Long-term overgrazing has significantly changed plant species composition in rangeland ecosystems, and this change may alter ecosystem functioning remarkably. In this study, decomposition rates and nutrient dynamics of pure litter (leaf, stem, or root litter) and 11 litter mixtures (from two to five litter components), including nine aboveground litter mixtures and two root litter mixtures, of five common plant species in degraded semiarid rangelands of northern China were studied for 1 yr. We found that fine root litters generally decomposed faster and had faster nutrient turnover rates than leaf and stem litters. The decomposition rates of leaves and stems were significantly and positively correlated with initial litter nitrogen (P < 0.01) and phosphorus contents (P < 0.05), and the decomposition rates of fine roots were significantly and negatively correlated with initial litter carbon:nitrogen ratios (P < 0.05). Nonadditive effects were found in six out of the nine aboveground litter mixtures (three positive and three negative). There were only additive effects on decay rates and nutrient fluxes in the two root litter mixtures. The occurrence and direction of nonadditive effects were dependent on the properties of component litters and had no obvious correlations with litter diversity. Negative mixing effects on nutrient immobilization can facilitate the release of some important nutrients during litter decomposition processes, and further help to accelerate nutrient cycling in such semiarid rangeland ecosystems. Our results indicate that change of plant species composition by overgrazing may slow down the mass loss rates, but may not necessarily impact the release of some nutrients.