Low substrate concentration intensifies the earthworm-driven increase in temperature sensitivity of SOM decomposition

Abstract Background and aims Substrate concentration is the central driver of temperature sensitivity (Q₁₀) in soil organic matter (SOM) decomposition. While microbial processes underlying this relationship have been extensively investigated, the role of soil fauna, particularly earthworms, in shaping temperature response remains largely unexplored. Methods In this study, we examined how earthworms and their deposits (excreta and secretions) influence SOM decomposition rate (Rs) and Q₁₀ across substrate concentration gradients. We combined stratified sampling across a 1-m soil profile (representing natural variation in substrate concentration gradients) with controlled incubations simulating natural diurnal temperature fluctuations. Using a high-frequency (CO₂ concentration captured at one-second intervals) automated system, we quantified how earthworms and their deposits altered Rs and Q₁₀. Results The results showed that earthworm deposits alone modestly increased Rs by 22–56% and Q₁₀ by 4–14%, especially in surface soils with high substrate concentrations. In contrast, earthworm continued presence magnified these responses substantially, with Rs increasing by 194–277% and Q₁₀ by 26–63%, the greatest increase in Q₁₀ occurred under low substrate concentrations. Furthermore, earthworm activity intensified the dependence of Q₁₀ on both substrate concentration and microbial activity; however, earthworm endogenous Q₁₀ remained stable across conditions. Conclusion Together, these findings reveal that earthworms act as potent regulators of Q₁₀ particularly under low substrate concentrations. This study highlights the critical role of earthworms in mediating soil carbon (C) cycling processes under varying substrate availabilities and offers novel insights into the biological controls on soil C–climate feedbacks.