Status assessment of biological soil health indicators in surface and subsurface soils of farmers’ sugarcane fields

Soil biological health is central to the sustainability of sugarcane-based agroecosystems, yet depth-wise variation in biological soil quality indicators under farmer-managed conditions remains insufficiently characterized. This study assessed the surface-to-subsurface distribution of biological indicators and carbon pools in sugarcane fields of Samastipur district, Bihar, India. Soil samples were collected from 38 farmer-managed sugarcane fields at two depths, 0–15 and 15–30 cm. Soil protein, soil respiration, dehydrogenase activity, microbial biomass carbon, organic carbon, KMnO₄-oxidizable carbon, and carbon fractions of different lability were analyzed. The biological indicators were further integrated into a normalized biological soil quality index (BSQI), and depth-wise patterns were evaluated using paired distribution plots, block-wise violin plots, percentage-change analysis, PCA, correlation matrix, OC-based regressions, and stratification ratios. Biological indicators declined consistently with soil depth. Soil protein, soil respiration, dehydrogenase activity, and microbial biomass carbon decreased by 24.8, 18.5, 26.1, and 15.2%, respectively, in the 15–30 cm layer compared with the 0–15 cm layer. The integrated BSQI also showed a highly significant depth effect, confirming lower biological functioning in subsurface soil. Active carbon fractions were more depth-sensitive than total organic carbon and passive carbon pools. Organic carbon showed strong positive relationships with biological indicators in the surface layer, whereas these relationships were weak and non-significant in the subsurface layer. PCA and stratification-ratio analysis further confirmed that surface soil was more closely associated with microbial activity and active carbon pools, while subsurface soil showed weaker biological expression. The study demonstrates that biological soil quality in farmer-managed sugarcane systems is strongly depth-dependent and should be assessed using biological indicators, active carbon fractions, and integrated indices rather than relying only on surface soil or total organic carbon. Biological soil quality indicators declined markedly from 0–15 cm to 15–30 cm soil depth. Dehydrogenase activity, soil protein, soil respiration, and microbial biomass carbon were more depth-sensitive than total or passive carbon pools. Surface organic carbon strongly predicted biological indicators, whereas subsurface organic carbon showed weak biological coupling. Integrated BSQI and stratification ratio confirmed stronger biological functioning in surface sugarcane soils.