Objective Plantation forests play a critical role in the global carbon cycle. However, traditional management practices aimed at enhancing timber productivity may lead to soil organic carbon loss, creating a trade-off between vegetation carbon and soil carbon. Harvest residue management is a key measure influencing the carbon sink function of plantations, yet its synergistic effects on above- and below-ground carbon pools remain unclear. This study aims to investigate the long-term effects of two residue management treatments—retention and removal—on vegetation and soil carbon densities in Chinese fir (Cunninghamia lanceolata) plantations, so as to provide a scientific basis for enhancing carbon sequestration through improved silvicultural practices.

Method A randomized block design was adopted, with two treatments—harvest residue retention and removal—each replicated 10 times. After 14 years of treatment (in 2022), tree layer biomass was estimated using tree-by-tree surveys combined with species-specific biomass equations. Shrub, herb, and litter layer biomass were determined using the harvest method. Soil bulk density and carbon content were measured using the cutting ring method and stratified sampling, respectively. All plant and soil samples were processed, and organic carbon content was analyzed using a carbon-nitrogen elemental analyzer. Carbon density of each component was subsequently calculated.

Result Under the residue retention treatment, the total tree layer carbon density (55.85 t/ha) was 5.14% higher than that under the removal treatment (53.12 t/ha), though the difference was not statistically significant. However, residue retention significantly increased tree root carbon density by 37.23% (from 5.64 t/ha to 7.47 t/ha). Carbon densities in the shrub and herb layers also showed increasing trends under residue retention, but not to a significant extent. No significant difference was observed in total soil carbon density (0-60 cm) between the two treatments. However, residue retention significantly increased carbon density in the 0–10 cm surface soil layer by 16.08% (from 22.90 t/ha to 26.57 t/ha), while no significant effects were detected in deeper soil layers.

Conclusion Long-term retention of harvest residues is an effective silvicultural practice for synergistically enhancing both vegetation and soil carbon storage in Chinese fir plantations. This practice markedly promoted carbon accumulation in tree roots and effectively increased organic carbon storage in surface soil, which holds significant practical value for the sustainable management of plantations under the goal of "carbon neutrality".