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Role of microorganisms in reclamation of coal-mine spoils
By: R S Upadhyay1 and Richa Raghuwanshi2
Introduction
The fossil fuel coal is largely valued for its energy content. Since the 1880s it has been primarily and widely used for electric/power generation. The industrial revolution, which started in Great Britain in the 18th century and from there, it later spread across all over continental Europe and North America, was based on the availability of
coal-powered steam engine which expanded the international trade. Coal exploitation, however, has led to negative consequences on the terrestrial and aquatic environments causing soil, air and water pollution and biodiversity loss. Mining activities result in rapid and dynamic changes in land-use patterns as coal-mined lands, albeit prior to mining operations, may have been a forest or an agricultural land with abundant biodiversity. Mining activities have serious impacts on the top fertile soil as the beneficial microbial community is lost and the heavy metal toxicity, altered pH, high temperature and low water holding capacity make the coal-mines area an unsuitable habitat for plant growth leading to the creation of a degraded barren land (Fig. 1). Mining activities cause loss of litter layer, which is an integral storage and nutrient exchange site. Thus, the coal mine spoils become chemically, physically and biologically disturbed and unstable.
Coal
extraction processes are typically differentiated by whether they prevail on the surface or beneath the earth. Most coal seams are buried too deep under the earth not suitable for opencast mining and indispensably require underground mining, a process that currently accounts for nearly 60 percent of world’s coal output. The soil debris is heaped in the form of dumps known as mine spoil. The surface coal mining completely eliminates or harms the existing
flora, destroys the horizons of a soil profile altering microbial communities, displaces or destroys fauna and their habitat, reducing biodiversity, degrades air quality, deteriorates current land use patterns, and to a certain extent permanently transforms the general topography of the entire area. Because of adverse soil conditions, the coal-mine spoils require reclamation for their speedy recovery. Different physical and chemical remediation technologies have been developed over the years but are less feasible on a large scale as they may be site-specific or have an economic concern. Biological reclamation involves the returning of the coal-mine spoil to near about its original stage by restoring the nutritional property of soil through re-vegetating the land. Reclamation through the plant and microbial communities on these sites not only has significant effects on soil fertility, which enhance the ecological succession, but are also sustainable and economically feasible. It makes the ecosystem healthy by creating proper vegetation and microbial flora that improves the soil structure and quality so that it can support plant growth and reduce pollution.
Microbes and plant growth
Ecosystem stability depends on the interactions between the abiotic components with biotic components which include the diverse primary producers i. e plants, and microbes. Plant growth promoting microorganisms mainly comprise bacteria, fungi and arbuscular mycorrhizal (AM) fungi. The application of plant growth promoting rhizobacteria (PGPR) mitigate the effect of stress in plants by various direct and indirect mechanisms like production of plant growth regulators, nitrogen fixation, solubilisation of inorganic phosphate, mineralization of organic phosphate, siderophore production, exopolysaccharide secretion and their role in various ecosystem processes like nutrient cycling, seedling establishment and in soil structure formation. These PGPR have a potential to fight with diverse environmental stresses such as drought, high and low temperatures, soil salinity and heavy metals. Their root colonizing ability and different mechanisms of action have made them applicable as a reliable component in the management of degraded ecosystem including the coal mine spoils. Screening an efficient microbe with stress tolerance potential and competence in the restoration of coal mine spoils is the most important aspect of the reclamation programs.
Arbuscular mycorrhizae are the mutually beneficial symbiotic relationship of soil-borne non-pathogenic fungi with the roots of terrestrial higher plants and are found in almost 80% of plant species. The AM fungi help in increasing the phosphorus uptake and other nutrients, enhance plant growth directly through production of hormones, increase the protein, lipid, sugars, and amino acid levels, providing biologically fixed nitrogen, alleviating tolerance to heavy metals, salinity and provide resistance to root-borne phytopathogens. During this symbiotic
relationship, the fungal hyphal network functionally extends the root system area of their host plants. Thus, plants in symbiotic association with AM fungi have potential to take up mineral nutrients from an enlarged accessible soil volume and are estimated up to 47-fold compared to a normal plant. Most of the naturally growing plant species surveyed at coal-mine spoils show mycorrhizal infection in roots. The reservoir of the mycorrhizal inoculum is generally the adjoining vegetated areas from where the inoculum spreads. The inoculum is chiefly comprised of spores, hyphae or the root debris which can easily spread. As the succession proceeds at the mine spoils, there is an increase in the AM inoculum level in the soil which helps in the further establishment of new plants through various mechanisms as depicted in Fig 2.
In older plantations, five genera of AM fungi including
Acaulospora,
Glomus,
Gigaspora,
Scutellospora and
Sclerocystis
are reported. Out of these,
Acaulospora and
Glomus are found in high frequency. The mycorrhizae colonization are reported to reduce the
detrimental effects of increased temperature, alter plant–water relations, improve their resistance to drought, increase stomatal conductance and
transpiration rates, besides suppressing the various soil-borne diseases and thus facilitate the establishment as well as the survival of pioneer vegetation during the stress conditions. Mycorrhizal fungi also influence the microbial diversity and abundance in the mycorrhizosphere. The abundance of trace elements in the coal mine spoils is the most challenging part in the process of reclamation. Phytoremediation is a sustainable and inexpensive technology of removing pollutants including heavy metals from the environment. Fungi colonizing the roots are capable of accumulating large amounts of heavy metals which may be up to 20% of dry mass production in plants, indicating that microbial biomass may promote the mobilization of metals in the soil system. In addition, they can alienate metals from the soils by
biosorption and/or
bioaccumulation processes. The fungal cell wall components can be very effective in heavy metals binding due to the presence of free amino, hydroxyl, carboxyl,
amine and other functional reactive groups. Some of the microorganisms can also mediate heavy metals precipitation process by producing a larger quantity of various organic acids or autolytic enzymes such as the acid phosphates or pigments, which additionally check the leaching of metals.
