Biodiversity And Tolerant Plant-Microorganism Remediation Potential On Bauxite Residue Disposal Areas

Bauxite residue is the solid waste generated during alumina extraction.It is high alkaline and saline,which is difficult to utilize.Currently,the majority of the bauxite residue is stored in disposal areas（bauxite residue disposal areas,BRDAs）,not only occupying land resources,but also possessing relatively serious environmental risks to surrounding areas.Recently,the ecological disposal of bauxite residue has attracted the attention in the academia and industry field.The transformation of bauxite residue into soil-like materials is the promising way forward to remediate the residues on a large scale and reduce potential environmental risks.Microbial communities are an important component of terrestrial ecosystems and directly affect the geochemical cycle and the soil formation of bauxite residue,which has been largely neglected.In this paper,the evolution of microbial communities and its potential functions during soil formation in bauxite residue were studied by the combination of field investigation and soil cultivation experiments.The main conclusions are as follows:（1）During the natural soil formation of bauxite residue,the ratio of carbon to phosphorus and the ratio of nitrogen to phosphorus gradually increased,and the ecostoichiometric ratio of carbon to nitrogen and phosphorus of red mud gradually approached the soil near the BRDAs.The contents of carbon,nitrogen and phosphorus on the surface of the newly stacked bauxite residue（stacked for less than 1 year）were 11.09 mg/kg,2.13 mg/kg and 0.48 mg/kg,respectively,and increased to 141.2 mg/kg,24.4 mg/kg and 4.59 mg/kg after 18 years stacked.After 18 years,pioneer plants invaded the BRDAs,and herbaceous plants were the dominant species.The enrichment coefficients of K,Ca,Na,Mg of the four salt ions in dominant plants ranged from large to small K（0.84）>.Ca（0.73）>Mg（0.54）and Na（0.28）,and the enrichment ability of K and Ca in dominant plants was higher than that of Mg and K.The transport coefficient of the dominant plants to the four salts was K（1.04）>Ca（0.87）>Mg（0.62）>Na（0.32）,and the enrichment ability of K and Ca in dominant plants was higher than that of Mg and K.Rhizoctonia dactyloides,Herba virginis,Digitaria sanguinalis and Festuca arundinacea have strong ability of salt ion enrichment and transport,and belong to the enrichment plants.Based on the comprehensive consideration of the salt content,enrichment coefficient and transport coefficient in plants,it is concluded that Digitaria sanguinalis and Festuca arundinacea have a good ability to enrich salt ions in bauxite residue,so they can be used as the pioneer plants in BRDA remediation.（2）Compared with fresh residue,the physical and chemical properties of the rhizosphere areas were improved.The microbial biomass,enzyme activity and bacterial community richness and diversity were increased significantly.Proteobacteria,Acidobacteria,Bacteroidetes and Actinobacteria were the dominated taxonomy group.Organic carbon,total nitrogen and p H were the main driving factors of bacterial community in the process of natural formation of bauxite residue.（3）Collected bauxite residue from the plant growth area on BRDA,isolate strains of indigenous microorganisms,and screened out a salt alkali tolerant acid-producing bacterium that can adapt to strong alkaline conditions and alleviate salinity stress through acid production.The salt alkali resistant bacteria were identified as Bacillus thuringiensis.The optimal acid production conditions include an initial p H of 9 and a salt concentration of 0.8%.When cultured with glucose as a carbon source and peptone as a nitrogen source for 11 days,the biomass of the bacillus reached its maximum,and the p H of the culture medium was reduced to3.6.（4）Gypsum was added at a ratio of 2%to reduce the alkalinity of bauxite residue.Next,use a combination of plants and microorganisms to improve the bauxite residue.The results showed that after planting Cynodon dactylon/Festuca arundinacea,the p H of the bauxite residue decreased to about 8,and the nutrient components such as carbon,nitrogen,and phosphorus significantly increased,and the physicochemical properties of the bauxite residue met the requirements of ecological restoration of the BRDAs.Compared with single plant cultivation,the addition of bacillus significantly increased the microbial biomass and enzyme activity of the bauxite residue.The microbial community diversity of the bauxite residue was significantly increased,and the abundance of bacterial groups such as Rhizobiaceae,Sphingobacter,Pseudomonadaceae,Xanthomonadaceae,Saprospiraceae,and Flavobacteriaceae increased,and the ecological functions of the microorganisms changed significantly.（5）After the addition of Bacillus subtilis,the contents of active oxygen components such as malondialdehyde,O₂^-,and H₂O₂ in the aboveground and underground tissues of Cynodon dactylon/Festuca arundinacea were significantly reduced（p<0.05）,while the activities of antioxidant enzymes such as superoxide dismutase,peroxidase,catalase,and ascorbate peroxidase were significantly increased（p<0.05）.This indicates that Bacillus subtilis can effectively promote the expression of antioxidant enzymes in Cynodon dactylon and Festuca arundinacea,and alleviate the toxic effects of bauxite residue salinity on plants.The combined action of plants and microorganisms can improve the succession of microbial community and accelerate the soil generation process of bauxite residue.Compared with the natural weathering process of bauxite residue,human disturbance can reduce the salinity of bauxite residue in a short time,improve the physical and chemical properties of bauxite residue,increase the richness and diversity of bacterial community,and promote the soil generation of bauxite residue and the ecological restoration of the storage site.