Diversity and Efficiency of Rhizobia from a Revegetated Area and Hotspot-Phytophysiognomies Affected by Iron Mining as Indicators of Rehabilitation and Biotechnological Potential.

This study aimed to evaluate the resilience of phytophysiognomies under influence of iron mining by assessing the occurrence, diversity, and symbiotic efficiency of native communities of nitrogen-fixing bacteria that nodulate leguminous plants (rhizobia) in soils of an area revegetated with grass after iron mining activities and in the phytophysiognomies in adjacent areas (Canga, Atlantic Forest, Cerrado, and Eucalyptus-planted forest). Experiments for capturing rhizobia through two species of promiscuous plants, siratro (Macroptilium atropurpureum) and cowpea (Vigna unguiculata), were conducted in a greenhouse. The rhizobial strains isolated were characterized phenotypically, genetically (16S rRNA sequencing and BOX-PCR fingerprinting), and regard symbiotic efficiency of biological nitrogen fixation (BNF) compared to mineral nitrogen and reference strains. Cowpea captured a higher density of rhizobia than siratro did. However, most of the strains captured by siratro had greater symbiotic efficiency. The revegetated area proved to be the community most efficient in N2 fixation and was also the most diverse, whereas Canga was the least diverse. For the two trap species, the predominant genus captured in the revegetated area and in the phytophysiognomies was Bradyrhizobium. The greater symbiotic efficiency and the high genetic diversity of the rhizobial community in the revegetated area indicate the effectiveness of the soil rehabilitation process. The revegetated area and the phytophysiognomies proved to harbor strains with high biotechnological potential. Results indicate that the high functional redundancy of this group of bacteria contributes to the resilience of these phytophysiognomies and the revegetated area.

Chemical, physical, and biological attributes in soils affected by deposition of iron ore tailings from the Fundão Dam failure.

Monitoring degraded areas is essential for evaluation of the quality of the rehabilitation process. In this study, we evaluate how the physical and chemical characteristics of the mixture of iron ore tailings with the soil have affected the soil microbial biomass and activity in areas along the Gualaxo do Norte River after the Fundão Dam disaster. Composite soil samples were collected from areas that were impacted (I) and not impacted (NI) by the tailings. The following attributes were evaluated: chemical element content; soil density, porosity, and texture; microbial biomass carbon; basal respiration; and enzyme activity and density of microbial groups (bacteria, actinobacteria, fungi, arbuscular mycorrhizae, phosphate solubilizers, cellulolytic microorganisms, nitrifiers, ammonifiers, and diazotrophs). According to result, the deposition of tailings increased the pH and the soil available P, Cr, Fe, and Mn content and reduced organic matter. The physical and biological attributes were negatively affected, with increases in the silt content and density of the soil, and reduction in macroporosity and in the microbial biomass and activity of the soil (respiration and enzymes) in the impacted area. However, the impacted areas exhibited greater densities of some microbial groups (cellulolytic microorganisms, nitrifiers, and diazotrophic bacteria). Modifications in the organic matter and silt content are the main attributes associated with deposition of the tailings that affected soil microbial biomass and microbial activity. This may affect erosive conditions and the functionality of the ecosystem, indicating an imbalance in this environment. In contrast, the higher density of some microbial groups in the impacted areas show the high rehabilitation potential of these areas.

Soil biological attributes in arsenic-contaminated gold mining sites after revegetation.

Recovery of arsenic contaminated areas is a challenge society faces throughout the world. Revegetation associated with microbial activity can play an essential role in this process. This work investigated biological attributes in a gold mining area with different arsenic contents at different sites under two types of extant revegetation associated with cover layers of the soil: BS, Brachiaria sp. and Stizolobium sp., and LEGS, Acacia crassicarpa, A. holosericea, A. mangium, Sesbania virgata, Albizia lebbeck and Pseudosamanea guachapele. References were also evaluated, comprising the following three sites: B1, weathered sulfide substrate without revegetation; BM, barren material after gold extraction and PRNH (private reserve of natural heritage), an uncontaminated forest site near the mining area. The organic and microbial biomass carbon contents and substrate-induced respiration rates for these sites from highest to lowest were: PRNH > LEGS > BS > B1 and BM. These attributes were negatively correlated with soluble and total arsenic concentration in the soil. The sites that have undergone revegetation (LEGS and BS) had higher densities of bacteria, fungi, phosphate solubilizers and ammonium oxidizers than the sites without vegetation. Principal component analysis showed that the LEGS site grouped with PRNH, indicating that the use of leguminous species associated with an uncontaminated soil cover layer contributed to the improvement of the biological attributes. With the exception of acid phosphatase, all the biological attributes were indicators of soil recovery, particularly the following: microbial carbon, substrate-induced respiration, density of culturable bacteria, fungi and actinobacteria, phosphate solubilizers and metabolic quotient.