Comparison of Soil Microbial Communities in Opencast Limestone Mine and Mine Rehabilitation Sites

Opencast mining is considered as an extreme form of land degradation, requiring an intensive array of rehabilitation practices. The mine rehabilitation practice, in the semiopencast limestone mine – northern Thailand, was operated by replacing the mine surface with soil and plating framework tree species. This method was practised to contribute positive feedback on plant and animal diversity. However, information about soil microorganisms, which play important roles in ecosystems, is still scarce. This study revealed a microbial perspective on limestone mine rehabilitation.

Soil physicochemical properties, soil microbial biomass, activity and community composition under the semi-opencast limestone mine and mine rehabilitation plots were investigated as compared to those in adjacent remnant forest. Soil samples were collected twice: i) before tree planting, to compare differences in soil and microbial properties between mine and rehabilitation substrates; and ii) nine months later, to monitor the response of both properties to mine rehabilitation. Microbial biomass and activities were measured based on phospholipid fatty acid (PLFA) analysis and extracellular enzyme analysis (soil enzyme activity). This comparison of soil and microbial properties, between mine and rehabilitation substrates, revealed the potential of substrate brough in to support mine rehabilitation processes. Some nutrients in the rehabilitation substrate were presented at significantly greater levels than those of the mine floor substrate. Rehabilitation substrates contained 0.05 % and 67.40 mg/kg of total nitrogen and potassium respectively, whereas levels in mine substrate were 0.02 % and for 26.44 mg/kg, respectively. After nine months of rehabilitation, approximately 9% of rehabilitation sites were covered by naturally re-established ground flora, yet no plant species were found in mine area. Moisture and organic matter were significant higher in the rehabilitation plots, compared with the mine substrate. Whilst the rehabilitation substrate contained 5% and 1% of soil moisture and organic matter respectively, levels of the same were only 2% and 0.4 % in mine substrate. No significant changes in microbial biomass and soil enzyme activities were detected in mine and rehabilitation substrates, before and after tree planting. However, moisture, organic matter, nutrients, microbial biomass and soil enzyme activity were markedly higher in forest soil than in the mine and rehabilitation plots; moisture content, total biomass and β-glucosidase activity were10 times higher in forest soil than the other two substrates.

High quality and quantity of DNA could not be obtained from the mine and rehabilitation substrate, which prevented investigation of overall microbial community at a deep taxonomic level. Therefore, amplicon sequencing of enrichment culture was used, which revealed 17% of the total microbial community and detected some taxa that cannot be detected by eDNA. Although the proposed method has limitations, it can overcome the eDNA issues (low quality and quantity). This method revealed significant differences in community composition of bacteria and fungi in in mine and rehabilitation plots compared with forest soil, yet no differences were found between the mine substrates. Information about soil microbes that are able to survive in the extreme conditions of mine areas was revealed. Microbial taxa, such as Bacillus, Streptomyces and Aspergillus, could be of particular interest for microbial inoculation to facilitate mine rehabilitation. This study increased understanding of the soil microbial community and microbial activity, during mine rehabilitation. The findings of this study could be useful in developing future mine rehabilitation plans and soil ecosystem health in mine rehabilitation program.