Effects of Differing Exotic Earthworm Assemblages on Soil Microbial Biomass, Carbon Substrate Utilization, and Microbial Community Composition

Abstract

Four European exotic earthworm fronts, two in Minnesota and two in Wisconsin, were used to examine the effect of earthworm guild assemblages (treatments) on the composition of the bacterial and fungal communities. Four transects, each containing 5 treatments based on ecological guild dominance (epigeic, endogeic, anecic, anecic 50 m, anecic 100 m) were constructed perpendicular to four advancing exotic earthworm fronts. The activity of each dominating guild was confirmed by a continual decrease in the O horizon depth and subsequent increase in A horizon depth. Soil samples collected in the fall of 2013 were used to examine the effect of guild assemblages on the soil microbial community. No biological differences were observed in microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), MBC: MBN, and carbon substrate utilization using Biolog Eco- and FF plates. TRFLP community profiles indicated the bacterial communities among treatments were highly similar, but a partial shift in the fungal community composition was observed between epigeic and anecic dominated treatments. I suspect the partial shift occurred from the previous and continued activities of epigeic and endogeic worms, and an established anecic worm population; although microbial community function may have been retained. No differences were observed in Total % C, Total % N, or C: N between transects, suggesting earthworm assimilation of O horizon did not substantially redistribute C and N into the A horizon. The study suggests the alteration of fungal food sources in the O horizon by exotic earthworm feeding and the disruption of the A horizon structure by exotic earthworm burrowing and casting alters fungal community composition. Further studies are required to evaluate what fungal phyla are most affected by exotic earthworm activity.