Distinct nature of soil organic carbon pools and indices under nineteen years of rice based crop diversification switched over from uncultivated land in eastern plateau region of India

•Changes in depth distribution of SOC was investigated under rice based sequential cropping systems over fallow.•Least change of reactive C stock was observed with increasing depth under D-4 system.•Highest rate of humification and microbial activity were observed in D-4 system.•Highest rate of humification and microbial activity were observed in D-4 system. Crop diversification and their impact on soil C pools and C indices in Eastern plateau region of India is yet to be clearly understood. To explore this, a nineteen years field experiment was carried out to identify suitable diversified cropping system of this region with assessing the SOC pool, C and humification indices. Selected cropping systems were, uncultivated grassland (D-0), rice-fallow (D-1), rice-fallow/rice-okra/rice-maize (D-2), rice-fallow/rice-capsicum/rice-mungbean (D-3) and rice-fallow/rice-capsicum/rice-babycorn/rice-groundnut systems (D-4). Total organic carbon (TOC), various fractions of reactive and recalcitrant C pools, humification indices, dehydrogenase activity and carbon management index (CMI) were determined for 0−15, 15−30 and 30−45 cm soil layers. After 19 years of cultivation, SOC content (g kg−1) of the soil was improved by 22.9 % in D-1 system in 0−15 cm soil depth. Depth dynamics revealed decrement of SOC fractions and stocks with increasing soil depth, although minimum decrement has noticed in D-3 and D-4 systems. D-1 system had the highest very labile C (VLC), labile C (LC) stock, whileless labile C (LLC) and humus C (Hum-C) was found to be highest in D-0. In deeper soil layers (30−45 cm), D-4 system had the highest reactive C pool (RP) stock. Percent deviation of RP-C stock under D-4 system had also the lowest across the 45 cm soil profile. D-2, D-3 and D-4 systems had ∼34, 52 and 40 % higher C lability index (CLI) than D-0 in 0−15, 15−30 and 30−45 cm soil depths, respectively. Besides, D-1 system had similar and ∼ 63 % higher CMI value at 0−15 and 15−30 cm, while, ∼76 % higher CMI value was observed in D-4 system at 30−45 cm in respect to the reference. At lower depth, rate of humification and dehydrogenase activity were enriched in D-4 system. Thus D-4 can be considered as efficient agro-ecosystem to maintain temporal soil fertility and soil health. At regional scale, more diversified rice-based cropping system with legumesappears to be more promising cropping practice that sustains SOC with higher rate of humification and microbial activity.