Synchronic shifts in phenolic compounds and fungal communities during litter decomposition in boreal forests

Phenolic compounds in plant litter influence decomposition process both directly by their own degree of decomposability and indirectly by affecting decomposer processes. While phenolic characteristics of aboveground plant biomass are well-known, the fate of litter phenolics and their regulatory effects on belowground decomposition remain unclear. We conducted a two-year decomposition experiment distributing litterbags of spruce, pine, and bilberry along a gradient from mesic spruce-dominated to xeric pine-dominated forests. Litter material was analysed for mass loss, low molecular weight phenolic compounds, condensed tannins, fungal biomass, and fungal community composition (using DNA metabarcoding). Bilberry litter initially decayed more rapidly than the conifer litter, but the remaining mass of bilberry litter was the same as for pine and higher than that of spruce litter after two years. The concentration and composition of phenolic compounds decreased with time, with mainly MeOH-insoluble condensed tannins remaining at the end of the experiment. Correspondingly, fungal biomass increased in the first year, while all litter types underwent rapid changes in fungal community composition. The species-specific pattern of temporal change in mass loss, as well as in the synchronic shift in composition of phenolic compounds and fungal community, give new insights in the importance of litter quality for the decomposition process.