The study, conducted by Saqilafu Bao and Gang Dai of the College of Chemistry and Environmental Science at Inner Mongolia Normal University in Hohhot, set out to answer a deceptively simple question: what happens to mineral-associated organic matter when soils are exposed to elevated concentrations of dissolved inorganic carbon? This form of carbon, which includes carbonate and bicarbonate ions dissolved in soil water, is abundant in alkaline and calcareous soils around the world, and its concentrations can rise as atmospheric carbon dioxide dissolves into soil pore water or as carbonate minerals dissolve. Yet its influence on the stability of organo-mineral associations had remained largely unexplored.

The researchers hypothesized that dissolved inorganic carbon would promote the desorption—that is, the release—of organic matter from soil mineral particles. The logic behind this hypothesis draws on well-established competitive adsorption chemistry. Carbonate ions, like phosphate and other oxyanions, can bind strongly to the surfaces of iron and aluminum oxides, occupying adsorption sites that organic molecules would otherwise hold. When carbonate floods into the soil solution, it can displace bound organic matter through direct competition for mineral surface sites, a mechanism previously documented for phosphate and for low-molecular-weight organic acids released by plant roots.

To test this idea, the team combined two complementary experimental approaches. First, they performed batch desorption experiments, mixing soil mineral particles with solutions containing dissolved inorganic carbon at concentrations ranging from zero to 0.06 moles per liter. Second, they conducted soil incubation experiments using 13C-labeled glucose, a stable isotope tracer that allowed them to follow exactly what happened when microbes metabolized a fresh carbon input in the presence of elevated dissolved inorganic carbon. The organic matter released from mineral surfaces was then characterized with two of the most powerful analytical tools available in modern environmental chemistry: Fourier-transform ion cyclotron resonance mass spectrometry, which resolves the molecular composition of complex organic mixtures at extraordinary precision, and X-ray photoelectron spectroscopy, which probes the chemical states of elements at mineral surfaces.

The results were unambiguous. At the highest concentration tested, 0.06 moles per liter, dissolved inorganic carbon released 10.6 percent of the mineral-associated organic carbon from the soil particles. That figure places dissolved inorganic carbon in the same league as phosphate, a well-known and widely studied driver of organic matter desorption, which released 12.8 percent in comparable experiments. In other words, a form of carbon that had been largely overlooked in discussions of soil carbon stability turns out to be nearly as effective at mobilizing protected organic matter as one of the most famous destabilizing agents in soil chemistry.

Perhaps even more revealing was the selectivity of the release. When the researchers examined the molecular fingerprints of the liberated compounds, they found that dissolved inorganic carbon preferentially mobilized lignin-derived and condensed aromatic-like molecules. This is a significant finding because lignin, the complex polymer that gives wood its rigidity, and condensed aromatic structures, which include char-like compounds, are typically considered chemically recalcitrant—molecules that resist microbial breakdown and that scientists often associate with stable, long-lived carbon pools. Phosphate, by contrast, released a more diverse suite of biomolecules, suggesting that these two anions destabilize mineral-associated organic matter through distinct pathways and with distinct consequences for which carbon compounds re-enter the soil solution.

The incubation experiments with isotope-labeled glucose added a dynamic dimension to the picture. Under elevated dissolved inorganic carbon concentrations, the incorporation of carbon from the labeled glucose into the mineral-associated organic matter pool was inhibited. Microbes metabolizing the glucose would normally contribute a portion of their processed carbon to the mineral-bound pool, either directly through sorption of microbial metabolites or indirectly through the so-called microbial carbon pump. The presence of high levels of dissolved inorganic carbon suppressed this pathway, meaning that not only was old carbon being released, but new carbon was being prevented from taking its place.

More troubling still, the researchers documented a decline in total mineral-associated organic carbon under high dissolved inorganic carbon conditions via a priming effect. Priming refers to the phenomenon in which the addition of fresh organic matter stimulates microbial activity in ways that accelerate the decomposition of pre-existing soil organic matter. In this case, the combination of fresh glucose and elevated dissolved inorganic carbon appears to have triggered microbes to mine the mineral-associated pool itself, consuming carbon that would otherwise have remained locked away. The net result is a double blow to soil carbon sequestration: destabilization of existing protected carbon and obstruction of new carbon entering the protected pool.

