Integrating Microbiological Indicators and Shotgun Metagenomics for the Assessment of the Rhizosphere Microbiome of Medicinal Plants

Medicinal plants are rich sources of bioactive secondary metabolites, yet their long-term effects on the rhizosphere (RS) microbial communities remain poorly understood, particularly with respect to microbial selection and functional potential. This study evaluated the number of selected groups of microorganisms culturable in vitro in the RS and bulk soil (BS) within 10-year monocultures of 11 medicinal plant species, and as a targeted case study, we performed shotgun metagenomic profiling for Allium ursinum. The abundance of microorganisms differed markedly among plant species, indicating species-specific RS selection. Azotobacter spp. showed the strongest variation: they were not detected in the RS of Allium ursinum, Thymus vulgaris, and Carum carvi, whereas higher counts were observed under Artemisia dracunculus (135.1 × 102 CFU g−1 DM), Melissa officinalis (67.1 × 102 CFU g−1 DM) and Calendula officinalis (38.8× 102 CFU g−1 DM). Azotobacter spp. may serve as a sensitive candidate indicator of RS imbalance. Metagenomic analysis of the A. ursinum-associated soil revealed fine-scale taxonomic restructuring, while major functional categories remained broadly similar between the RS and BS. The novelty of this study lies in the development of the Integrated Microbiological Health Soil Index (IMHSI) and the proposal of a Nitrogen Enrichment Index (NEI) as exploratory composite metrics that integrate selected functional microbial groups.