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Abstract The storage of organic carbon in the soils of salt marshes and other coastal blue carbon ecosystems has gained considerable attention by the scientific community for more than a decade now, while the relevance and mechanisms of soil inorganic carbon accumulation remain poorly understood. Using long‐term annual accretion monitoring over 17 years in N = 50 permanent plots distributed across a 1050‐ha salt‐marsh complex of the European Wadden Sea, we identified clear relationships between salt‐marsh vertical growth rates and the soil densities of inorganic and organic carbon. Specifically, we demonstrate a strong positive correlation between vertical accretion and inorganic carbon density while observing a strong negative correlation between vertical accretion and organic carbon density. This decoupling observed between inorganic and organic soil carbon stocks was governed by plant community composition and associated plant traits, which controlled sedimentation processes.