Soil stressors on ecophysiology of bauxite mine impacted soil: Heavy metal-acidity-organic matter nexus

Soil ecophysiology is adversely affected by various environmental hazards, particularly in mining regions. While there has been substantial research on the effects of coal, mica, copper (Cu), and manganese (Mn) mining on soil quality, the impact of bauxite mining operations on nearby soils has largely been overlooked in the literature. Therefore, this study aims to investigate how microbial activity and dynamics are influenced by soil stressors, such as acidity and heavy metals, in areas adjacent to active bauxite mines. Soil samples were collected from three adjacent locations of an active bauxite mine area at distances of <100 m (S1), 100-500 m (S2), and >500 m (S3). The samples contained chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn), manganese (Mn), and cadmium (Cd), as well as elevated acidity and aluminum (Al). These conditions adversely affected the soil microbial indicators, including fluorescein diacetate (FDA), microbial biomass carbon (MBC), and enzyme activity. The highest concentrations of labile metals (i.e., water-soluble and exchangeable) were found in soil mixed with mining waste (S1), whereas acidity and Al were highest in sparsely vegetated soil (S3). Total acidity, total potential acidity, pH-dependent acidity, and Al were significantly positively correlated. Moreover, the significant positive correlation among organic carbon (OC), acidity, Al, and microbial properties (FDA, MBC, and microbial enzymes) suggests a potential effect of OC in mitigating acidity in S3. The ratios of microbial properties with OC depicted a significant negative correlation with acidity and Al fraction, denoting that acidity and Al posed a deleterious effect on soil microbial health. The similarity percentage analysis identified acid phosphatase as the key enzyme accounting for similar to 78% of the observed differences in enzyme composition across the sites. Visual MINTEQ modeling revealed that the sites were saturated with different Al-bearing minerals. Pollution load index (PI) and the geo-accumulation index (I-geo) values identified the region as heavily contaminated (PI > 1). Finally, the health risk analysis revealed that Ni posed a potential carcinogenic risk for both adults and children.