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).