INDUCTION
India produced 606.89 million tonnes of coal during 2017-18 (MOC, 2019), of which more than 92% was produced by open strip mining. During open strip mining process, entire vegetation cover is removed, and topsoil is scraped out to reach the coal seam. It results in extensive soil degradation, loss of microbial population, severe loss of soil organic carbon which leads to destruction of vast amounts of vegetative area. Indorante et al. observed that SOM (soil organic matter) content declined drastically in soils disturbed by mining (Indorante et al.,1981). With the adoption of appropriate reclamation strategies, post-reclamation land management practices, and increasing time since reclamation, reclaimed mine soils can sequester significant amounts of SOC (Jacinthe & Lal, 2007). Reclamation of mine soils could be done by physical and biological methods. Physical reclamation, which is costly, aims at creation of suitable landforms, compatible with the landscape. Biological reclamation is concerned with establishing and maintaining vegetation cover on the overburden dump, which is compatible with surrounding landscape, stable and self-sustainable. Revegetation is a useful way to reduce erosion and protect soils against deterioration and improving SOC stock during reclamation. Thus soil reclamation and re-establishment of vegetation cover on disturbed land could lead to C sequestration.
Trees, being efficient biomass generators, add more organic material (both above-and below-ground) to the soil. Their deep roots involve a greater depth of raw mine stones in the soil organic system (Singh et al., 2015). The restored site has a large potential to sequester atmospheric C that may vary with the climatic conditions and the plant species used for reclamation (Lal, 2005; Pietrzykowski & Daniels, 2014). Soil organic carbon is a useful indicator of soil quality and contributes largely to the global carbon pool. Recalcitrance indices can serve as an indicator of stable carbon in the soil (Datta et al., 2018). Soils hold one of the largest terrestrial reservoirs of organic carbon (OC), and while most of this pool cycles on very slow time scales (centuries to millennia), climate change and landscape disturbance can affect the proportion of soil organic carbon (SOC) with the atmosphere (Houghton et al., 2001). Since post-mining soils are depleted of carbon, chronosequence based approach to understand the effects of time since reclamation on development of different SOC pools can be easily studied under this condition.
SOC represents a complex assemblage of polyphenols, amino acids, ketones, esters, carbohydrates, and a wide variety of different moieties with highly variable and complex molecular properties (Chin et al., 1998). Spectroscopic techniques provide useful information about the structural and compositional characteristics of SOC molecules (Muñoz et al., 2009), based on the intensity and position of different absorption bands, diagnostic to the structure and composition of specific chromophores (functional groups) (Yu et al., 2010). A variety of ultraviolet-visible (UV- vis) (Jiang et al., 2011) and Fourier transform infrared (FT- IR) (Haberhauer et al., 2000) spectroscopic indices have been devised to relate molecular characteristics of SOC to its source, quality, and decomposition pathways. For spectroscopic techniques, sample requirement is less and sample preparation is easy.
Spectroscopic assessments of SOC in mine soils are relatively scarce as compared to those in undisturbed soils and also data on the quality and amount of carbon sequestration through reclamation of mine lands in Gevra coalfields and elsewhere in India were scanty. Therefore, the objectives of this study were to assess the different carbon pools in soil under three trees Azadirachta indica, Dalbergia sisoo, Gmelina arborea and quality of carbon sequestered in a chronosequence comprising two reclaimed mine soils having similar soil-forming conditions except for time (since reclamation) and compared with recently dump mine soil. Therefore, the objectives of the experiment were: (i) isolate different SOC pools in each year (8 and 25year) mine soil; (ii) characterize temporal changes in molecular properties of each SOC pools along the chronosequence; and (iii) identify interrelationships between SOC molecular properties and SOC sequestration. The governing idea was to gain insight into the different carbon pools present and broad classes of functional groups associated with the SOC molecules in these mine soils and present a qualitative assessment of the changes in molecular properties along the chronosequence. The indices were selected to obtain complementary information about the SOC molecular properties and thus to gain a holistic view of the overall structural and compositional details of the organic molecules. Outcomes of this study will provide clues to understanding the influences of time on the SOC molecular properties and on SOC pools to gain insight into SOC sequestration processes in reclaimed mine soils under three tree species.