Abstract: CO₂ released into the atmosphere through soil respiration represents the second-largest carbon flux between terrestrial ecosystems and the atmosphere. While extensive research has concentrated on surface soils, limited studies have explored CO₂ emission patterns and their primary drivers across varying soil depths. In this study, soil CO₂ emissions were measured using static chambers at six different depths (10 cm, 50 cm, 100 cm, 200 cm, 300 cm, and 400 cm) in a primary forest. Additionally, potential influencing factors, including soil physical and chemical properties, microbial diversity, and community structure and function, were assessed. The results demonstrated that soil nutrients, along with fungal and bacterial diversity, generally declined with increasing soil depth. Soil CO₂ emissions also decreased significantly with depth, driven primarily by biotic factors like fungal and bacterial alpha diversity and abiotic factors such as ammonium nitrogen and available phosphorus. These findings provide new insights into the mechanisms of carbon cycling within deep soil layers in forest ecosystems.

Key words: Subtropical forest, CO₂ emission, Soil depth, Soil properties, Microbial diversity