Weathering-derived calcium supports carbon storage in grassland soils

Dark colored grassland soils, known as Mollisols, are intensively farmed and rich in organic matter, and hence have an important role in the carbon cycle. Organic carbon storage in Mollisols may result from interactions between organic matter and calcium released by carbonate and silicate weathering. Here we test this hypothesis by using a geochemical model to simulate calcium release from weathering in soils across the USA. We find that calcium released by mineral weathering is associated with soil organic carbon storage and that this effect is stronger in Mollisols than in non-Mollisols. Our model also shows that agriculture has increased calcium inputs to cultivated USA Mollisols 4-5 fold, indicating that humans have fundamentally changed the geochemical conditions that sustain organic carbon in these soils. By implication, moderating the quantity of and timing of calcium release may help to preserve and restore carbon in the world’s most intensively farmed landscapes.