Illegal gold mining filters soil bacterial communities and enhances mercury mobility across Brazilian biomes: A multi-season study

Artisanal gold mining is a major source of mercury (Hg) contamination in humid tropical soils, but its combined effects with seasonal climatic variation on soil biogeochemistry and bacterial communities remain unclear. In this study, we evaluated whether mining-induced degradation of soil organic matter (SOM) quality and seasonal increases in Hg bioavailability act as environmental filters that reduce bacterial diversity, favor stress-tolerant taxa, and impair soil functioning and resilience. We evaluated four Brazilian biomes (Amazon, Cerrado, Pantanal, Atlantic Forest) under contrasting land uses (pasture vs. mining) during wet and dry seasons, integrating SOM characterization, sequential Hg extractions, 16S rRNA sequencing, and functional prediction. Seasonality strongly influenced soil moisture, SOM composition, and Hg mobility, with the dry season increasing active and bioavailable Hg. Bacterial diversity showed a marked negative correlation with aromatic SOM (R = –0.44) and active Hg (R = –0.32), and positive associations with SOC, WEOC, WETN, and pH (R > 0.6). Mining consistently reduced SOC, TN, and labile C–N fractions while increasing bioavailable Hg. Functional prediction revealed pronounced reductions in functions linked to OM decomposition and nitrogen cycling in mining soils, whereas pastures, especially in the Atlantic Forest and Pantanal, retained higher functional potential. No taxa associated with direct microbial Hg methylation (hgcAB carriers) were detected, although several stress-tolerant groups may indirectly modulate Hg reactivity. Collectively, these patterns highlight the risk that continued mining activity and intensified seasonal extremes may accelerate soil degradation and increase Hg mobility, posing long-term threats to ecosystem recovery.