Despite their low primary production, ombrotrophic peatlands have a considerable potential to store atmospheric carbon as a result of their extremely low litter decomposition rates. Projected changes in temperature and nitrogen (N) deposition may increase decomposition rates by their positive effects on microbial activity and litter quality, which can be expected to result in enhanced mass loss and N release from Sphagnum and vascular plant litter. This is the first study that examines the combined effects of increased temperature and N deposition on decomposition in bogs. We investigated mass loss and N release at four bog sites along a gradient from north Sweden to northeast Germany in which both temperature and N deposition increased from north to south. We performed two litterbag experiments: one reciprocal experiment with Eriophorum vaginatum litter and one experiment using recalcitrant (Sphagnum fuscum) and more degradable (Sphagnum balticum) Sphagnum litter collected from the most northern site. We measured mass loss and N release during two (Sphagnum) and three (E. vaginatum) years. The N concentration and decomposability of the E. vaginatum litter did not differ between the sites. Mass loss from E. vaginatum litter increased over the gradient from north to south, but there was no such effect on Sphagnum litter. N loss of all litter types was affected by collection site, incubation site and time and all interactions between these factors. N release in Sphagnum was positively related to N concentration.
We conclude that decomposition of vascular plants and Sphagnum litter is influenced by different environmental drivers, with enhanced temperatures stimulating mass loss of vascular plant litter, but not of Sphagnum. Enhanced N deposition increases Sphagnum litter N loss. As long-term consequences of climate change will presumably entail a higher vascular plant production, overall litter decomposition rates are likely to increase, especially in combination with increased temperature.