Objective Monitoring and evaluating carbon sink in small watersheds of the purple soil region in southwestern Hunan Province provides techniques and methods for assessing the carbon sink capacity of soil and water conservation projects, and offers a reference for carbon sink research in soil and water conservation in the purple soil region of southwestern Hunan.

Methods Through field sampling of 30 plots, soil bulk density and organic carbon content were measured, and the diameter at breast height of each tree was recorded. The vegetation carbon pool, soil carbon pool, and soil and carbon retention in the small watershed from 2004 to 2024 were analyzed and quantified.

Results 1) After 20 years of comprehensive management, the average annual increase in carbon storage in the Huangniqiao and Sipingxi small watersheds of Cili county was 4.46 × 10³ t/a. 2) Compared with the baseline scenario, soil carbon storage under the project scenario increased by 11.89%, and vegetation carbon storage increased by 54 078.62%. The soil and carbon retention amount in the project area reached 52.54 t. 3) Different soil and water conservation measures, including exclosure management, afforestation of soil and water conservation forests, and slope-to-terrace projects, varied in their capacity to enhance carbon sink. Specifically, exclosure management of Phyllostachys edulis and Bambusa spp. stands demonstrated the most significant enhancement in soil carbon sink. Meanwhile, afforestation of soil and water conservation forests with P. elliottii had the highest vegetation carbon sink density. The slope-to-terrace project was of great significance for soil protection and erosion reduction.

Conclusions Various soil and water conservation measures demonstrate significant effects in carbon preservation, carbon sequestration, and carbon sink enhancement. Among them, afforested areas with species such as P. elliottii, Pinus massoniana, and Cunninghamia lanceolata effectively improve the overall benefits of comprehensive soil and water conservation management.