In previous studies, the rate of soil carbon (C) sequestration decreased with increasing age of post‐mining soils. It was also shown to depend on plant biomass and earthworm bioturbation. Here, a soil transplant experiment was used to determine whether this decrease is caused by soil C saturation or other factors (such as bioturbation and litter input). Soils collected on 15‐, 25‐ and 50‐year‐old successional sites, dominated by willow (Salix caprea L.) and birch (Betula pendula Roth), and on a 50‐year‐old site reclaimed by the planting of alder (Alnus glutinosa L.) were placed in plastic boxes that were accessible to soil macrofauna. The boxes were buried in the 50‐year‐old reclaimed site and supplemented with either alder litter or successional (willow and birch) litter. Soil C content and soil C fractions (hot water C, particulate organic C, particulate organic C bound in aggregates and C bound to mineral soil) were studied. After 1 year, the C content increased by 2–5%, but there was no effect of soil source or litter type. For all C fractions, the relationships between change in C content and initial C content were described by bell‐shaped curves. Easily available C fractions were saturated earlier than more recalcitrant fractions. Despite these saturation tendencies in individual soil organic matter pools, the soil was evidently far from saturation after 50 years of soil development. The decrease in C sequestration with soil age previously observed for this soil was probably caused by a decrease in litter input rather than by C saturation. Copyright © 2016 John Wiley & Sons, Ltd.