Calcium promotes carbon rich grassland soils

Dark colored grassland soils, known as Mollisols, are intensively farmed and exceptionally rich in organic matter, and hence have an important role in the carbon cycle. Elevated carbon storage in Mollisols may be facilitated by calcium (Ca) released by carbonate and silicate weathering. This synergy between carbon and Ca cycling has not been definitively quantified. To close this knowledge gap, we developed a novel geochemical model and leveraged continental-scale datasets to simulate Ca release in soils across the USA. We found that Ca released by mineral weathering helps to predict the distribution of Mollisols and soil organic carbon storage. Our model also indicates that agriculture has increased Ca inputs to cultivated USA Mollisols 2-5 fold, demonstrating that humans have fundamentally changed the geochemical conditions that sustain carbon storage in these soils. By implication, moderating the quantity of and timing of Ca release may be essential for sustaining soil carbon storage in the world’s most intensively farmed landscapes.