The implications of these findings ripple outward into several domains of Earth system science. Soil contains more carbon than the atmosphere and all vegetation combined, and the mineral-associated fraction represents one of the largest and slowest-turnover components of that stock. Global estimates suggest that mineral-associated organic carbon accounts for a substantial majority of total soil organic carbon in many ecosystems, and models of the global carbon cycle depend critically on assumptions about how stable this pool is. If dissolved inorganic carbon concentrations in soil solutions are high enough to mobilize this carbon—particularly in alkaline soils, saline soils, and regions undergoing changes in hydrology, irrigation, or acidification—then carbon cycle models may be overestimating the security of a major terrestrial carbon reservoir.

The findings may be especially consequential for arid and semi-arid regions, where calcareous soils are widespread and where dissolved inorganic carbon concentrations in soil water can be naturally elevated. Inner Mongolia, the home region of the research team, is characteristic of such landscapes. Climate change and land management practices that alter soil moisture, carbon dioxide fluxes, or carbonate weathering could shift dissolved inorganic carbon concentrations in either direction, and this study suggests that such shifts would carry consequences for organic matter stability that have not been captured in existing frameworks. Irrigation with carbonate-rich waters, for instance, or changes in soil pH that enhance carbonate dissolution, could inadvertently accelerate the release of protected carbon.

The study also adds an important nuance to the growing literature on how anions compete for mineral surfaces. For years, researchers have documented that phosphate fertilization can desorb organic carbon from iron oxides and clays, and that root exudates containing organic acids can do the same. Carbonate was known to adsorb to ferrihydrite and other iron minerals, competing with phosphate, but its role as a driver of organic matter release at ecologically relevant concentrations had not been quantified in this way. By demonstrating that carbonate performs comparably to phosphate in desorption experiments, and by using advanced molecular spectrometry to show that the two anions mobilize chemically distinct compound classes, Bao and Dai have refined the mechanistic understanding of the mineral-organic interface—a zone that a landmark 2021 review in Nature Reviews Earth and Environment described as fundamentally dynamic rather than static.

There remain important caveats and open questions. The experiments were conducted under controlled laboratory conditions, with concentrations chosen to span a defined range, and the authors note that data will be made available on request. Translating these batch and incubation results to field conditions will require measurements of dissolved inorganic carbon concentrations in soil pore waters across diverse landscapes, along with studies of how seasonal wetting and drying, plant root activity, and microbial community composition modulate the desorption process. It is also not yet clear whether the released lignin and condensed aromatic molecules are subsequently mineralized to carbon dioxide, leached to deeper soil horizons, or re-adsorbed elsewhere—fates with very different implications for net carbon storage.

Nevertheless, the study delivers a clear message to the soil science and climate communities: the inorganic carbon dissolved in soil water is not a bystander in the carbon cycle. It is an active chemical agent capable of destabilizing the very carbon pools on which long-term terrestrial sequestration depends. As researchers refine strategies for building soil carbon—from regenerative agriculture to enhanced rock weathering to biochar amendment—the findings argue for treating dissolved inorganic carbon dynamics as a first-order consideration rather than a footnote. The new work was supported by the Natural Science Foundation of Inner Mongolia Autonomous Region of China, and it arrives at a moment when the scientific community is urgently seeking to understand every pathway that might tip soils from carbon sinks into carbon sources.

The distinction matters because soils hold more carbon than the atmosphere and vegetation combined, much of it in mineral-associated organic matter, or MAOM. This carbon can remain protected for centuries to millennia when organic molecules become attached to clay minerals, iron compounds and other reactive surfaces. Yet soil is not chemically static. Rainfall, flooding, drainage, root activity and microbial respiration can repeatedly switch microscopic environments between oxygen-rich and oxygen-poor states. Those changes alter the oxidation state of iron, the solubility of minerals and the ability of microorganisms to break down organic compounds. When iron-bearing minerals are reduced, they may dissolve and release previously protected carbon. When oxygen returns, minerals can re-form and potentially capture carbon again. The new experiment tested whether these redox oscillations—the chemical equivalent of repeatedly flipping an oxygen switch—interact differently with carbon molecules that have contrasting properties.

