Abstract: Soil organic carbon (SOC) is closely related to forest land maintenance and carbon sink function. Small fluctuations in soil SOC can cause significant changes in atmospheric CO2 concentrations. As a main management measure of plantation, how and why litter management affects the chemical structure and stability of SOC remains unclear. In this study, a field control experiment was conducted in a Chinese fir (Cunninghamia lanceolata) forest with three treatments, namely, litter addition, litter removal and control, following a completely randomized block design. After 6 years of treatment, soil samples from three layers (0-10 cm, 10-20 cm, and 20-40 cm) were collected, and the chemical structure of SOC was determined by transmission Fourier-transform infrared spectroscopy (T-FTIR). The thermal stability of SOC was analyzed by thermogravimetry (TG) and differential scanning calorimetry (DSC), and the soil physicochemical properties were determined. The results showed that (1) Litter removal significantly decreased NH₄⁺-N concentration in the 0-10 and 20-40 cm layers and NO₃^--N concentration in the 10-20 cm layer; (2) Litter addition significantly decreased the relative proportion of alcohols and phenols in SOC and increased the relative proportion of aromatics; (3) Litter addition significantly decreased the thermal stability coefficient (H), and litter removal treatment significantly increased the temperature corresponding to 50% mass loss during SOC combustion (Tg-T50); (4) The NH₄⁺-N and NO₃^--N concentrations were significantly negatively correlated with Tg-T₅₀. Alcohol phenols and aromatic groups were positively and negatively correlated with H, respectively. In summary, litter removal improved the thermal stability of SOC by decreasing N effectiveness, while litter addition improved thermal stability of SOC by decreasing the readily decomposable carbon and increasing the formation of refractory carbon in the chemical molecular structure of SOC. Our results can provide a scientific basis for subtropical plantation to achieve “carbon neutrality” by improving soil carbon sink function through litter management.

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