The quality of organic soil amendments such as composts and vermicomposts can be improved by using additives during their production. However, little is known about the impact of these materials on carbon flow in the plant-soil system. We investigated the impact of organic amendments produced through composting and vermicomposting in the presence of clay minerals (montmorillonite) and/or earthworms on plants (perennial ryegrass, Lolium perenne and common bean, Phaseolus vulgaris) and carbon flow in soil. We carried out a growth chamber experiment with continuous 13C labelling and analysed above-/below-ground biomass production, as well as biogeochemical parameters of plant and soil (rhizospheric and non-rhizospheric) compartments. Quantitative biogeochemical analyses were further merged with microscale elemental and isotopic information (NanoSIMS) to investigate carbon transfer in the rhizopheric soil. Our results showed lowest biomass production for treatments amended with organic amendments produced without minerals probably related to the release of harmful substances (excess of dissolved organic carbon, salts and inorganic nitrogen) due to the immature nature of these materials. The use of montmorillonite as additive during the preparation of the amendments alleviated these adverse effects and resulted in a significant increase of above-/below-ground plant biomass production. When the organic amendments were produced in the presence of eathworms and montmorillonite, the higher plant biomass promoted the release of root-derived carbon within the rhizospheric soil (compared with amendments without clay minerals), which was used as an energy-rich substrate by microorganisms. We conclude that the use of mineral additives during composting or vermicomposting favours plant biomass production and carbon transfer to the soil and its microorganisms, which could enhance soil carbon storage in the longer term.