Influence of biochar and vermicompost on labile and stable carbon pools in degraded jhum soils of northeast India

Degraded jhum (shifting cultivation) soils are characterized by rapid losses of soil organic carbon (SOC), yet limited information is available on how organic amendments regulate SOC fractions and carbon stability. A two-year field experiment was conducted under soybean cultivation in northeast India to evaluate the effects of biochar, vermicompost and their combined application on total SOC and its labile and recalcitrant fractions. The experiment followed a randomized block design with eight treatments, including sole and combined applications of amendments. Compared with the control, the combined application of biochar and vermicompost increased total SOC by 2.33–3.86%, with a concurrent increase of 0.78–0.94% in less labile and non-labile carbon fractions, indicating enhanced carbon stabilization. Vermicompost alone primarily increased very labile and labile SOC pools, reflecting enhanced microbial activity and short-term carbon turnover. In contrast, biochar promoted the accumulation of recalcitrant carbon through sorption, aggregation, and physical protection mechanisms. Depth-wise analysis showed that SOC enrichment was more pronounced in surface soils layer (0–15 cm) than in the subsoil (15–39 cm). Overall, the results demonstrate that integrating biochar with nutrient-rich organic amendments facilitates a shift from active to more stable SOC pools, thereby improving carbon persistence in degraded jhum soils. This study highlights the potential of combined amendment strategies for enhancing soil carbon stability and supporting sustainable soil management in shifting cultivation systems.