The balance between accumulation and loss of soil organic matter in subarctic forest is related to ratios of saprotrophic, ecto- and ericoid mycorrhizal fungal guilds

Free-living saprotrophic fungi and symbiotic mycorrhizal fungi affect organic matter dynamics differently because of contrasting ecological adaptations. We investigated how mass-loss, C:N-ratio and stable isotope dynamics of leaf litter and humus substrates depended on presence of living tree roots and associated fungal communities in a forest-to-tundra ecotone over three years. Litter mass-loss was stimulated by tree roots, contrary to a Gadgil effect. Increases in the litter nitrogen pool and 815N suggested import of nitrogen from deeper soil by the dominating saprotrophic fungi. Over time, humus first lost, then gained, mass, and corresponding shifts in 815N and 813C suggested fluctuating pools of fine roots and fungal mycelium. Ectomycorrhizal tree roots consistently reduced longer-term humus mass-gain, counteracting positive effects of ericoid roots and associated fungi. Across all substrates, mass dynamics correlated with the balance between ectomycorrhizal and littersaprotrophic fungi, both linked to mass-loss, and ericaceous shrubs and associated fungi, linked to mass-gain.