In high-latitude ecosystems climate change induced plant community shifts toward dominance of shrubs and trees will potentially have large consequences for soil carbon dynamics. Changes in the litter layer due to an altered quantity and quality of litter input, or by its indirect effect on the microclimate, might affect the decomposer community. To be able to predict the effects of increased litter input on decomposers and consequently on soil carbon dynamics, we studied the contribution of Collembola to carbon processing in a high-latitude peat bog system. Moreover, we assessed the effects of changing litter inputs on their abundance, diversity and diet choice, using a 13C tracer approach. The δ13C signatures of Collembola in peat moss (Sphagnum fuscum) showed that species differed in their diet. However, when vascular plant litter (Betula pubescens) entered the Sphagnum peat ecosystem, the δ13C signatures of the Collembola, changed and species-specific differences disappeared. There were no significant changes in Collembola species composition and density after Betula litter addition, but all species showed a strong dietary preference for Betula-associated food sources over Sphagnum; 67% of their diet contained carbon originating from Betula litter. Decomposition patterns corresponded to these findings; mass loss (after 406 days of incubation) of Betula increased from 16.1% to 26.2% when decomposing in combination with Sphagnum, and Sphagnum decomposed even slower in combination with Betula litter (from 4.7% to 1.9%). Our results indicate that the change in litter quality rather than its effects on microclimate is the main way in which vascular litter inputs alter the role of Collembola in carbon turnover. Collembola are plastic in their diet choice, which implies that changes in carbon turnover rates in situations where vegetation shifts occur, might well be due to diet shifts of the present decomposer community rather than by changes in species composition.