1. Invasive plant effects on litter decomposition tend to be idiosyncratic among species and ecosystems, which may arise from variation in the invader’s relative abundance (mass ratio effect), its relative functional difference to other species (trait divergence effect), and/or from species’ litter mixing that causes non-additive decomposition rates relative to single-species decomposition. 2. We use experimental microcosms to quantify the potential for mass ratio and trait divergence to explain effects of invasive litters on carbon mineralization rates of decomposing litter mixtures and to generate non-additive dynamics in two distinct forest litter types. 3. The relative strength and direction of the mass ratio and trait divergence effects on carbon mineralization values were moderated by interactions between each effect and the canopy litter mixture. In both maple-poplar and oak hickory litter mixtures, we found that the strength of the trait divergence effect increased with increasing proportions (mass ratio) of understory litter in the litter mixtures, suggesting that interactions between these two mechanisms are important for moderating the influence of understory invasions. Similarily, the addition of understory litter always caused synergistic, non-additive impacts on carbon mineralization, increasing carbon mineralization on average by 34%. For both litter mixtures, non-additivity increased with increasing trait divergence between understory and canopy litters. 4. Our findings show the potential for pronounced context-dependency in the impact of nonnative invasive species that arise via multiple mechansism and their interactions. As such, they suggest the necessity of accounting for frequently-theorized but infrequently-tested mechanisms relating to mass ratio and trait divergence. Accounting should improve understanding of when mass ratio and trait divergence affect ecosystem process rates, and hence build confidence in mechanistic projections of invader impacts.