Peatlands are wetlands characterised by an accumulation of peat thanks to organic matter (OM) production higher than decomposition. This unbalanced budget of OM allows peatlands to store a third of the soil carbon (C) on only 3% of land area. Their role as global C sink is due to their particular environmental conditions (waterlogging, low pH) and their ecosystem engineers: Sphagnum species. However, with climate change and human disturbances, vascular plants invade peatlands to the detriment of Sphagnum mosses. A shift of peatland vegetation could reduce the C sink function of peatlands through an increase of OM decomposition and higher emissions of CO2 and CH4. Our study aims to assess the effect of vascular plant (Molinia caerulea) occurrence in Sphagnum mesocoms on (i) greenhouse gas emissions (CO2 and CH4), (ii) dissolved organic C (DOC) dynamics and to relate C flux changes to environmental conditions. Experimental design was set up in March 2015. Cylindrical peat mesocosms (30 cm diameter and depth) from La Guette peatland (France) were installed and environmentally monitored (radiation, air relative humidity and air and soil temperature). 2 groups of 6 mesocosms were randomly drawn and received two treatments: i) 6 mono-specific mesocosms containing only Sphagnum rubellum (called “Sphagnum”) and ii) 6 mesocosms containing Sphagnum rubellum and Molinia caerulea (called “Molinia”). From April 2015, CO2 (Net Ecosystem Exchange and Ecosystem Respiration) and CH4 emissions were weekly measured (with static chambers) as well as DOC contents, water table level, vegetation height and the number of Molinia caerulea leaves. Molinia caerulea occurrence significantly affects both CO2 and CH4 emissions, as well as DOC contents. CO2 was significantly more absorb in Molinia mesocosms during the growing season, but it was more released during Molinia caerulea senescence. Higher CO2 uptake is correlated to the number of Molinia caerulea leaves (r2:0.84; p<0.001). After senescence, the leaves decomposed, which generated a higher CO2 flux than in “Sphagnum” treatment. Sphagnum mesocosms acts as a CO2 sink, but not as efficiently as the “Molinia” mesocosms. The sink function of “Sphagnum” was relatively constant in time, with just a weak decrease during the growing season. It is rather the variation of ecosystem respiration that predominantly determined the CO2 balance in Sphagnum mesocosms in relation with air temperature (r2:0.6; P<0.001). CH4 emissions significantly increased in both treatments during summer in relationship with soil temperature at 20 cm (r2: 0.78; P<0.001), with a higher sensitivity of CH4 fluxes to the soil temperature with Molinia caerulea occurrence. DOC contents were significantly lower in “Molinia”. The difference in DOC between “Molinia” and “Sphagnum” plots is related to the difference in ecosystem respiration (r2:0.41; P<0.001) suggesting that Molinia presence increase the consumption of DOC to form CO2. Molinia caerulea occurrence affects C cycle in peatlands by increasing C fluxes (CO2 and CH4) and reducing DOC contents. Variations of CO2 and CH4 fluxes are mainly explained by vegetation and environmental parameters. Specific mechanisms of these findings will be further investigated through the study of the belowground compartment (MicroResp, enzymes, PLFA and mycorrhiza).