Assistant Professor Williams College, United States
As powerful ecosystem engineers, introduced earthworms can dramatically change plant community composition and soil carbon (C) cycling in northeastern North America. While earthworm introductions are associated with accelerated soil C cycles and loss of stored organic matter, their presence can also promote soil aggregate formation, providing alternative mechanisms for soil C stabilization. As global soils store nearly 80% of terrestrial C , understanding how introduced invertebrates influence soil C cycling is critical for understanding soil C feedbacks to ongoing global change. We evaluated earthworm community composition, soil chemistry, and soil C cycling at Hopkins Memorial Forest (Williams College; Northwestern Massachusetts; 42.7235oN/73.2227oW) in 48 plots, evenly divided between two sites of differing elevation (high elevation, 700 m; low elevation, 250 m), soil pH, and mineral parent material. We investigated how earthworm populations varied with soil C content and chemistry and quantified the contribution of invasive Amynthas agrestis earthworms to cross-site variation in mass-adjusted soil respiration. We then compared laboratory-based microbial respiration rates across soils with varying earthworm density to identify the relationship between soil C loss rate and earthworm presence.
We identified the novel presence of highly invasive Amynthas agrestis (Megascolecidae) earthworms in Berkshire County, MA, which are especially associated with dramatic soil litter and organic horizon loss within introduced ranges. We found substantial nonnative earthworm populations, including highly invasive A. agrestis, at the 200m elevation site, which contains marble-derived bedrock, circumneutral pH, and limited soil organic horizon. In contrast, few earthworms and no A. agrestis individuals were present at the higher elevation site, which contains phyllite-derived bedrock, acidic soils, and high soil organic matter content. Relative to the higher elevation site, the 200m site contained greater Ca2+, Mg, and Na cation concentration but lower K cation concentration. Under constant laboratory conditions, soils with high earthworm density experienced more rapid microbial consumption of soil C (p = 0.0009). This analysis suggests that the distribution of earthworm populations at Hopkins Memorial Forest is highly dependent on background soil chemistry, but earthworm establishment disrupts soil physical protection and facilitates faster C loss. Next steps in this investigation include conducting lab incubations of A. agrestis earthworms to measure their respiratory contribution to soil CO2 fluxes directly.