Some of the world’s best farmland may hold onto carbon not just because of its grasses, but because minerals slowly release calcium into the soil. Farming may have dramatically changed that hidden support system. Temperate grasslands feature fertile, dark-colored soils known as Mollisols.

These soils cover only a small part of Earth’s land but support a large share of farmland and food production. They also contain a large share of the organic carbon held in agricultural soils, with Mollisols containing thirty-two percent of it.

That makes them critical to land-based climate efforts, even as environmental threats, including soil erosion, and cultivation-driven losses of soil organic carbon threaten that store. The central idea is that organic carbon storage in Mollisols may reflect interactions between organic matter and calcium released as carbonate and silicate minerals weather.

The study tested whether calcium released by mineral breakdown is linked with stored soil carbon, and whether that link is stronger in these dark grassland soils than elsewhere. To test that idea, the researchers built a model covering soils across the United States.

It tracked how minerals break down, how calcium enters the soil, and how much remains available. The analysis then separated calcium’s relationship with stored carbon from the effects of vegetation, climate, and other soil conditions, while comparing different sources of calcium.

The carbon measure focused on the thick upper layer of these soils, including both how deep that layer is and how much carbon it contains. The study then compared that stored carbon with the amount of exchangeable calcium modeled as available on soil surfaces across the United States.

After accounting for other conditions, available calcium was positively linked with stored carbon. The combined influence of other charged substances was weak, pointing specifically to calcium rather than to a general soil property. Here is the surprise: when the model removed calcium supplied by either of two kinds of mineral breakdown, the predicted carbon storage fell.

Removing both had a larger effect than removing either one alone. The effect was especially strong in these dark grassland soils, where stored carbon was particularly sensitive to calcium from carbonate weathering and to the combined action of carbonates and silicates. The combined loss of mineral breakdown approached the impact of removing grassland vegetation, with both changes substantially reducing stored carbon.

The findings show calcium was not the whole story: vegetation also mattered, but calcium inputs still helped support carbon storage in Mollisols. The results do not replace the role of plants. Grassland and savanna vegetation promoted stored carbon independently of the other factors, and fire can help grasses persist where forests might otherwise take over.

But tropical grasslands often lack these carbon-rich soils. The study links that difference to weaker calcium supply, more acidic conditions, and minerals that are less able to support the same soil structure. Agriculture has changed this calcium cycle in two ways: acid inputs have sped up calcium release from some minerals, and applying lime has greatly increased calcium entering cropland soils.

Using modeled liming rates as the reference, farming raised calcium inputs to Mollisol cropland by approximately fourfold relative to preindustrial levels. Using nineteen eighty-seven agricultural census liming rates instead, the estimate rose to fivefold, relative to preindustrial levels, for Mollisol cropland soils.

The spread of possible calcium inputs is especially wide for liming, while the mineral-weathering estimates remain clustered near the bottom in both soil groups. That changes the practical lesson. More roots, more plant diversity, and less soil disturbance may not be enough by themselves to preserve carbon in these soils.

Farmers may also need to protect the soil’s calcium reserve, because the study says that reserve likely helps protect stored carbon in these soils. They could also adjust when and how much lime is applied, so the managed cycle better resembles the natural weathering regime.

Calcium released as minerals break down is linked to carbon stored in these grassland soils, especially the richest farm soils. Protecting that carbon may require managing both plants and the soil’s natural calcium supply.