Peatlands represent only 3% of terrestrial surface area but they stock one third of the world's soil carbon stock. Sphagnum genus have a fundamental role in peatlands, because it forms dense carpets that slowly decompose into peat, sequestering carbon. Belowground interactions between plants and microorganisms play a key role in ecosystem functioning and constitute a crucial challenge for peatland responses to climate warming.

Previous works clearly demonstrated that phenolics compounds from living Sphagnum are involved in microbial community structure, in the control of enzymatic activities and affect the germination of vascular plants. According to the key role of mycorrhizal fungi in the development of vascular plant, we investigated the interaction between Sphagnum phenolics and the fungal colonization of an Ericale species (Andromeda polifolia). A climate warming treatment (by around + 1°C) were initiate since 2008 by Open-Top Chambers in a Jura mountain peatland (France) in two microhabitats (lawns and hummocks). We hypothesize 1) an allelopathic interaction of Sphagnum fallax phenolics on A. polifolia mycorrhizal colonization and that 2) a long experimental warming affects such interactions.

Results show that phenolic compounds was related to the microhabitat (0.6 mg g-1DW in lawns and 1.3 mg g-1DW in hummocks respectively). We observed a significant correlation of Sphagnum phenolic production between the capitulum and the senescent stems. Mycorrhizal colonization, including ericoid mycorrhiza and Dark Septate Endophyte (DSE) was also related to the microhabitat. In ambient plots, a positive correlation between DSE colonization and Sphagnum phenolics was observed whereas there was a negative correlation between mycorrhizal colonization and Sphagnum phenolics in warming plots. These results strongly suggest that allelochemical interactions play a key role in peatland functioning and confirm that belowground biological activities are crucial in ecological processes.