Earthworms play crucial roles in the regulation of soil organic carbon (SOC) and greenhouse gas emissions within forest soils. In laboratory studies, they have been proven to promote soil carbon dioxide (CO2) emissions. However, the effects of earthworm activity on forest soil CO2 emissions and organic carbon (C) have not yet been quantified in situ, and the impacts of different densities remain unclear. We investigated how earthworm (Pheretima guillelmi) activity of different populations (one- and double-fold numbers) influenced SOC and its labile fractions, C-related enzymes activities and soil CO2 emissions in a Moso bamboo forest ecosystem over a six-month field experiment. Results revealed that a one-fold increase in earthworm density resulted in a 23.4% increase in soil CO2 emissions, while double-fold of earthworm density resulted in a 9.6% reduction compared to the former. Additionally, SOC and its various labile fractions increased significantly, and the augmentation of SOC density (SOCD, SOC stock per land area, kg C ha-1) induced by earthworm activity substantially exceeds (>6 times) the earthworm-induced increase in CO2-C emissions. Furthermore, our results identified dissolved organic C (DOC) as the most critical labile organic C influencing soil CO2 emissions associated with earthworm activity, while β-glucosidase (BG) was identified as the most pivotal C-related enzyme affecting soil CO2 emissions driven by earthworm activity. These findings are pivotal for understanding the role of soil fauna in CO2 emissions and SOC increase in subtropical forests.

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