Pragmatic categorization of moist woodland ecosystems of the world matching the pattern of earthworm biomass

Predicting changes in global carbon dynamics is urgently required in a climatically changing world. Earthworms, the most powerful ecosystem engineers, play a dominant role in the regulation of soil processes, including carbon sequestration and decomposition. Delineating the regional and global patterns of earthworm biomass is vital to predicting changes in global carbon dynamics. Recent meta-analyses of soil macrofaunal metrics (biomass, abundance, and diversity) attempted to determine the global patterns of earthworm biomass by applying different models using different ecosystem categorizations. The observed or predicted patterns were inconsistent, and therefore inconvenient in regard to their practical use application, such as global carbon modelling and sustainable forest management planning. The standardization of ecosystem categorization is expected to lessen these inconsistencies. This study has aimed to provide an optimal categorization of moist woodland ecosystems that would better match the patterns of earthworm biomass than previous categorizations. To this end, published data on earthworm biomass in moist woodland ecosystems were resynthesized into a default category system, considering tree functional group classification and earthworm ecology as well as conventional climate and forest type classifications. The overall effectiveness of the default categorization was evaluated based on the significance of the variations in earthworm biomass among the categories and then improved in order to better reflect the significance/insignificance of the differences between paired categories. This resulted in the following three climate × two vegetation categories: boreal/cool temperate conifer/beech/oak and mull forming deciduous, warm temperate deciduous broadleaf and evergreen broadleaf, and tropical undisturbed and tree plantation/disturbed secondary. The former vegetation category within each climate category exhibited significantly lower earthworm biomass than the latter. This categorization revealed a reversal in the relationship between litter type (recalcitrant versus labile) and earthworm biomass along the global temperature gradient; that is, higher biomass was found to be associated with labile litter in boreal/cool temperate forests, versus higher biomass with recalcitrant litter in warm temperate forests. A possible explanation for this reversal is that labile litter promotes earthworms through synergistic interactions with microbes in the regions at lower temperatures, but impedes them through competitions with microbes in the regions at higher temperatures. A dual pattern of earthworm biomass in moist woodland ecosystems of the world consisting of recalcitrant litter producing forest series and labile litter-producing forest series was proposed as a novel basis for global carbon modelling and also sustainable forest management planning.