Impacts of invasive earthworm community composition on carbon cycling in a sandy north temperate forest soil

Background/Question/Methods

Sandy forest soils in the Great Lakes region store ~80 Mg C ha^-1, functioning with vegetation as part of the Northern Hemisphere carbon (C) sink. Our study focuses on the functional diversity of exotic earthworm communities in sandy temperate forest soils and on whether these communities alter the amount and distribution of C as has been shown for finer-textured forest soils. No study to date has examined changes in all major forest soil C budget components (particulate, dissolved, and gaseous) associated with earthworm activity.  The goal of this study was to examine individual and interactive impacts of three earthworm species (Lumbricus terrestris, Aporrectodea trapezoides, and Eisenia foetida) belonging to different functional groups on forest soil C budget components.  We used a 12-month mesocosm experiment with a full factorial design (all combinations of earthworm species). Relationships among earthworm community composition, changes in soil C content and distribution, cumulative CO₂ losses, and dissolved organic C losses were resolved using generalized linear models (GLM) and Kruskal-Wallis tests. Correlational analyses were used to assess relationships between burrow system structure, as determined by X-ray computed tomography, and soil C budget components.

Results/Conclusions

GLM repeated measures analysis showed significant effects of earthworm community treatment over time (p < 0.05). Control mesocosms and mineral soil-dwelling (endogeic) species monocultures showed the lowest CO₂ losses. The highest CO₂ losses were observed in mesocosms with paired surface-dwelling (epigeic) and vertical-burrowing (anecic) species, paired epigeic and endogeic species, and all functional groups combined. Kruskal-Wallis tests showed no significant differences in mean rank distributions of total CO₂-C or DOC loss across treatments. Earthworm community effects on leaf litter mass loss were significant and largely driven by anecic species presence (Kruskal-Wallis, p<0.05). No significant impacts were observed on total soil C mass. However, earthworms drove vertical redistribution of soil C by transferring litter and subsoil C into the A horizon. Also, soil del¹³C signatures showed burrow soils are depleted relative to non-burrow soils, indicative of leaf litter incorporation into forest soils. Lastly, correlational analyses showed no relationships between burrow system structure and soil CO₂ and DOC losses. This is likely explained by the well-drained nature of these soils, where C losses are controlled by production rather than diffusion/infiltration rates. Our results suggest changes in earthworm community composition could have non-additive effects on soil C budget components, and establish fundamental baseline data to compare earthworm community impacts on soil C cycling in north temperate forests.