Salt marshes, a type of coastal wetland, store “blue” carbon. At the same time, these ecosystems can release the greenhouse gases carbon dioxide (CO₂) and methane (CH₄) via microbial decomposition of stored carbon. In this study, we studied what drives the release of CO₂ from mineral-rich salt marshes and found that the quantity and form of carbon are the most important factors. Our results improve understanding of salt marsh carbon cycling, allowing better prediction of future changes.