How water-level drawdown modifies litter-decomposing fungal and actinobacterial communities in boreal peatlands

Microbes decompose fresh organic carbon inputs, such as plant litter, to CO2. Due to a high water-level (WL) in pristine peatlands the decomposition process is retarded and therefore, northern peatlands contain about 30% of the global soil carbon (C) pool. Global warming will decrease the WL of these soil C hotspots and therefore, the responses of the decomposition process of plant litter to shifts in WL regime is of interest. Two important decomposer communities, fungi and actinobacteria, were studied in a diverse set of litters incubated over two years in a boreal peatland. The experimental design involved a non-treatment control and plots experiencing short-term (ca 4 yrs) and long-term (ca 40 yrs) WL drawdown. The responses of litter-inhabiting fungal and actinobacterial decomposers to environmental variables (WL regime, site type, litter quality) were evaluated using PCR-DGGE fingerprinting and direct sequencing of regions within the 16S and 18S ribosomal RNA genes after reverse transcription of RNA into its complementary DNA. ngal sequences found in the decomposing litters represented organisms from a variety of fungal taxa, whereas most of the actinobacterial sequences were only distantly similar to known actinobacteria. Our results indicated that: (1) WL regime and site type had minor impacts on the litter-inhabiting microbial community composition, (2) litter quality had the strongest impact on the communities, especially on fungi; (3) decomposition stage had an impact on the fungal community composition, showing a possible change from mycorrhizal to saprotrophic fungi and (4) mass loss of the litters after the two-year-decomposition period was not related to the variation in fungal community composition and only to a negligible extent to the actinobacterial community composition. In relation to the decomposition process these results align with the idea of functional redundancy in microbial communities, which does not seem to be modified by moderate WL drawdown. Overall, our results indicate that the WL regime modifies the litter-inhabiting microbial communities largely indirectly via litter quality.