<p>The addition of composted sludge to soil is an effective measure for replenishing soil organic matter and improving soil nutrient status. However, the effects of composted sludge addition on the mineralization of soil organic carbon (C) and nitrogen (N) and their correlations have rarely been studied, limiting the scientific application of composted sludge in degraded soil systems. In this study, composted sludge was added to degrade grassland soil at mass ratios of 0, 2, 5, 10, 15, and 20% and incubated at 5, 15, 25, and 35 ℃ for 50 days. Results: Soil cumulative C mineralization and the potentially mineralized C content significantly increased with increasing sludge dose. The soil cumulative N mineralization and potentially mineralized N content first significantly increased and then decreased with increasing sludge dose, with the highest values observed in the 5% addition treatment. Sludge addition significantly decreased the temperature sensitivity (Q₁₀) values of soil C mineralization and increased those of soil N mineralization. Soil C and N mineralization were significantly correlated throughout the incubation period. The negative correlation between the activation energies of soil C and N mineralization and the soil C and N quality indices supported the applicability of the C quality–temperature hypothesis to C and N mineralization in composted sludge-amended soil. Conclusions: The correlation between soil C and N mineralization is influenced by the sludge addition dose and incubation temperature. The addition of 5% composted sludge is recommended to reduce soil C emissions and increase the available N nutrient supply.</p>