Rhizosphere soil indicators for carbon sequestration in a reclaimed coal mine spoil

•Rhizosphere process influences carbon density of tree species in mine spoil.•Rhizosphere effects of trees in mine spoil were explored to develop carbon index.•Carbon accumulation index based on rhizosphere effects has been developed.•Rhizosphere C, microbial biomass, and N are indicators for carbon sequestration.

Re-vegetation of mine spoil enhances carbon storage in both above-ground plant biomass and mine soil. The current study was conducted at the coalmine overburden dumps of Jharia Coalfield (India), with the aim to evaluate the effect of different tree species on the rhizosphere soil properties and to identify key rhizosphere soil indicators that influence tree biomass and carbon density. Rhizosphere soil samples were collected from five tree species (Acacia auriculiformis, Albizia lebbeck, Cassia siamea, Delonix regia, and Dalbergia sissoo) of the same age. An area without ground vegetation was selected as a non-rhizosphere soil. The carbon density was higher for D. sissoo and A. auriculiformis (39.6-43.7 kg C/tree) and lowest for A. lebbeck (20.7 kg C/tree). Except for C. siamea (4.38%), the total C (TC) content was lower in the rhizosphere than the non-rhizosphere soil. About 50% reduction in TC was observed for A. auriculiformis and A. lebbeck and 75% for D. sissoo. Labile C and microbial biomass carbon (MBC) were significantly higher in the rhizosphere than the non-rhizosphere soil. Dehydrogenase enzyme activity was higher in all the rhizosphere soils with the maximum activity under C. siamea (88.48 μg/TPF/g/24 h) and D. sissoo (71.95 μg/TPF/g/24 h). Three types of carbon accumulation indices (CAI) were calculated: CAI-1, based on TC and labile C; CAI-2, TC, and MBC; and CAI-3, TC, labile C, and MBC. CAIs depending on rhizosphere effect were generally higher for D. sissoo and C. siamea. Principal component analysis showed that the tree carbon density is closely associated with CAI-3, CAI-2, carbon lability index (CLI), available N, and MBC. Thus, an integrated rhizosphere carbon accumulation index (CAI-3, based on rhizosphere effects) and N could be considered as indicators for carbon sequestration in reclaimed mine spoils.