Soil microaggregate bacterial communities following Amynthas tokioensis and Amynthas agrestis earthworm co-invasion

Earthworms are soil ecosystem engineers due to burrowing, consumption, and casting behaviors that restructure the soil environment. Though non-native European Lumbricid earthworms are well-studied in North American soils, the Asian pheretimoid Amynthas tokioensis and Amynthas agrestis earthworms exhibit distinct ecological patterns that threaten invaded habitats. Our goals were to determine the effects of the co-invasion by A. tokioensis and A. agrestis in woodlands in Madison, WI, U.S. on the soil bacterial community and edaphic characteristics including soil carbon, nitrogen, and aggregation. Using 16S rRNA gene sequencing, we found that the presence and activity of these Amynthas species earthworms affected bacterial community composition. At one site, there was a decrease in sample-to-sample dissimilarity (i.e., decreased beta diversity), with concomitant increases in homogenizing community assembly processes, however, at another site, we found opposite trends with evidence for increased compositional dissimilarity between samples and decreased evidence for homogenizing community assembly processes. Overall, inconclusive support for the hypothesized homogenization of bacterial community composition driven by homogenizing community assembly processes indicates that the effects of Amynthas pressure in these systems represent a departure from previously established soil disturbance paradigms. Instead, we conclude that aggregate formation via A. tokioensis and A. agrestis casting activity does not consistently impose a strong selective filter on soil bacterial communities, nor does the heightened earthworm activity necessarily act to homogenize soil communities via dispersal. Overall increases in soil C and N under Amynthas spp. activity support previous works that indicate enhanced decomposition and incorporation of soil litter, but future work could focus on the long-term fate of microaggregate-protected C.