While invasive European earthworms belonging to the family Lumbricidae have known impacts on the temperate deciduous forests of North America, little is known about earthworms originating from other areas, despite increasingly common reports of thriving populations. Amynthas hilgendorfi, an Asian species with established populations in North American forests, is characterized by an annual life cycle, high metabolic and growth rates, and an ability to thrive at very high densities – traits that distinguish this species from many European earthworms. We hypothesized that the ecological impacts of this species on forest soils also differ, and here we present the results of field and laboratory experiments designed to 1) evaluate the potential impacts of A. hilgendorfi on forest-floor nutrient cycling and leaf-litter decomposition, 2) compare these impacts to those of a better-understood European species, Lumbricus rubellus,and 3) test for interactive effects between these two species. We conducted a replicated manipulative field experiment in which 1.5 m2 enclosures were treated with either A. hilgendorfi, L. rubellus, both species together, or no earthworm additions as a control. We also applied these same treatments to laboratory mesocosms, and in both experiments evaluated soil nitrate, ammonium, and soluble reactive phosphorus (SRP) concentrations, and organic-matter decomposition.
In the field we observed no difference in soil nitrate concentrations across earthworm treatments, while in laboratory mesocosms, nitrate concentrations in soil containing A. hilgendorfi was 43% greater than that mesocosms treated with L. rubellus,and almost double the nitrate concentration of earthworm-free mesocosms. Soil ammonium concentrations were greatest in field plots treated with A. hilgendorfi, but no differences were observed among treatments applied to laboratory mesocosms. A. hilgendorfi increased SRP concentrations relative to soils treated with L. rubellus and controls, in both the lab and the field. L. rubellus was associated with greater organic-matter decomposition rates than A. hilgendorfi in both the field and laboratory experiments. No interactive effects between the two species were apparent for any of the response variables we examined. Overall these results suggest that the effects of A. hilgendorfi on forest ecosystems differ from those of L. rubellus, and for nutrient mineralization in particular, can be more intense. Present knowledge of earthworm invasion ecology in North America is largely based on studies of European lumbricids, which may be inappropriate for understanding the impacts of earthworms of other families from other regions.