Diverse plant litter mixtures frequently decompose substantially differently than expected, compared to the average of the individual component species. These strong “non-additive” effects constitute an important way in which biodiversity influences key components of below-ground ecosystem function like soil C and N cycling, and it remains unclear which plant traits drive diversity effects on soil C and N cycling. In this study, we measured non-additive soil respiration, net N mineralization and microbial biomass N responses to litter mixtures comprised of up to four alpine plant species. We found that litter chemical composition and litter chemical diversity had a greater influence on soil C and N responses to litter mixtures (0.11 ≤ R² ≤ 0.52 for the measured response variables) than did more traditional ways of describing litter mixture chemistry like %N, C:N, lignin:N, phenolic:N, and species richness (0.00 ≤ R² ≤ 0.10 for the measured responses variables). Notably, net N mineralization responses to litter mixtures were significantly influenced by the concentration of litter N, sugars, condensed tannins (CT) and low molecular weight (LMW) phenolics as well as litter chemical diversity (0.11 ≤ R² ≤ 0.26 for these variables). Microbial biomass N responses to litter mixtures were influenced to a lesser degree by litter CT, LMW phenolics, and the acid soluble fraction (R² = 0.093 for these variables). The effect of a given species’ loss from the litter mixtures was dependent on whether a particular species contained compounds that significantly influenced soil C and N responses. In accordance, we found that loss of some species did not significantly affect soil respiration, net N mineralization, or microbial biomass N accumulation while loss of other species had strong effects on soil responses.