Leaf and root litter profoundly impact soil carbon sequestration and nutrient cycling in terrestrial ecosystems. Recent evidence indicates that within single-species contexts resource traits are coordinated between leaves and roots driving parallel decomposition dynamics of leaf and root litters, yet it remains unclear whether this coordination also underlies parallel mixing effects in leaf and root litter mixture decomposition. In a 501-day field experiment in a temperate steppe, we incubated leaf and fine root litters from six species alone and in all pairwise mixtures. We assessed the relationship between leaf and fine root litter decomposition responses to litter mixing, and examined how trait dissimilarity between component species and decomposition responses of four carbon fractions (soluble compounds, hemicellulose, cellulose and lignin) shape this relationship. We found litter trait dissimilarities drove contrasting fraction-level responses to litter mixing. Most leaf and fine root litter mixtures exhibited non-additive effects in soluble-compound and cellulose decomposition, with soluble compounds contributing most to the overall non-additive effects of mixed leaf and fine root litters. Coordinated dissimilarity in leaf and root traits led to parallel decomposition responses of leaf and root soluble compounds to litter mixing, but to negative correlations for hemicellulose and cellulose and no correlation for lignin. These divergent fraction-level relationships blurred overall coordination of decomposition response between leaf and fine root litters to litter mixing, causing uncoordinated bulk-litter mixing effects. Our results demonstrate that resolving fraction-level processes is critical for understanding mixed-litter decomposition and for predicting ecosystem carbon and nutrient fluxes under changing plant communities.