Exogenous nitrogen (N) input was the key factor driving vegetation restoration and soil carbon (C) sequestration in Karst grassland ecosystems, which were undergoing N limitation. Soil organic carbon (SOC) mineralization was an important pathway for soil C output. However, most studies focused on the effects of N addition on SOC mineralization, while less attention was paid to the impacts of nitrogen-fixing plants. Thus, the effects of N input types, including N addition and introduction of nitrogen-fixing plants (Indigofera atropurpurea and Amorpha fruticosa), and N input amount (control, low N, and high N) on SOC mineralization and Q₁₀ were investigated in a four-years experiment in a karst natural restoration grassland. The results showed that (1) Exogenous nitrogen input significantly increased soil ammonium nitrogen and available nitrogen contents, with maximum increases of 245.2% and 152.3%, respectively. Compared with low-density introduction of nitrogen-fixing plants, high-density introduction showed a decreasing trend in soil available phosphorus content. (2) N input amount significantly affected SOC mineralization, and low N input significantly increased SOC mineralization, especially in N addition and introduction of Indigofera atropurpurea, with increases of 14.4% and 21.5%, respectively. However, high N input trended to reduce SOC mineralization. N input reduced Q₁₀, and the effect of low N input on reducing Q₁₀ was more obvious than that of high N input. Especially for Indigofera atropurpurea, low-density planting significantly reduced by 19.0% compared to high-density planting. Redundancy analysis results showed that soil physicochemical properties explained 45.3% of the changes in SOC mineralization characteristics, with ammonium nitrogen and available phosphorus accounting for 15.4% and 18.8%, respectively. Soil available phosphorus was a major factor influencing SOC mineralization. This might be related to the phosphorus consumption during the biological N fixation by nitrogen-fixing plants. Soil N availability was a major factor influencing Q₁₀, which might be related to the enhanced stability of SOC under increased N availability. (3) The form of N input had no significant effects on SOC mineralization and Q₁₀. The effects of N addition and the introduction of Indigofera atropurpurea were similar, whereas Amorpha fruticosa showed a weaker impact. In summary, N input amount was more important for affecting SOC mineralization in the early stage of exogenous N input in Karst grassland ecosystems. Future increases in N deposition and the introduction of nitrogen-fixing plants were found to favor SOC retention in Karst grasslands.Additionally, Indigofera atropurpurea was identified as a preferred model for vegetation restoration in Karst grassland ecosystems.