Microbial Indicators of Soil Health Along Forest Rewilding Trajectories in Europe

Natural rewilding of abandoned agricultural land and proforestation (the practice of leaving existing, mature forests to grow undisturbed and unmanaged) are increasingly promoted as low-cost nature-based solutions for biodiversity conservation and climate mitigation. Yet their outcomes remain debated across scientific, policy, and societal domains. In particular, Europe lacks harmonized, large-scale evidence on how rewilding influences soil biodiversity and its relationship to forest recovery and carbon sequestration. Existing studies are fragmented across regions, methods, and disciplines, leaving, especially, belowground responses insufficiently understood and poorly integrated into environmental monitoring frameworks.The WILDCARD project, an EU Horizon 2020 initiative, addresses these gaps through a coordinated, cross-European assessment of forest ecosystems undergoing natural succession and proforestation. Using high-throughput DNA metabarcoding of bacterial and fungal ribosomal markers, we characterize soil microbial communities along rewilding gradients spanning from managed croplands to old secondary and primeval forests. Microbial datasets are combined with measurements of forest structure, soil properties, and carbon accumulation trajectories to identify microbial indicators that reflect ecosystem recovery processes.Among others, we expect to observe belowground recovery patterns along the rewilding gradient, including increases in microbial diversity and network connectivity, which are associated with higher functional complexity and overall ecosystem resilience. Reductions in agroecosystem-associated pathogens and increases in symbiotic and saprotrophic microorganisms are anticipated to enhance nutrient cycling, support forest regeneration capacity, and influence carbon persistence. Exploring these patterns will help determine whether and how rewilding shapes belowground biodiversity, and what implications this may have for long-term ecosystem functioning and carbon stabilization.By identifying robust microbial indicators linked to forest rewilding and soil carbon dynamics, this work provides a foundation for developing soil health metrics suitable for large-scale monitoring. The resulting bioindicators will strengthen the scientific basis needed to evaluate the contribution of rewilding to EU climate and biodiversity goals and further support evidence-based decision-making in ecological restoration.