Rock-Weathered Calcium Emerges as Hidden Guardian of Carbon in Grassland Soils

Some of the most fertile and carbon-rich soils on Earth owe their remarkable ability to store organic carbon not just to plants and microbes, but to a quiet geological process happening deep below the surface: the weathering of rocks. A new study published in Communications Earth & Environment suggests that calcium released by the slow chemical breakdown of carbonate and silicate minerals plays a fundamental role in stabilizing organic matter in grassland soils, particularly in the dark, deep, organic-rich soils known as Mollisols that underpin much of the world’s agriculture. The finding, led by E. W. Slessarev of Yale University and the University of California, Berkeley, together with colleagues at UC Davis, Lawrence Livermore National Laboratory, UC Merced, and Oregon State University, reframes how scientists think about the geochemical foundations of soil carbon storage in some of the planet’s most intensively farmed landscapes.

Mollisols are the deep, dark-colored grassland soils that stretch across the American Midwest, the Eurasian steppe, the Argentine pampas, and other temperate grassland regions. They are extraordinarily valuable to humanity: rich in organic matter, naturally fertile, and heavily cultivated for grain production. Because of their immense organic carbon stocks, these soils matter enormously for the global carbon cycle. Yet for decades, researchers have debated why Mollisols hold onto so much carbon while other soils, subject to similar climates and plant productivity, do not. The new work points to a geochemical answer rooted in the bedrock itself.

The central hypothesis tested by the team is that organic carbon storage in Mollisols arises from interactions between organic matter and calcium released by the weathering of carbonate and silicate minerals. In soils derived from calcium-rich parent material, dissolved calcium can bind with organic compounds, promoting the formation of stable associations that resist microbial decomposition. Calcium can act as a bridge between negatively charged organic molecules and mineral surfaces, and it can also precipitate with carbonates in ways that physically protect organic matter. In effect, the slow leaching of calcium from rocks provides a continuous chemical subsidy that helps lock carbon in place over centuries to millennia.

To test this idea across broad geographic scales, the researchers employed a geochemical model designed to simulate the release of calcium from mineral weathering in soils across the United States. Rather than relying solely on field measurements, which are expensive and spatially limited, the model allowed the team to estimate weathering-derived calcium inputs across a wide range of soil types and environmental conditions. This modeling approach made it possible to ask a question that would be difficult to answer with observations alone: does the calcium supplied by rock weathering systematically track the amount of organic carbon stored in soils?

The answer, according to the study, is yes. The model results show that calcium released by mineral weathering is associated with soil organic carbon storage, and that this association is stronger in Mollisols than in non-Mollisol soils. In other words, the grassland soils that are most famous for their carbon richness are precisely the ones where weathering-derived calcium appears to exert the greatest influence. This pattern supports the idea that calcium-organic matter interactions are a key mechanism sustaining the deep carbon stocks that make Mollisols distinctive among the world’s soil orders.

The implications extend beyond basic soil science into the urgent territory of climate mitigation. Soil organic carbon represents one of the largest terrestrial carbon pools, and strategies to rebuild carbon in agricultural soils—through cover cropping, reduced tillage, grazing management, and soil amendments—are central to many national and corporate net-zero plans. If weathering-derived calcium helps determine how much carbon a soil can hold, then the geochemical properties of a landscape may set an upper limit on how much carbon restoration efforts can realistically achieve. Soils with abundant calcium-bearing minerals may be inherently better at preserving newly sequestered carbon than soils lacking that geological endowment.

Perhaps the most striking finding of the study concerns the scale of human interference in this natural system. The model indicates that agriculture has increased calcium inputs to cultivated Mollisols in the United States by a factor of four to five. Lime applications, irrigation water carrying dissolved minerals, and other management practices have flooded these soils with far more calcium than natural weathering alone would supply. This means that modern farming has fundamentally altered the geochemical conditions that sustain organic carbon in some of the world’s most important agricultural soils—a change so large that it rivals or exceeds the natural weathering flux that shaped these soils over millennia.

That four-to-five-fold increase is a double-edged result. On one hand, the additional calcium may currently be supporting carbon stabilization in cultivated Mollisols, potentially buffering some of the carbon losses caused by tillage and cropping. On the other hand, it means that the carbon stocks we observe in farmed grassland soils today are being maintained under artificial geochemical conditions that would not persist without continued human inputs. If calcium additions were reduced or if management practices changed, the stabilizing influence on organic matter could weaken, with consequences for how much carbon these soils retain. The study’s authors suggest that moderating both the quantity and the timing of calcium release may help preserve and restore carbon in the world’s most intensively farmed landscapes.

The research also carries lessons for emerging carbon removal technologies. Enhanced rock weathering, in which crushed silicate or carbonate rocks are spread on fields to accelerate mineral dissolution and capture carbon dioxide, is attracting major investment as a climate strategy. This study adds a potentially significant co-benefit: the calcium released by deliberately enhanced weathering could simultaneously strengthen organic carbon stabilization in soils, particularly in calcium-poor agricultural lands. Conversely, the findings suggest that enhanced weathering projects should consider not only how much carbon dioxide the dissolved minerals consume, but also how the released calcium reshapes the soil’s capacity to protect its existing and newly added organic carbon.

For a study built on a geochemical model, the path from simulation to real-world management will require further field validation, and the authors note that their work was supported by the Department of Energy, including the Laboratory Directed Research and Development program at Lawrence Livermore National Laboratory and the ‘Terraforming Soil’ Carbon Negative Energy Earthshot Research Center. Still, the conceptual shift is clear. The dark breadbaskets of the world are not merely products of climate and grassland vegetation; they are also products of bedrock, slowly dissolving and delivering calcium that binds carbon in place. Recognizing that hidden geological partnership may prove essential as humanity attempts to manage, restore, and expand the carbon stored beneath its feet.

Subject of Research: The role of weathering-derived calcium in soil organic carbon storage in grassland Mollisols

Article Title: Weathering-derived calcium supports carbon storage in grassland soils

Article References: Slessarev, E. W., Goertzen, H. R., Lybrand, R. A., McFarlane, K. J., Pett-Ridge, J., Sokol, N. W., Zaharescu, D. G., & Georgiou, K. (2026). Weathering-derived calcium supports carbon storage in grassland soils. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04142-2

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04142-2

Keywords: soil carbon, Mollisols, calcium, mineral weathering, grassland soils, carbon cycle, geochemical modeling, agriculture, carbon sequestration, enhanced rock weathering, soil organic matter, climate mitigation