The researchers chose glucose and oxalate because they represent two common forms of low-molecular-weight carbon released by plants and roots. Glucose is relatively chemically reduced and yields substantial energy when microorganisms oxidize it to carbon dioxide. Oxalate, an organic acid with two carboxyl groups, is more oxidized and provides less energy during further oxidation, but it has a strong affinity for binding to soil minerals. In simplified terms, glucose is attractive food for microbes but a relatively weak mineral glue, whereas oxalate is less rewarding food but a powerful competitor for mineral binding sites. The study therefore set up a direct test of two possible routes to long-term soil-carbon storage: microbial processing followed by incorporation into organic matter, or direct sorption of an intact molecule onto mineral surfaces.

For the experiment, the scientists collected silty clay loam from the A and upper B horizons of a mixed deciduous forest soil at Margery Gallogly Nature Preserve on Grand Island, New York. The soil contained 4.8 percent organic matter and substantial free iron, providing the clay and iron-rich surfaces needed for mineral-organic interactions. They added either glucose or oxalate labeled with carbon-13, a non-radioactive isotope that allowed the researchers to distinguish newly added carbon from carbon already present in the soil. Each gram of dry-equivalent soil received 2 milligrams of labeled carbon—about 4 percent of the soil’s existing carbon content. The amended material was sealed in airtight glass jars and incubated under four regimes: continuously aerobic, oxygenated and oxygen-free on a weekly cycle, oxygenated and oxygen-free on a two-week cycle, or continuously anaerobic. Ten replicate jars were prepared for every substrate and oxygen combination, alongside unamended controls.

Over 20 weeks, the team repeatedly sampled gases in the jar headspaces, measuring carbon dioxide, methane and carbon dioxide containing the carbon-13 label. At the end of five months, the researchers separated the remaining soil carbon into chemically and physically distinct pools. A density separation divided relatively light particulate material from the heavier MAOM fraction. The heavy fraction was then subjected to sequential chemical extractions designed to identify carbon associated with polyvalent cation bridges, reducible minerals, non-reducible short-range-order minerals and crystalline mineral phases. Water extractions measured carbon that remained dissolved or readily mobilized. This approach did not simply ask how much carbon remained in the soil; it tracked where the added carbon went, whether it was respired as gas, dissolved in water, loosely retained in particulate matter or attached to mineral surfaces. Statistical comparisons used two-way analyses of variance, testing the independent and interactive effects of carbon substrate and redox regime.

The central result was stark. Under continuously aerobic conditions and under both intermittent oxygen treatments, about seven times more added glucose carbon was retained as soil organic carbon than added oxalate carbon. The difference appeared in both the light fraction and MAOM, with glucose retention averaging 7.4 times higher in the light fraction and 6.4 times higher across MAOM fractions. Oxygen did not need to be continuously present for this pattern to emerge. Weekly or biweekly anaerobic intervals did not significantly change the total amount of glucose or oxalate carbon stabilized compared with persistent aerobic incubation. The researchers interpret this as evidence that anaerobic periods temporarily pause microbial transformation and mineral-associated carbon formation, rather than creating a lasting storage advantage. Once oxygen returns, aerobic metabolism appears to dominate the overall carbon trajectory.

The explanation lies partly in microbial carbon-use efficiency, a measure of how much consumed carbon microorganisms convert into biomass rather than releasing as carbon dioxide. Oxygen is an energetically favorable terminal electron acceptor, enabling microbes to extract more energy from many substrates. Glucose, with its relatively high bioenergetic yield, can therefore support greater microbial growth under oxygenated conditions. Microbial cells and their residues are increasingly recognized as important sources of MAOM: organisms consume plant-derived compounds, build biomass and eventually leave behind chemically altered remains that bind to minerals. Oxalate follows a different path. Although it can bind strongly to minerals, its lower energy yield limits the microbial biomass produced per unit of carbon under aerobic conditions. In the experiment, much more oxalate remained dissolved in water—about five times more than glucose—and more was respired under continuously aerobic conditions. Its mineral-binding ability did not compensate for its weaker capacity to fuel microbial processing.

