Climate change causes upward shift of forest lines worldwide, with
consequences on soil biota and carbon (C). Using a space-for-time
approach, we analyse compositional changes in the soil biota across the
forest line ecotone, an important transition zone between different
ecosystems. We collected soil samples along transects stretching from
subalpine mountain birch forests to low-alpine vegetation. Soil fungi
and micro-eukaryotes were surveyed using DNA metabarcoding of the 18S
and ITS2 markers, while ergosterol was used to quantify fungal biomass.
We observed a strong shift in the soil biota across the forest line
ecotone: Below the forest line, there were higher proportions of
basidiomycetes and mucoromycetes, including ectomycorrhizal and
saprotrophic fungi. Above, we observed relatively more root-associated
ascomycetes, including Archaeorhizomycetes, ericoid mycorrhizal fungi
and dark septate endophytes. Ergosterol and percentage C content in soil
strongly and positively correlated with the abundance of root-associated
ascomycetes. The predominance of ectomycorrhizal and saprotrophic fungi
below the forest line likely promote high C turnover, while
root-associated ascomycetes above the forest line may enhance C
sequestration. With further rise in forest lines, there will be a
corresponding shift in the belowground biota linked to C sequestration
processes.