DNA Metabarcoding of Soil Microbial Communities in a Postvolcanic Region: Case Study from Băile Lăzărești, Romania

This study investigates the impact of post-volcanic gas emissions on soil microbial communities in the Băile Lăzărești region (Romania). Nineteen soil samples across a CO2 gradient ranging from background levels to ≈46,221 ppm. Methane and hydrogen sulfide showed localized peaks (CH4 up to 8271 ppm; H2S up to ~10.12 ppm), with CH4 contributing to outlier community patterns. eDNA metabarcoding identified 3064 OTUs, (2463 bacterial and 601 fungal). Bacteria were dominated by Proteobacteria, fungi by Ascomycota, with Thelebolales nearly ubiquitous. Alpha diversity (Chao1, Fisher) declined significantly in high-CO2 soils (>3000 ppm), while intermediate concentrations (1000–3000 ppm) showed heterogeneous responses. Beta-diversity analyses (PCoA, clustering) revealed distinct grouping of high-CO2 soils, with sample P16 (CH4-rich) forming an outlier. A PCA including CO2, CH4, and H2S confirmed CO2 as the main driver of variance (>65%), with CH4 accounting for local effects. At the genus level, Acidobacterium, Granulicella, Streptomyces, and Nocardia increased with CO2, while Rhizobium and Pseudomonas declined. Fungal responses were mixed: Thelebolus and Cladosporium increased, whereas Mortierella and Cryptococcus decreased. Overall, elevated soil CO2 reduced microbial richness and reorganized communities, while CH4 shaped local niches. These findings provide key natural analog insights for assessing ecological risks of CO2 leakage from geological storage.