species. In these mineral-rich chars, capture is a partnership between physical pore-filling and mineral-assisted interaction.

The practical lesson is that biochar design must be matched to feedstock chemistry. For lignocellulosic wastes like onion peels, the goal should be preserving narrow micropores rather than maximising total surface area, and avoiding the pore widening and structural collapse that come with excessive heat. For manures, engineers should exploit the inherent calcium, magnesium and potassium phases that add a chemical dimension to capture. Converting two problematic wastes into tailored carbon sorbents in a single thermal step offers a double dividend—waste valorisation and carbon management—grounded not in exotic chemistry but in a careful reading of what heat does to the organic and mineral halves of agricultural residue.

Subject of Research: Thermal conversion of chicken manure and onion peels into biochars for carbon dioxide adsorption

Article Title: Effect of thermal treatment on the structure and CO2 adsorption capacity of biochar from chicken manure and onion peels

Article References: Effect of thermal treatment on the structure and CO2 adsorption capacity of biochar from chicken manure and onion peels. (n.d.). https://doi.org/10.1007/s10098-026-03598-2

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03598-2

Keywords: biochar, pyrolysis, CO2 capture, chicken manure, onion peels, carbon adsorption, microporosity, mineral phases, waste valorisation, X-ray diffraction, Raman spectroscopy, surface area

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