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An incubation study was carried out to investigate the influence of nitrogen rates to determine optimum C/N ratio under various moisture levels for straw decomposition and sequester carbon (C) in the soil. The aim was to observe straw carbon mineralization through measuring the amount of CO2 evolution. A clay loam topsoil mixed with maize straw was supplied with four nitrogen rates (0.04, 0.08, 0.16, 0.32 g N/kg) using (NH4)2SO4 to adjust C/N ratios at 82, 42, 20, and 10. Soil moisture was maintained at 55%, 70%, 85%, and 100% of field capacity incubated at 25°C for 53 days. The experiment was set up with 16 treatments arranged in complete randomized design. Results showed that mixing of straw with soil increased 50% cumulative CO2-C compared to controls. Averagely, about 44% of added maize straw C was mineralized to CO2-C. Straw addition along with nitrogen and moisture had significant relationships (P < 0.05) to cumulative CO2-C, soil organic C and microbial biomass C. There was a highly significant relationship (R2 = 0.99) between CO2-C emission and incubation time.