Long term effects of Lespedeza bicolor revegetation on soil bacterial communities in Dexing copper mine tailings in Jiangxi Province, China

Highlights

• Direct L. bicolor revegetation increased bacterial diversity.

• Bacterial communities dominated by acid tolerant and nutrient regulated species.

• Olsen-P and pH were main regulators of bacterial composition.

• Long term L. bicolor revegetation facilitated bacterial community development in mine tailings.

Abstract

Soil microbial communities are important for ecological restoration and succession on mine tailings. In the present study, we performed Illumina sequencing to investigate the effects of long term Lespedeza bicolor revegetation on bacterial diversity and community structure in mine tailings under subtropical and moist climatic conditions. Microbial diversity indices (Shannon, OTU number and coverage estimator) of the revegetated soils were higher than that of the control, and increased over sampling period compared to the decreased pattern in the control. Species within known acid tolerant and nutrient regulated genera dominated both revegetated tailings and the control, and exhibited more abundant in the revegetated tailings. At the phylum level, percentage of the Proteobacteria and Actinobacteria were remarkedly higher in revegetated tailings than that in the control, while Chloroflexi performed reversely. Overall, this study found the positive role of L. bicolor revegetation in bacterial diversity development in the acidic mine tailings. Furthermore, the 30-year L. bicolor revegetation made the microbial community structure more homogenizable as a decrease of dissimilarity in tailings was identified over sampling times. Redundancy analysis at the OTU level indicated that Olsen-P and pH were the main regulators of microbial composition, suggesting that soil P and pH are determinant for ecological restoration and microbial community development in acidic mine tailings.

Introduction

In the light of large quantities of mineral solid wastes (such as tailings and gravel) produced by metal mining and their derived environmental problems across the world, ecological restoration in mining areas and the surrounding areas has been causing great concerns (Bes et al., 2014, Doe et al., 2017). Governance of mining area and disposal of solid wastes remain a tremendous challenge despite decades of research efforts (Mendez and Maier, 2008). Soil is important in ecosystem functioning, as it can supply habitat space and nutrient support for various plants, soil animals and innumerable microbes, therefore soil remediation in mining area is the basis of ecological restoration. In comparison, mine tailings disposal is especially urgent since piled tailings occupied huge of cultivated land and caused serious environmental problems on soils and water streams, e.g. heavy metal pollution, soil acidification, soil erosion, etc. (Rodríguez et al., 2009). Compared with physical and chemical ways of reclamation, bioremediation is a prior one due to its lower cost, effectual outcome and no secondary pollution. In recent years, phytoremediation attracts more and more attentions, and it refers to the technologies that utilize living plants to clean up soil, air and water contaminated with hazardous chemicals (Reichenauer and Germida, 2008). As an important strategy of bioremediation, phytoremediation is a cost-effective, environmentally-friendly and a long-term sustainable approach due to the ability of plants and their associated microbiota to stabilize soil structure, and to remove, accumulate, immobilize, or render harmless environmental contaminants (Chaney et al., 1997). Over the past 20 years, this technology has become increasingly popular and has been employed on the rehabilitation of soils contaminated with lead, uranium, and arsenic (Alkorta et al., 2004, Alsabbagh and Abuqudaira, 2017). Phytoremediation usually requires a long-term commitment, as the process is dependent on the plant’s ability to grow and thrive in an environment that is not ideal for normal plant growth. Previous studies found that phytoremediation strategies with shrub or trees had promoted the ecological restoration and succession in many polluted and degraded soils, such as Jatropha curcas (Marrugo-Negrete et al., 2016), Nerium oleander, Cistus albidus and Pistacia lentiscus (Parra et al., 2016). In the current study, phytoremediation was implemented in an acid mine tailing to investigate the effects of a leguminous shrub, Lespedeza bicolor, on soil ecological restoration. Microbial community in the mine tailings had extremely low biomass and activity due to lack of ripening soils and essential nutrition. Plants in the phytoremediation strategies might improve soil structure and facilitate microbial community development through providing organic compounds or exudates (Chapon et al., 2002). The successful development of a diverse soil microbiota is in turn important for subsequent ecological restoration and succession in mine tailings (Huang et al., 2012). Thus, in mine tailings rehabilitation practice, soil microbial diversity and composition are essential indicators for soil quality and plant fitness, and furthermore for the successful vegetation restoration (Bhatia, 2008).

