Soil microbial communities dynamic in spontaneous afforestation: a comparative analysis between the Casentino Forests and the Julian Prealps

In this study we investigate the effects of rewilding, a spontaneous process ongoing since decades after land abandonment at national and European levels, with a focus on the replacement of former grasslands and pastures by tree forest. In particular, we explored the ecological dynamics occurring within the topsoil. The main objectives are: i) to clarify how topsoil physico-chemical properties change along the successional gradient, ii) to provide an overview of soil microbial communities response along the same gradient, and iii) assess causal relationships among soil predictors and microbial response, in terms of community composition and diversity, as well as abundance of bacterial and fungal taxonomic groups. The study areas were the Foreste Casentinesi National Park and the Julian Prealps Regional Park (Italy), In both areas we identified by historical ortophotos (period 1954-2020) five successional stages replicated in four chronosequences: grassland-pasture (G), shrubland (S), early (E), intermediate (I), and late afforestation (L). Replicated topsoil samples (0–10 cm) were analysed for pH, bulk density (BD), and organic carbon (OC), and total nitrogen (N) contents. Microbial communities were assessed from environmental DNA extracted by the fine soil fractions followed by DNA metabarcoding using ITS and 16S markers for fungi and bacteria, respectively. Results showed that as the succession progresses, soil acidification and a reduction in bulk density occur, coupled with an increase in soil organic matter at later stages in mature soils. However, such trends are quantitatively affected by site-specific variability. Bacterial and fungal communities respond differently to secondary grassland afforestation: fungi, mainly Ascomycota and Basidiomycota, exhibit greater specialisation in mature successional stages, while bacteria, dominated by Proteobacteria and Verrucomicrobiota, show more site-specific traits. Comparisons between the two study areas showed a lower variability in microbial diversity in the Casentino National Park, likely due to its more homogeneous environmental conditions, including plant cover. Our study underlines the functional importance of soil biota in enhancing and sustaining carbon storage in forest soils undergoing natural afforestation. On a broader scale, the study highlights the value of nature-based solutions such as rewilding for climate neutrality and biodiversity conservation.