Differential Influence of Soil Organic Carbon and Calcium on the Community of Lumbricid Earthworms as Ecosystem Engineers in Cool Temperate Forests of Hokkaido

Earthworms are important ecosystem engineers that stabilize soil aggregates and increase the size of aggregates. The major determinants for the biomass, density, and mean individual weight of lumbricid earthworms are considered to be the availability of soil organic carbon (SOC) and calcium (Ca). However, the importance of SOC and Ca availability for lumbricid earthworms has not been tested simultaneously in the field. In addition, the ripple effects of SOC and Ca on the soil aggregate stability and relative abundance of larger aggregates (mean weight diameter; MWD) via earthworm communities are poorly understood. To fill these knowledge gaps, we conducted field research across a spatially wide range of cool temperate forests, where soils were classified as Cambisols and Andosols and evaluated the relationships among the biomass, density, and mean individual weight of lumbricid earthworms, soil chemical properties, and MWD by conducting structural equation modelling (SEM). Our results showed that SOC affected not the density but the mean individual weight of earthworms. On the other hand, interestingly, exchangeable Ca affected not the mean individual weight but the density of earthworms. These results suggest that SOC stimulates earthworm growth and that exchangeable. Ca enhances the fertility rate and/or survival rate in earthworm communities in forest ecosystems. Furthermore, SOC had a positive ripple effect on the MWD via an increase in the mean individual weight of earthworms. These results suggest that larger earthworms produce larger casts, which bind a larger amount of soil particles thus making larger aggregates.