Peatlands have stored a third of the soil organic Carbon (C) in only 3% of the land area. However, with global changes, vascular plants invade peatlands to the detriment of Sphagnum mosses that contribute to C storing. Our study aims to assess the effect of vascular plant (Molinia caerulea) occurrence on C storage in Sphagnum-dominated peatlands by monitoring CO2 and CH4 emissions and Dissolved Organic C (DOC) dynamics and to relate C flux changes to environmental variables.

Peat mesocoms were collected in La Guette peatland (France) in March 2015. Twelve mesocosms received randomly two treatments: 6 with only Sphagnum rubellum and 6 containing both Sphagnum rubellum and Molinia caerulea. From April 2015 to June 2016, mesocosms were environmentally monitored and DOC contents; CO2 and CH4 emissions weekly measured.

CO2 uptake was significantly higher with Molinia caerulea during the growing season and positively correlated with Molinia leaves number (r2:0.84). After senescence, the leaves decomposed, which generated a higher CO2 flux than in Sphagnum ones. CH4 emissions significantly increased during summer with a higher sensitivity and relationship to the soil temperature (r2:0.78) with Molinia. DOC contents were significantly lower with Molinia and was related to an increase of respiration (r2:0.41) suggesting an increase of DOC consumption by microorganisms to form CO2.

Molinia caerulea occurrence affects C cycle in peatlands by increasing CO2 and CH4 fluxes which are mainly explained by vegetation and environmental parameters. Specific mechanisms of these findings will be further investigated through the study of the belowground compartment.