Carbon Content of Mine Soil
Among four different pools of organic carbon, the carbon per cent was highest in the non-labile pool of carbon and it had increased with increase in year of reclamation except in the case of Dalbergia sissoo . Chaudhary et al. study, the average per cent resistant SOC pool increased significantly (p < 0.05) along the chronosequence (Chaudhary et al., 2015). This gave an early indication of the presence of a higher proportion of humic-like substances in the older mine soils. From the result of different pool of carbon, we can conclude that mean non-labile carbon pool was highest under Azadirachta indicia and mean very labile, labile carbon pool was highest under Gmelina arborea.
The recalcitrance index 1 showed variations among the three tree species. RI 1 decreased with increase in year of reclamation suggesting decline in quantity of less labile carbon under Azadirachta indicia and Dalbergia sissoo but increase of less labile carbon pool in case of Gmelina arborea. The recalcitrance index 2 decreased with increase in year of reclamation except in the case ofGmelina arborea . Recalcitrance index 2 was highest in the case ofAzadirachta indica followed by Dalbergia sissoo andGmelina arborea which suggested more stability of SOC in case ofAzadirachta indica .
The decrease in bulk density with increase in year of reclamation was mainly due to root system development, addition of biomass, and improvement in soil structure (Mukhopadhyay et al., 2014., Singh et al., 2015). In all three tree species bulk density was higher in lower layer as compared to surface soil and also the bulk density decreased with increase in age of reclamation. These results indicated that if there is no post reclamation management to restore the loose topsoil, this layer is vulnerable to heavy erosion and exposed the compact subsurface soil layer over time. Bi & Zhang (2014) reported that soil bulk density of surface soil decreased. The bulk density of soil can be used to calculate the total quantities of carbon sequestered at a particular time and soil depth. Correlation analysis (Table-6) showed that the MBC of the reclaimed soils was negatively correlated with the BD(Mg/m3). This indicates high BD decreases microbial activity. The TOC of reclaimed soil was significantly negatively correlated with BD (p< 0.05). Among three tree species decrease in BD was highest under Gmelina arborea .
Tripathi et al. reported that mine soil can act as a significant sink for atmospheric CO2 through revegetation (Tripathi et al.,2013). In the present study, the TOC content was found to gradually increase with the reclamation age. This indicates that in similar pedogenetic conditions, time has a positive effect on the evolution of the TOC in reclaimed soils. Several chronosequence studies have shown that SOC constantly accumulates in reclaimed soils over time, which shows great potential for carbon sequestration (Singh et al., 2015b). The low TOC content of recently reclaimed mine land was mainly due to increase in SOC mineralization, soil erosion due to low soil aggregation and high soil compaction, and leaching of SOC (Shrestha & lal,2006). Maximum accumulation of SOC was observed under Azadirachta indicafollowed by Dalbergia sissoo and Gmelina arboreaindicating high carbon accumulation potential of Azadirachta indica. It also suggests significant accumulation of organic residues has occurred in restored mine soils and presence of a higher proportion of humic-like substances in older mine soils.
According to Shukla & Lal, 2005, reclamation increased soil organic C stock. The lower soil organic C stock for newer reclaimed soils as compared with older sites showed the unfilled C-sink capacity of newer sites. An increase in organic matter usually is accompanied by a reduction of a soils bulk density (Tunstall, 2010; www.eric.com.au). In present study also we found as the age of reclamation increased the BD decreased. Soil organic matter plays a major role in soil development process as it influences other soil properties, such as nutrient accumulation, water-holding capacity, cation exchange capacity (CEC), and microbial activity (Vinduskova & Frouz,2013). Among three tree species, carbon stock was highest under Azadirachta indicia.