Building biological capacity including microbial diversity and functions have been studied on both nonacidic and acidic mine tailings through vegetation restoration (Tiwary, 2001, Wakelin et al., 2012). The studies revealed that the microbial community in these stressed environments was dominated by bacteria having metabolic impacts on the tailings geochemistry (Diaby et al., 2007, Zhang et al., 2007, Mendez and Maier, 2008). The shifts of the microbial community following the establishment of plants in tailings have also been studied by either terminal restriction fragment length polymorphism, 16S rRNA gene clone library, or denaturing gradient gel electrophoresis (Rosario et al., 2007, Rastogi et al., 2010). However, much of these methods performed for profiling the microbial communities have limited ability to permit detailed and comprehensive phylogenetic or taxonomic surveys of microbial communities, especially in these stressful environments such as metallic mine tailings, abandoned mining area and severely degraded acidified soil in subtropical zone (Roesch et al., 2007). It is therefore necessary to examine microbial composition and diversity in mine tailings using newly advanced techniques. To achieve the detailed information about the microbial compositions of the whole communities accurately, the cloning-independent and massively high-throughput sequencing technology including 454 pyrosequencing, ion torrent semiconductor sequencing and Illumina sequencing were developed to study the microbial composition and genetic diversity of the target ecosystem. Among these high-throughput sequencing technologies, Illumina sequencing is popularly adopted due to its predominant sequencing quality and reasonable price (Cheung et al., 2010). Illumina sequencing technique allows in-depth analysis of the abundance of the detailed taxonomic profiles in complex environments (Kozich et al., 2013), but this type of research was limited (Li et al., 2014), especially in metallic mine tailings in response to long term direct revegetation.

Acidic mine tailings represent a typical extreme environment on earth and are characterized by unstable geochemistry associated with the weathering of sulphides and carbonates (Dold and Fontboté, 2001). Compared to the restoration of other terrestrial habitats, like highly degraded or contaminated soils, acidic mine tailings associated with low pH and high levels of iron, toxic metals such as aluminum, manganese, lead, cadmium, and zinc, as well as metalloids caused high toxicity to plants and microbial community (Neel et al., 2003, Alkorta et al., 2004, Alsabbagh and Abuqudaira, 2017, Macdonald et al., 2017). The microbes play key roles in the biogeochemistry of metal(loid)s and mineral nutrients, promoting plant growth and ecosystem stability. In these serious stressful environment, for better survival and development, microbes in the soil and associated with multiple pioneer plants have evolved certain resistance and detoxification mechanisms to various pollutants. Microbial community composition in acidic mine tailings and their shifts following vegetation restoration, are unique representativeness of these habitat and much different from other environments. Studies on microbial composition and the influenced factors in acid mine tailings will enlarge our understanding of soil micro ecosystem and provide basis for utilization of environmental microbial resources.

A comparative field study on the microbial community was carried out, taking advantage of Illumina Hiseq sequencing, on an acidic mine tailings with/without revegetation by L. bicolor, a leguminous forage species with capacity of nitrogen fixation that can survive in infertile acid or eroded soils (Sun et al., 2008). According to Dong et al. (2008), L. bicolor also developed heavy metal resistance through secreting malate and citrate. The effects of L. bicolor on soil microbial community accompanied by secretion of secondary metabolites are not consistent. It was reported that pterocarpans isolated from the roots of L. bicolor was found to inhibit bacterial neuraminidase activity (Woo et al., 2011). However, a study showed that long term re-vegetation with L. bicolor recovered microbial biomass and diversity in the severely eroded soil (Deng et al., 2010). It is yet not clear whether nitrogen fixation and chemical induction of L. bicolor regulate the microbial community in the serious stressful environment. The objective of this study was to examine bacterial community in acidic metal mine tailings in response to long term direct revegetation with L. bicolor and detect seasonal variation of microbial diversity. The bacterial communities in the revegetated and un-revegetated tailings were profiled using 16S rRNA gene based Illumina HiSeq sequencing. The outcomes of this study will provide insight into the efficacy of direct L. bicolor revegetation in ameliorating tailings’ soil biological capacity, and expand the current knowledge of acidophilic microbial community structure and seasonal variations of microbial diversity under subtropical and moist climatic stressful conditions.