Temperature-dependent modulation of soil organic carbon by earthworm (Pheretima guillelmi) bioturbation in a subtropical forest: Topsoil versus Subsoil

Earthworms play a crucial role in pedogenesis through bioturbation, shaping soil structure and driving the sequestration of soil organic carbon (SOC). Although extensive studies have documented earthworm-mediated carbon cycling in croplands and forestlands under optimal temperatures, their impact on carbon dynamics under suboptimal temperatures remains poorly understood, particularly in forest soils. This study conducted a one-year field experiment in a bamboo forest across different temperature phases (warm and cool conditions) to investigate the density-dependent effects of earthworms ( Pheretima guillelmi ) on soil carbon preservation capacity (CPC), quantified as sequestered SOC, in both topsoil (0–15 cm) and subsoil (15–30 cm). Results indicated that earthworm bioturbation significantly increased coarse particulate organic carbon (by 8.5–24.9 %), fine particulate organic carbon (by 11.3–19.7 %), and mineral-associated organic carbon (by 11.9–22.9 %). Under warm conditions, earthworm bioturbation significantly increased the proportion of large macroaggregates (> 2 mm) by 35.1–62.8 % and promoted aggregate stability (mean weight diameter and geometric mean diameter) in the topsoil. However, under cool conditions, there were no significant changes in aggregate in the topsoil and subsoil. Earthworm-mediated CPC exhibited significant temperature dependence: CPC significantly increased by 12.3–25.3 % under warm conditions but only by 6.0–15.9 % under cool conditions. Notably, earthworms enhanced CPC mainly through improving aggregate stability and promoting SOC fractions during warm conditions. In contrast, the efficacy of earthworm bioturbation diminished as temperatures dropped, with CPC variation becoming primarily affected by SOC fractions. This mechanistic understanding establishes earthworm bioturbation as a temperature-sensitive driver of carbon stabilization and provides theoretical insights into the potential of earthworms to enhance soil carbon protection under climatic variability in agroforestry ecosystems.