Synergism is not the primary aspect of microbe-earthworm interactions in warm temperate forest soil food webs

Background and Aims Understanding carbon dynamics in forest ecosystems—driven primarily by trees, soil microbes, and earthworms— is indispensable in developing ecological strategies to mitigate global warming. A recent systematic review revealed that the effects of tree functional types on earthworms differ between boreal/cool-temperate and warm-temperate forests. The hypothesized driving force of this difference is a phase shift in microbe-earthworm interactions between synergy and competition along the global temperature gradient. This study aims to empirically verify this hypothesis. Methods In a warm temperate region, the six components of decomposer food webs, annual litterfall mass, fine root mass, forest floor turnover rate, soil organic matter content, CO ₂ emission rate from the soil, and endogeic earthworm biomass were assessed in each four stands of two forest types—labile litter-producing deciduous and recalcitrant litter-producing evergreen broadleaf forests. Results The two forest types were distinguished without misclassification using a significant linear combination of the six variables of interest. However, regarding the latter three variables (soil, microbes, and earthworms related-variables), the magnitude relationships between the forest types were opposite to those commonly observed in boreal and cool temperate regions. Conclusion The reversal in the relationship between tree functional types and decomposer sub-systems between boreal/cool temperate and warm temperate forests empirically supports the hypothesis that could provide a novel basis for global carbon modelling. Therefore, the dual nature of microbe-earthworm interactions (synergistic versus competitive) and the effects of temperature on the relative importance of the two aspects merit future experimental studies.