Root and leaf turnover is a major contributor to soil organic carbon (SOC) dynamics and storage. However, while roots of multiple species occur in soils, and leaves are often incorporated to soil by bioturbation processes, it remains unknown how litter mixing in soils influences litter decomposition processes, and the associated carbon (C) fluxes controlling the SOC balance. To address this issue, we studied the (almost complete) decomposition of ¹³C labeled single leaf and root litters and 15 realistic litter mixtures incorporated into soil in the laboratory. We studied litter mass loss and litter nitrogen concentration as litters decomposed, tracked litter ¹³C into the mineral-associated organic matter (MAOM) and particulate organic matter (POM) fractions of the SOC, and quantified litter influence on native SOC mineralization (i.e., priming). Litter diversity increased litter decomposition rates, yet not due to nitrogen transfer and immobilization in decomposing litters. Litter tannins dissimilarity enhanced decomposition rates. Plant litter diversity enhanced soil MAOM-C formation, but had a negligible impact on overall SOC stocks. This occurred because SOC formation and priming mixture effects in the MAOM fraction were counterbalanced by those in the POM fraction. In mixed litters, the dissimilarity in litter manganese (Mn) was responsible for non-additive MAOM-C gains and POM-C losses, suggesting Mn transfer between litters that foster lignins degradation and incorporation into soil MAOM. Litter dissimilarity in Mn also reduced MAOM-C and increased POM-C priming. Overall, we demonstrate that diversity effects, potentially implying nutrient transfer between litters, can occur belowground among root litters, as well as biologically incorporated leaf litters. It further shows that different nutrients are implicated in diversity effects controlling litter decomposition rate and soil C storage.