Quality of Soil Carbon Sequestered
Higher EET: EBz ratios observed in the
HAs fractions in older mine sites indicated that SOM molecules became
more enriched in polar functional groups as mine soils aged along the
chronosequence and probably became more reactive over time. A vast
majority of the chromophores that absorb light in the ultraviolet region
(l < 400 nm) are aromatic groups comprising phenols and
various aromatic acids with varying degrees of substitution (Chin et
al., 1994). According to Weishaar et al. absorption features between the
200 and 380 nm are indicative of conjugated systems, commonly associated
with aromatic moieties (Weishaar et al., 2003)). The magnitude of the
EET: EBz ratio also offers an insight
into the type of substitution occurring in the aromatic C species. The
EET: EBz represents the ratio of
absorption intensities of the electron in the transfer band (around 250
nm) and the benzenoid band (around 220 nm). The intensity of the former
band is primarily affected by the presence of hydroxyl, carbonyl,
carboxyl, and ester moieties and therefore indicates the extent of
substitution in the benzene nucleus by these polar functional groups
(Fuentes et al., 2006). For unsubstituted benzene, EET:
EBz is low and increases with increasing substitution by
polar functional groups (Kim & Yu, 2005).
The absorbance intensities at 465 nm correspond to organic carbon
molecules of intermediate structural complexity including carbohydrate
and proteins, whereas the band at 665 nm arises from more structurally
complex and humified biomolecules (D’Orazio & Modelli, 2010). The
magnitude of E4: E6 is inversely related
to the extent of aromatic C=C absorption, with a low ratio corresponding
to a higher degree of aromaticity, molecular condensation, and high
molecular weight (Chen et al., 2002). In present study, we found higher
E4/E6 ratio in 8years restored soil than
in 25years of reclamation. It represents high aromaticity and molecular
weight in 25 years old restored soil. The result obtained indicated that
TOC content was highest under Azardichta indica but aromaticity
was highest under Gmelina arborea .
FT-IR spectra of restored mine soil in the case of Azadirachta
indica, Dalbergia sissoo and Gmelina arborea indicated that
relative proportions of aromatic groups along the chronosequence
increased indicating higher proportion of resistant SOC components in
the older mine soils. This probably was due to fresh litter input in the
younger mine soils that comprised of more labile organic matter. In case
of Dalbergia sissoo and Gmelina arborea COO-, -C-NO2, C=C and aromatic C-H group
were highest in 25years old reclaimed soil than in 8 years old reclaimed
soil which was in agreement with the results obtained from the UV- vis
spectroscopic methods. SOC molecules became increasingly enriched in
aromatic C species with higher degree of substitution by quinones and
ketonic moieties in older mine soils.
The spectroscopic indices used in this study used to obtain
complementary information about the structure and composition of the
broad classes of functional groups associated with the organic molecules
in the mine soils and gain a holistic view of the overall SOC molecular
characteristics in the chronosequence. Results indicated that the SOC
molecules in the older mine soils were comprised of highly humified
polyaromatic and polycondensed species with higher proportions of
O-containing and N-containing functional groups. Significant
(p<0.05) positive relationships were found between the
spectroscopic index EET: EBz and carbon
stock indicating that humification of SOC molecules probably has
progressed with increase aromatization of the SOC molecules(Table-8).