Calculations by Söderström show that the surface of interaction between
soil–fungal communities may be up to 0.14 m2 in 1 g of soil. Glomalin, a protein produced by AM fungi, seems to be an effective candidate in sequestering Cu, Cd, Pb and Mn found in abundance in coal mine spoils. Retention of heavy metals in the extraradical mycelium of ectomycorrhizae has been proposed as a tolerance mechanism by these fungi which support plant growth under stressed soil condition. The fungi can detoxify metals by intracellular precipitation processes. In recent years, a plethora of transmembrane transporters controlling the intracellular trafficking and distribution of metal ions have been detected in plants and microorganisms. The preferential intracellular detoxification strategy in fungi as well as plants is mainly attributed to metal chelation by cysteine-rich peptides (CRPs) such as reduced glutathione (GSH), phytochelatins (PCs) and metallothioneins (MTs). AM fungi are thus an attractive microbial system that may help in plant-based environmental clean-up.
It has been demonstrated by various studies that vegetation has a significant influence on the abundance of microorganisms in surrounding bulk soil and rhizosphere. Plants adapted to grow on polluted sites can be used for restoration of coal-mine spoils to improve soil structure and organic content. Most reclamation models utilizing plants like
Pongamia pinnata,
Gmelina arborea,
Dalbergia sissoo,
Dendrocalamus strictus,
Azadirachta indica,
Albizia procera,
Albizia lebbeck,
Emblica officinalis, Jatropha curcas,
Terminalia arjuna, Pennisetum pedicellatum,
Tectona grandis,
Eucalyptus camaldulensis,
Acacia auriculiformis,
Acacia catechu,
Acacia nilotica, Casuarina equisetifolia,
Azadirachta indica,
Cassia siamea,
Eucalyptus hybrid,
Grevelia pleridifolia
and Leucaena leucocephala have shown excellent improvement in soil organic matter. Most studies have shown that the primary colonisers of a coal mine have a dominance of Asteraceae species. To increase the microbial population in a coal-mine spoil, the organic matter and humus need to be increased. Different plant species that are cultivated in various combinations with endemic grasses have a significant impact on the structure of rhizospheric microbial community that can beneficially affect the nutrient cycling and plant establishment on coal-mine spoils. Grasses after death and decomposition contribute to soil organic matter and also provide mulching to the soil.
Acacia mangium and
Acacia catechu have potential to acclimatize to stress conditions originated due to coal mining activities. The plant root exudates have the potential to disintegrate the coarse particles into smaller particles such as clay which is important for maintaining soil aggregation and structure. The rhizosphere of these plants usually harbours useful microbes like symbiotic nodular bacteria, mycorrhiza helper bacteria, saprobic fungi and many PGPR. These organisms not only influence the plant growth by stimulating the production of vitamins and hormones and acquisition of nutrients but also help in overcoming the abiotic stress components of the soil and hence are widely exploited in the coal-mine restoration programs.
Microbial community and soil health
Ecosystem functioning is primarily governed by soil microbial dynamics as they have an important role in linking the chemical, physical and biological components of the soil and thereby making it fertile. Microbial biomass acts as a sink as well as a source of nutrient in dry tropical environment. The cycling of nutrients by microbes regulates the sustainability of any plant community in absence of which plant community will not be able to regenerate. Soil microbial biomass varies according to the land uses which are important parameters to get an idea about the soil health as illustrated in Fig. 3. Increase in soil organic content and microbial biomass results in increased functionally diverse microbes which make an ecosystem stable and functional.
Soil properties and the plant cover determine the microbial community of a place and together they determine the trajectory of ecosystem development. Microbial associations are critical for ecosystem functioning as they are directly involved in organic matter decomposition and nutrient cycling. Microbial communities play a crucial role in improving the soil fertility and productivity by breaking down organic matter into inorganic forms of mineralized nutrients accessible for plant uptake. During opencast mining, the mine spoils or overburdens created are often devoid of essential nutrients and have relatively lower water holding capacity. The gradual improvements in physical and chemical soil properties after reclamation of overburden dumps through re-vegetation aided by microbes is due to control of soil erosion, increasing organic matter and formation of humus with the increasing age of plantations and decrease in heavy metal content. The gradual reduction in the bulk density values of the reclaimed sites has been found with increasing the age of plantations. Studies have shown that depending upon the coal-mine site; complete microbial recovery may take up to 26 years after surface coal mining, with an average time of 20+ years. In general, the most important phase of microbial community recovery is between 5 to 14 yr after the initiation of reclamation programs.
Conclusions
Mining activities lead to significant degradation of the ecosystem. Long-term and large-scale measures are required in order to build greater resilience in a coal-mine degraded soil. The most suitable approach for reclamation of coal-mine spoils is through re-vegetation by use of native and indigenous plant species harboring plant growth promoting microbes in their rhizosphere which help in establishing stable biogeochemical cycle that further improves soil quality. Soil quality, microbial biomass and their diversity are the major factors governing ecosystem succession on a fresh coal-mine dump and its ecological restoration.
1Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, UP, India.
E-mail: [email protected]
2Department of Botany, Mahila Maha Vidyalaya, Banaras Hindu University, Varanasi, UP, India
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