This study investigates interactions between soil organic carbon (SOC) and calcium (Ca). It reveals that Ca contributes to SOC stability even in acidic soils, a finding that departs from previous assumptions that Ca's role is limited to neutral or alkaline soils. It also investigates the formation mechanisms driving the association of Ca with a characteristic fraction of SOC, highlighting the importance of decomposition processes in its formation. In this work, the authors employ advanced spectromicroscopy and targeted experiments to confirm that Ca binds SOC compounds rich in aromatic and phenolic groups after decomposition, preventing their loss as dissolved organic carbon. The identification of this biogeochemical mechanism has direct implications for improving soil carbon models and guiding Ca amendment practices in agriculture, enhancing soil carbon retention and contributing to climate resilience.