Global warming induces complex interactions which could have a positive or a negative feedback on C storage in soils. Peatlands have stored one-third of global C stock of soils because of low rates of plant residue decomposition due to oxygen limitation resulting from flooding which inhibits microbial activities. Given the multiple possible responses of peatlands to global warming, it is presently impossible to predict their future contribution to the global C cycle. Several approaches have attempted to assess the effect of warming on peatlands: mesocosms, latitudinal transects, and in situ field experimental warming. Most of these studies only assessed the warming impact on vegetation and surface gas emissions, and so far, no study to date has addressed the entire range of biodiversity, community structure and ecosystem function responses to increased temperature, particularly in temperate peatlands. Within the PEATWARM project (ANR-07-VUL-010), we investigated the response of a Sphagnum peatland in the Jura Mountains (France) to climate change using an experimental system (Open Top Chambers – OTC’s) to simulate in situ an increase in average temperature. We examined the effects of temperature increase along a narrow fen-bog gradient on the vegetation, above- and belowground gas fluxes (CO2 and CH4), microbial diversity and activity in Sphagnum mosses and in peat, CNS interactions between macro- and microorganisms in the peat biota, and the dynamics of labile and recalcitrant peat organic matter. We showed that warming modifies the C and nutrient cycling of the peatland via changes in above- and belowground interactions. Furthermore, the contrasted responses to warming between the fen and the bog sites suggest that the spatial variability of moisture and hydrochemistry is critical for understanding the impact of global warming on the fate of organic matter and the C cycle in peatlands.