Carbon Content of Mine Soil
Distribution of organic carbon (%), very labile pool of carbon was highest in the case of 25years of reclamation except in the case ofGmelina arborea (Table-2). It had increased non-significantly from 3.57 to 4.59%, 5.32 to 6.24% in Azadirachta indica andDalbergia sissoo respectively but it had decreased significantly in the case of Gmelina arborea with the increase in year of reclamation from 17.09 to 7.51%. In case of labile carbon pool, it had increased non significantly from 3.06 to 3.44% and significantly from 3.08 to 8.10% in Azadirachta indica and Dalbergia sissoorespectively but in case of Gmelina arborea it had decreased significantly from 12.39 to 5.34% with the increase in year of reclamation in surface soil. In the case of less labile pool of carbon, it had increased from 8.41 to 15.24% and 6.00 to 28.72% inAzadirachta indica and Dalbergia sissoo but in case ofGmelina arborea with increase in year of reclamation in surface layer, it had decreased from 17.18 to 11.10%. In case of non-labile carbon pool, there was an increase under Azadirachta indica andGmelina arborea except for Dalbergia sissoo, it had decreased significantly with the increase in year of reclamation. 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 an increase in year of reclamation except in the case of Dalbergia sissoo .
The recalcitrance index serves as an indicator of stable carbon in the soil (Datta et al., 2017,2018). The recalcitrance index 1 (RI1) was in the order of Azadirachta indica >Gmelina arborea > Dalbergia sissoo at 0–15cm soil depth (Table-3). The recalcitrance index 2 (RI2) was in the order of Azadirachta indica > Dalbergia sissoo> Gmelina arborea at 0–15cm soil depth. There was a significant difference in bulk density(BD) for all species. BD decreased with an increase in year of reclamation from 0-25years (Figure 2). BD increased with increase in depth. BD was improved from 1.64 % to 1.40%, 1.33% to 1.22% and 1.49% to 1.46% over the year of reclamation in 0-15 cm depth(Table-5).
An increase in organic carbon content of the soil was found along the re-vegetation age gradient. An increase in organic carbon content of soil by two times occurred along the re-vegetation age gradient of 0 to 25years (Figure 3). Significant difference in TOC was observed in surface soil between 25 and 8years of reclamation. With increasing depth from 0–15 to 30-45cm, there was a decline in soil organic carbon content of 76%, 79% and 59% in Azadirachta indica ,Dalbergia sissoo, and Gmelina arborea tree, respectively in 25yeras of reclamation (Figure 3). Soil total organic carbon (SOC) was 2.16,1.67 and 6.96 times higher in surface soil of the oldest reclaimed site than the young reclaimed mines under Azadirachta indica, Dalbergia sissoo and Gmelina arborea respectively (Table 5); maximum accumulation of SOC was observed under Azadirachta indica followed by Dalbergia sissoo and Gmelina arborea . TOC content was higher in surface than subsurface soil which was probably due to leaf litterfall and its conversion to humus in the surface soils.
Total soil C stock in restored mine soil under different tree plantation was given in Table-4 and 5. Mean total C stock was estimated as 334.72,226.94 and 191.20 Mg C ha-1, inAzadirachta indica , Dalbergia sissoo, and Gmelina arborea plantation respectively. Carbon stock of the soil increased with increase in year of reclamation.