Peatlands are wetlands acting as carbon (C) sink at a global scale. Even if, it represents only 3% of the land area, these specific ecosystems have stored a third of the global soil C. Most peatlands (up to 80%) are located in the northern hemisphere and lie in cool temperate zone in association with waterlogged, nutrient poor conditions and Sphagnum mosses, which limit the activity of the soil decomposers. As a result, in spite of the relative small net ecosystem production in peatlands, the imbalance between the primary production and the decomposition is enough to allow organic matter (OM) accumulation as peat (Bragazza et al., 2009). However, global change are expected to cause a plant community shift in peatlands with an increase of vascular plants (such as graminoids at the detriment of Sphagnum species (Dieleman et al., 2015). Vascular plants invasion in peatlands have mostly been studied through a change of CO2 and CH4 emissions with different plant community composition. In spite of observed C fluxes modification, the role of vascular plant invasion on the C sink balance in peatlands are still to be elucidated. The aim of this study was to investigate how an invading graminoid plants, Molinia caerulea, can affect the C balance of Sphagnum-dominated peatland. CO2 fluxes and CH4 emissions were regularly measured in peat mesocoms during fourteen months and related to biotic and abiotic factor to calculate the annual C budget with and without Molinia caerulea. Twelve peat mesocosms (depth and diameter: 30 cm) were collected in March 2015 in La Guette peatland (France). This is an acidic fen invaded by Molinia caerulea, a graminoid plant invading numerous peatlands. The mesocosms were buried outdoor and surrounded with a tarpaulin containing water from the peatland. Air and soil temperature at 5 and 20 cm depth were monitored in each mesocosm. The mesocosms were separated into 2 treatments with 6 containing only Sphagnum rubellum ('Sphagnum' mesocosms) and 6 containing Sphagnum rubellum and Molinia