The effects of different forest types on the soil organic carbon pool and enzyme activity characteristics in sandy plain areas

The ecosystem in sandy plain areas is fragile, and enhancing the soil organic carbon pool is crucial for improving ecosystem functions and carbon sequestration. This study investigated sandy soils in a plain area of Zhejiang Province, examining four forest types: pure poplar forest, pure willow forest, mixed poplar-willow forest, and abandoned land (control). By collecting soil samples from 0–100 cm, the study systematically analyzed soil organic carbon content, active carbon components (easily oxidizable organic carbon, dissolved organic carbon, and microbial biomass carbon), and soil enzyme activities (invertase, polyphenol oxidase, β-glucosidase, and peroxidase), as well as their interrelationships. Results showed that different forest types significantly influenced soil organic carbon content and stock. Compared to abandoned land, afforestation significantly increased soil organic carbon stock, with the highest levels observed in pure poplar forests, where soil organic carbon content in the 0–100 cm layer increased by 45.4%–82.6%, and stock increased by 1.2–1.9 times. Active carbon components and soil enzyme activities varied significantly among forest types, with mixed poplar-willow forests showing the greatest increase in active carbon components, while enzyme activities in pure poplar and mixed poplar-willow forests were significantly higher than in abandoned land. Correlation analysis indicated that polyphenol oxidase and β-glucosidase played important roles in the formation of active carbon components, whereas invertase and peroxidase were key to organic carbon accumulation and stock. Redundancy analysis revealed that soil carbon-to-nitrogen ratio and fine root biomass were the primary driving factors of forest-type effects on soil organic carbon pool characteristics. This study provides an in-depth understanding of the mechanisms by which different forest types affect the soil organic carbon pool and active components in sandy plain areas, offering practical insights for ecosystem restoration and carbon sequestration.