Uncovering the root microbiome of woody plants in peatlands and its response to climate change

Background/Question/Methods

Boreal peatlands occupy 3% of the land surface but contain nearly 1/3 of the total terrestrial carbon stock. The rhizosphere, where carbon is released as root exudates and enhances microbial activity, is a hotspot of the peatland carbon cycle. Despite their potential global importance, our understanding of peatland root microbiomes and their response to climate drivers is limited. Warming and CO2 enrichment (eCO2) are expected to cause a shift in plant communities from Sphagnum moss to vascular plants. Since belowground plant traits can modulate microbial community and function, it is critical that we combine fine-root trait measurement with microbial community analyses to predict how plant-microbe interactions may shift in responses to climate change. Here, our objectives were two-fold: 1) characterize the root microbiomes of woody plants in a forested bog, and 2) investigate the response of their fine-roots and associated microbiomes to warming and eCO2. To address these objectives, we collected ingrowth cores from enclosures warmed from 0 to 9 ºC above ambient temperature and with or without eCO2. After removing and sorting woody plant roots from the cores, we quantified fine-root traits and characterized the roots and rhizosphere bacterial and fungal composition using an amplicon-sequence approach.

Results/Conclusions

In accordance with previous work on shrubs, trees heightened soil exploitation in response to warming, indicated by increased fine-root tip density and biomass. Furthermore, eCO2 stimulated tree ectomycorrhizal colonization (EC). Tree root microbiomes were dominated by the bacterial families Burkholderiaceae, Acidobacteriaceae, and Acetobacteraceae, and the fungal families Cenangiaceae, Thelephoraceae, and Hymenogastraceae. While bacterial diversity decreased with eCO2 and increased with warming (P < 0.001), fungal diversity increased with EC but decreased with root length density and eCO2 (P < 0.001). The relative abundance of ectomycorrhizal fungi increased with EC (P < 0.001), whereas the relative abundance of saprotrophs decreased with eCO2 but increased with moisture and warming (P < 0.005). Shrub microbiomes contained similar dominant taxa and showed a similar bacterial response to climate drivers as trees. However, no change was observed in shrub root fungal communities, which were dominated by the Suillaceae, Thelephoraceae, and Serendipitaceae. In conclusion, eCO2 destabilizes fungal communities, promoting ectomycorrhiza while reducing overall fungal diversity in tree roots. Warming further decreases fungal diversity through its positive effect on root production while stimulating bacterial diversity. The increased relative abundance of saprotrophs is of particular concern, as it indicates a potential for accelerated peat decomposition with warming.