Soil fungal diversity of birch plantations on former agricultural land resembles naturally regenerated birch stands on agricultural and forest land

Short-rotation forestry on former agricultural soils is a novel silvicultural system for producing woody biomass and mitigating climate change through CO2 uptake. However, tree plantations are often criticized for their low biodiversity and are sometimes referred to as "green deserts". We hypothesize that regarding biodiversity, the spontaneous natural regeneration of birch on former agricultural lands may be preferable to plantations. In this study, we compared soil fungal diversity and composition of fast-growing silver birch (Betula pendula Roth) plantations on former agricultural lands (n = 11) with naturally regenerated birch stands on former agricultural lands (n = 11) and native forest birch stands (n = 11) at the age of 21-28 years in hemiboreal Estonia. PacBio third-generation sequencing was carried out to analyse fungal species richness and composition in the soil samples. Sequencing of the fungal ITS region from 33 birch stand soil samples yielded 2830 fungal OTUs. We found that the total fungal species richness, per sample species richness, Shannon diversity index, and Pielou's evenness index did not differ significantly between the studied stand types. However, fungal community composition of both plantations and naturally regenerated stands on former agricultural lands differed significantly from the native forest stands, indicating that previous land use (agriculture vs. forest) had a stronger effect on fungal assemblages than the used regeneration method (planting vs. natural regeneration). The relative abundance of ectomycorrhizal and arbuscular mycorrhizal fungi was higher, and the relative abundance of saprotrophic fungi was lower in plantations than in native birch dominated forest stands. Soil chemical properties were found to be weak explanatory variables for describing soil fungal richness and composition. We identified historical surrounding forest cover, stand age, stand distance from a native forest, and soil Mg content as the factors significantly affecting soil fungal communities. These results indicate that fast-growing birch plantations established on former agricultural soils or naturally regenerated stands on such soils in the Northern Europe region harbour soil fungal diversity levels similar to those of same-aged native birch forests.