A laboratory study was conducted in an ongoing field experiment started during Rabi 1988-89 under AICRP on ―Soil Test Crop Response Correlation‖ in calcareous soil at Research Farm of Dr. Rajendra Prasad Central Agricultural University, Pusa, Bihar. The experimental design was split-plot with four fertilizer levels (0, 50, 100 & 150% NPK) in main plots and four levels of manures (no manures, compost @10 t ha-1, crop residues and compost @10 t ha-1 plus crop residues) in sub-plots. The present investigation was undertaken with objectives to investigate the effect of combined use of NPK and different organics (compost and crop residues) on soil organic carbon fractions, mineralization of carbon and carbon management index under rice–wheat-cropping system. Post-harvest surface soil samples after 57th wheat crop were analyzed for pH, EC, organic carbon (OC), total organic carbon, oxidisable organic carbon fractions under gradient of sulfuric acid, mineralizable carbon, potassium permanganate oxidisable carbon (KMNO4- C), soil carbon pools and carbon management index. Application of fertilizers along with compost and crop residues resulted in significant buildup of soil organic carbon fractions. Very labile soil organic carbon was the maximum (3.75 g kg-1) in the treatment receiving both compost (10 t ha-1) and i crop residues and it was 34.30% higher than that in the treatment receiving no manure application. The maximum non-labile soil organic carbon (6.06 g kg-1) was found to be in the treatment receiving both compost and crop residue and was 2.9, 3.41 and 36.48% higher than that in the treatments crop residue, Compost @ 10 t ha-1 and no manure application, respectively. The improvement in non-labile carbon fraction was more than labile carbon fractions suggesting higher carbon sequestration in the soil. The positive and significant correlation among soil carbon fractions viz. VL, L, LL, NL, KMnO4-C, SOC and TOC, suggested that they are in dynamic equilibrium in soil and enhance nutrient availability and uptake by plants. All the levels of fertilizers significantly affected the KMnO4-C content of soil. The maximum KMnO4-C (0.86 g kg-1) was found to be in the treatment receiving both compost and crop residue which was 45.76% higher than that no manure application. Effect of different levels of treatments of NPK and manures as well as their interactions effect on carbon dioxide evolution was significant. Carbon management index increased from 123.46 to 155.68 with increasing in fertilizer levels from No NPK to 150% NPK treatments. The maximum carbon management index (155.68 %), obtained in the treatment receiving 150% NPK, was 4.71, 15.10 and 26.09% higher than the treatments receiving 100%, 50% and No NPK, respectively. The treatment receiving 100% NPK was at par with the treatment receiving 150% NPK. Integrated nutrient management resulting in enhanced carbon management index indicated favorable impact of integrated nutrient management on carbon stabilization in soil.