Field experiment reveals varied earthworm densities boost soil organic carbon more than they increase carbon dioxide emissions

Abstract

Earthworms play crucial roles in regulating soil organic carbon (SOC) and greenhouse gas emissions in forest soils. Laboratory studies have proven that they 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 impact of different earthworm densities remain unclear. In this study, we investigated how earthworm (Pheretima guillelmi) activity at three densities (no earthworms, original density, and double the original density) affected SOC, its labile fractions, C-related enzyme activities, and soil CO₂ emissions in a Moso bamboo (Phyllostachys edulis) forest ecosystem over a six-month field experiment. Our results showed that the original earthworm density resulted in a 23.4 % increase in soil CO₂ emissions, while double the original earthworm density resulted in a 9.6 % reduction in emissions compared to the original density. Additionally, earthworms at both densities significantly increased the concentration of SOC and its labile fractions. Notably, the increase in SOC density (SOCD, SOC stock per unit land area, kg C ha⁻¹) induced by earthworm activity far exceeded the increase in CO₂-C emissions. Specifically, the earthworms at double the original density led to an 8.8-fold increase in SOCD, while the original density resulted in a 3.7-fold increase. Furthermore, our findings identified dissolved organic C (DOC) as the most critical labile organic C fraction influencing soil CO₂ emissions associated with earthworm activity, while β-glucosidase (BG) was the most significant C-related enzyme affecting soil CO₂ emissions driven by earthworm activity. These results provide important insights into the role of earthworms in both CO₂ emissions and SOC accumulation in subtropical forests.