Soil health and microbial diversity in early restoration sites affected by tailings from the Fundão dam failure

Abstract

The failure of the Fundão dam in 2015 resulted in the deposition of large volumes of iron mining tailings over soils in marginal sites throughout the Doce River basin in Mariana, Minas Gerais, severely compromising their physical, chemical, and biological properties and posing major challenges to environmental recovery. In this context, this study evaluated soil recovery in revegetated sites affected by iron mining tailings from the Fundão dam, integrating soil health, microbial community analysis, and functional prediction based on molecular data. Soil recovery was investigated through the application of a Soil Health Index (SHI), developed from physical, chemical, and biological indicators in sites with 3, 4, and 5 years of revegetation following tailings deposition, compared with a native vegetation reference site, using composite soil samples collected at a depth of 0–10 cm. The results demonstrated that revegetation promoted progressive improvements in soil health, including increases in available water content, soil organic carbon, carbon stocks, K and S availability, as well as enhanced microbial and enzymatic activity. Functions related to plant growth support, carbon storage, and habitat provision showed faster recovery, whereas nutrient cycling remained the most limited function, indicating non-linear recovery trajectories constrained by the tailings-derived substrate. The response of soil microbial communities to revegetation was evaluated using high-throughput sequencing of the 16S rRNA and ITS2 regions and microbial functional potential prediction with PICRUSt2, revealing that although revegetation promoted shifts in taxonomic composition and the emergence of microbial groups associated with plant–microorganism interactions, revegetated sites remained distinct from the reference site, indicating persistent limitations in soil biogeochemical complexity and the need for longer recovery periods. Functional predictions indicated constraints in carbon-related metabolic pathways and oxidative processes, consistent with the low organic matter availability in the tailings, while also revealing early signs of a return toward the functional state of the microbiota and its contribution to ecosystem services. Overall, the results demonstrate that revegetation promotes initial structural and functional improvements in soil, but that microbially mediated processes recover more slowly, highlighting the relevance of integrating soil health indices with microbiome-based approaches and functional prediction to diagnose limitations, monitor recovery, and guide restoration strategies in tropical soils impacted by mining tailings.