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.