Earthworms were subjected to four food source treatments using a subset of the litter types above (no litter control, sugar maple, basswood, and hemlock) under the same environmental conditions, on five different soil treatments (Table 1). Mesocosm containers were filled with equal parts A horizon and B horizon (15 cm each) from each of the four collection sites. Soils were chosen for their current presence or absence of earthworms, with two soils collected from each (Table 1). For the two uninvaded soils the Oa and E horizons were mixed to create an A for comparison with the invaded soils. To test for the effect of A horizon material on growth, the fifth soil type (Un1-no A) was created by using 30 cm of B horizon soil from the uninvaded Keweenaw–Kalkaska complex (Un1) (Table 1). By removing the A horizon we can control for other chemical or physical variables that might influence earthworm growth among soils, allowing insights into growth impacts of different horizons. Although the primary food source for L. terrestris is fresh litter, forest A horizons are typically more organic rich than B horizons, and hence could be used as a potential supplementary food source for L. terrestris, either on its own or co-metabolized with fresh litter. Additionally, A-horizon could have non-nutritional effects on earthworms, e.g. by buffering availability of metals in the soil. Each soil-type by litter-type combination was replicated three times.