Comparative Ecological and Metagenomic Characterisation of Wild Legume Habitats in Bulgaria: Kaliakra, Strandzha, and Eastern Rhodopes

Wild relatives of cultivated legumes represent valuable genetic resources for crop improvement, ecosystem resilience, and sustainable agriculture. This study presents a comparative ecological and metagenomic assessment of three biogeographically distinct regions in Bulgaria—Kaliakra, Strandzha, and the Eastern Rhodopes—where populations of wild legumes, including Pisum elatius, Cicer montbrettii, Vicia incisa, and Lupinus spp., occur. Field expeditions were conducted during flowering and seed maturation stages, followed by rhizosphere soil sampling and high-throughput sequencing targeting bacterial 16S rRNA and fungal ITS regions. Soil physicochemical properties, microbial diversity indices, and taxonomic composition were analysed and compared among regions. Distinct microbial community structures were identified. Kaliakra soils were dominated by Firmicutes (26–58%) and Proteobacteria (20–25%), while Strandzha soils showed higher abundance of Actinobacteriota (12–68%) and Proteobacteria (10–35%). The Eastern Rhodopes exhibited more balanced communities, with Proteobacteria (30–45%), Firmicutes (7–43%), and Actinobacteriota (3–11%). Fungal communities also differed significantly, with Nectriaceae dominating in Kaliakra, higher evenness in Strandzha, and intermediate diversity in the Eastern Rhodopes. Alpha diversity revealed the highest bacterial richness in Kaliakra, whereas the Eastern Rhodopes showed the greatest community evenness. Beta diversity analysis demonstrated clear regional separation driven by environmental filtering. These findings highlight the ecological and microbiological differentiation of wild legume habitats and support their conservation and utilisation in sustainable agriculture and breeding programs.