Belowground plant traits and hydrology control microbiome composition and methane flux in temperate fen mesocosms
The rewetting of formerly drained peatlands is a strategy to fight against global warming through the reduction of CO2 emissions, although this can lead to elevated CH4 emissions. The interplay between plants, hydrology and microbiomes as ultimate determinants of CH4 dynamics is still poorly understood, despite recent progress in field studies.  Using a mesocosm approach, we simulated the re-cultivation of a degraded temperate fen with three different water levels and two different plant over the course of a growing season.  Peat samples for microbiome analysis, above- and below-ground plant biomass and gas fluxes were measured in April, June, August and October. Microbiome composition in top and subsoils was determined using 16S rRNA gene amplicon sequencing. We found that peat depth and sampling time were the major factors shaping the microbiome composition dynamics. While plant species had a less strong impact, the difference to bare ground microbiomes was significant, especially in the lower layer. The water status also affected the microbiome, albeit to a much lesser extent. Methanogens were most abundant in the deeper peat and also more abundant in bare ground and Carex rostrata pots, as compared to Juncus inflexus or mixed pots. This was inversely linked to the larger root network size of J. inflexus. The methane emissions correlated positively with the abundance of methanogens and correlated negatively with the root network size.  In conclusion, this interdisciplinary study sheds light on how the complex interplay between plants, hydrology and the fen microbiome affect CH4 emissions. It showed that the presence of plants as well as the plant functional type determine the abundance of methanogens and microbiome composition and thereby the resulting CH4 fluxes accordingly.