Exotic earthworm invasion increases soil carbon and nitrogen in an old-growth forest in southern Quebec

To test whether invasion of exotic earthworms affects soil carbon (C) and nitrogen (N), we sampled the litter and upper mineral soil (to 30 cm) at a series of sites varying in their earthworm populations in an old-growth beech–maple forest at Mont St. Hilaire, southern Quebec. We measured earthworm abundance and biomass using hand-sorting and chemical extraction (allyl isothiocyanate) methods. They gave similar results, though there was evidence of size and species-specific biases. Abundance and biomass of the earthworms ranged from <10 to >100 earthworms·m–2 and from <10 to 125 g·m–2, respectively, and were correlated with distance from a nearby lake (negatively) and soil pH (positively). The presence of earthworms was associated with a decrease in the mass and thickness and an increase in the C/N quotient of the litter layer. There were no significant changes in C and N mass of the mineral soil between 0 and 10 cm, but the underlying layers (10–20 and 20–30 cm) in sites with >10 earthworms·m–2 showed significantly (p < 0.05) greater concentrations and masses of both C and N than did sites with <10 earthworms·m–2. The overall profile (litter plus soil to 30 cm) average C was 13.7 and 10.1 kg·m–2 with and without earthworms, respectively, and the equivalent figures for N were 1.01 and 0.68 kg·m–2. These results demonstrate that invasion of earthworms into deciduous forests affects both the litter and mineral soil, and sampling to a depth of 30 cm suggests that earthworm invasion (from <10 to >10 earthworms·m–2) may increase overall C and N.