Vascular plant litter input in subarctic peat bogs changes soil invertebrate diets and decomposition patterns

In high-latitude ecosystems climate change induced plant community shifts towards 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 microclimatic conditions, 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 soil invertebrates (springtails) 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 13 C tracer approach. The δ 13 C signatures of springtails showed that they contributed significantly to the decomposition of peat moss (Sphagnum fuscum) and that species feed on different carbon sources. However, when vascular plant litter (Betula pubescens) entered the Sphagnum peat ecosystem, the δ 13 C signatures of the springtails, and thus their role in organic matter processing changed and species-specific differences on decomposition disappeared. There were only slight changes in springtail species composition and abundances in Sphagnum-Betula mixtures, but more importantly all springtail species showed a strong dietary preference for Betula; 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 chemistry is the main way in which vascular litter inputs alter the role of high-latitude soil invertebrates in carbon turnover. Soil invertebrates 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.