Field Cultivation of Medicinal Earthworms Increases Soil Large Macroaggregates and Subsurface Organic Carbon Storage

Field cultivation of medicinal earthworms is a distinctive agricultural practice in South China, characterized by large-scale rearing of the anecic earthworm species through substantial organic matter input. However, the effects of varying cultivation durations on soil organic carbon (SOC) distribution across aggregates and soil layers remain unclear. This study compared commercial cultivation plots with adjacent controls at two sites with different cultivation histories: Yangshan (6 months) and Yingde (12 months). Soil samples from three layers (0–20, 20–40, 40–60 cm) were wet-sieved into aggregate fractions for SOC and labile organic carbon (LOC) analysis. Results indicated that earthworm cultivation significantly enhanced the proportion of water-stable large macroaggregates, increased the organic carbon content within them, and elevated the overall SOC storage, particularly in subsurface layers (20–60 cm). The responses of LOC exhibited temporal variation, with a significant reduction observed only at the sites with longer cultivation duration. Overall, cultivation duration modulates the responses of labile carbon pools, whereas field cultivation of medicinal earthworms consistently promotes large macroaggregate formation and their carbon enrichment, increases total SOC stocks, drives subsurface carbon sequestration, and improves aggregate stability. These findings offer a practical strategy for enhancing soil carbon sinks in subtropical red soil ecosystems.