Long-term effects of organic and inorganic fertilizers on the carbon management index in terrace soils

Carbon management index (CMI) provides a sensitive indicator of soil quality changes under different management practices. This study assessed the long-term (32-year) impacts of organic and inorganic fertilizers on CMI and soil carbon (C) fractions in terrace soils of Bangladesh under a rice–wheat cropping system. Treatments included cow dung, compost, green manure, rice straw, and a no-manure control, each combined with nitrogen (N) fertilizer and applied at variable rates to provide approximately equal C inputs (~ 0.5 t C ha –1 year –1 ) across treatments. Soil samples were collected after rice harvest in 2021 from three depths (0–20, 20–40, and 40–60 cm) and analysed for soil physicochemical properties, C fractions, and C indices including total C, total N, labile C, and non-labile C. Organic amendments were associated with higher soil C and N pools, with green manure showing the greatest CMI (147) followed by compost (137), rice straw (131), cow dung (129), and no-manure (105). In contrast, mineral N fertilizer alone was associated with comparatively smaller changes in CMI, suggesting a stronger contribution of organic inputs to soil C dynamics. Although C contents decreased with depth, CMI values varied less markedly, indicating potential persistence of management effect below the surface layer. Pearson correlation and principal component analyses revealed strong positive associations among labile C, non-labile C, total C, and soil N, suggesting coordinated responses of active and stable C pools under organic amendments. Overall, the results indicate that long-term organic inputs, particularly green manure, are closely linked with improvements in C fractions and CMI, while mineral N fertilizer showed more limited associations with C quality indicators. These findings underscore the potential importance of sustained organic inputs, together with balanced N fertilization, for improving soil C status and supporting soil health in degraded terrace soils. Incorporating organic amendments into soil management policies and using CMI as a monitoring tool may support efforts toward sustainable land management and relevant Sustainable Development Goals.