The picture changed completely in the permanently anaerobic jars. Respiration was strongly suppressed for both substrates, and much larger amounts remained in the aqueous phase: roughly 108 times more glucose carbon and 63 times more oxalate carbon remained dissolved than under the other redox regimes. Under these oxygen-free conditions, the difference in total solid-phase carbon between glucose and oxalate vanished. Oxalate-derived carbon entering MAOM increased compared with aerobic treatments, while glucose-derived carbon entering MAOM decreased. The researchers propose that suppressed microbial uptake under sustained anaerobiosis gave oxalate more time to associate directly with mineral surfaces. Because oxalate is a strong sorber, it may bind to clay or iron-containing minerals even when microbial transformation is energetically constrained. Glucose, by contrast, lost the microbial advantage that helped stabilize it under oxygenated conditions. The authors caution that isotope measurements cannot prove that labeled carbon recovered from MAOM remained chemically intact as oxalate or was converted into microbial residues, but direct sorption is a plausible explanation.

The experiment also exposed a potentially troubling side effect of oxalate. Under aerobic and fluctuating conditions, adding oxalate caused a strong priming effect: it stimulated the breakdown of carbon that had already been present in the soil. Compared with unamended controls, oxalate increased respiration of existing soil carbon by 1.8 times under persistent aerobic conditions and by about 1.9 times under the intermittent regimes. It also released far more pre-existing carbon into soil solution—9.1 times more under continuous oxygen, 24.7 times more under weekly cycling, 17.3 times more under biweekly cycling and 4.3 times more under persistent anaerobiosis. One likely mechanism is that oxalate’s small size and carboxyl groups allow it to displace older carbon from mineral surfaces, a process sometimes compared with molecular “unbuttoning” of mineral-organic associations. Once released into solution, that carbon becomes accessible to microbes. Fluctuating oxygen may intensify the process by dissolving iron-associated carbon during anaerobic intervals and exposing it to oxidation and microbial consumption when oxygen returns.

Glucose did not produce the expected positive priming effect. It neither increased respiration nor substantially increased solubilization of existing soil carbon under aerobic or fluctuating conditions, and it actually suppressed respiration of existing carbon under persistent anaerobiosis. The researchers suggest that microbes may have preferentially consumed the added glucose, reducing their need to attack older carbon. This behavior could be especially pronounced when oxygen and other electron acceptors are scarce. The results therefore complicate broad claims that adding labile carbon will universally accelerate the loss of stored soil carbon. The outcome depends on the molecular identity of the input, the minerals present, the availability of oxygen and the microbial community’s energetic constraints. A root exudate rich in organic acids may destabilize existing mineral-associated carbon even as it adds new carbon, while a sugar may be routed more efficiently through microbial biomass.

The study’s controlled jars cannot reproduce the full complexity of a living forest soil, where roots, fungi, fauna, water movement, temperature and nutrient availability interact over seasons and decades. The carbon addition was also deliberately high to ensure that the carbon-13 label could be recovered after five months. Even so, the findings offer a mechanistic warning for soil-carbon models and climate strategies: oxygen availability alone is not enough to predict whether new carbon will persist. Models must also account for the chemical identity of plant inputs and for the contrasting behavior of carbon compounds during microbial uptake, mineral sorption and priming. In particular, wetland soils, flooded agricultural fields, compacted soils and periodically saturated landscapes may respond differently depending on whether anaerobiosis is brief or sustained. The authors’ broader message is that soil carbon is not a single pool with a single fate. It is a moving network of molecules, microbes, minerals and redox reactions—and a small change in molecular structure can determine whether carbon is stored, dissolved, respired or used to unlock older reserves.