EXOTIC EARTHWORM INVASION: CONSEQUENCES FOR SOIL BIOGEOCHEMICAL CYCLES AND SOIL MORPHOLOGY IN A MINNESOTA SUGAR MAPLE FOREST

Earthworms, arguably the best known soil bioturbator, influence both the physical and chemical properties of soil. However, the impact of earthworms on soil biogeochemistry is difficult to quantify separately from their role in physically mixing soils. Earthworms also have a widespread distribution across the landscape, making it challenging to assess the influence of specific ecological groups on soil biogeochemical cycling and soil properties. The hardwood forests of the Great Lakes Region have evolved without native earthworms since the Last Glacial Maximum, but are now facing the invasion of exotic earthworms due to fishing, logging, and recreational activities. The Ottertail earthworm invasion transect in northern Minnesota provides an ideal natural laboratory to constrain the biogeochemical and physical changes associated with specific earthworm species and biomasses. We quantified soil total elemental chemistry (Ca, Mg, K, P, C, Fe, Al, Si) applied in a geochemical mass balance model, radioisotope 137-Cs activities, and earthworm population data to understand to what degree different earthworm populations altered soil mixing, and elemental enrichment and depletion of major soil elements. Soil morphology shows the disappearance of the O horizon and expansion of the A horizon after the appearance of endogeic species. Soil total elemental chemistry confirms increased soil mixing by different earthworm populations, but the altered elemental depth profile cannot be explained by mixing alone. Mass balance calculations show significant losses of Ca, Mg and P from the 0-10cm depths in soils heavily invaded by endogeic earthworms. The simultaneous loss of A horizon light fraction organic matter from 25% to 10% in most invaded soils suggests that the consumption and mineralization of soil organic matter may lead to the tenfold decreased enrichment of Ca and similar trends for Mg and P. The loss of base cations in this forest may have negative consequences for sugar maple trees that are largely dependent on soil and litter Ca. Although the weathering supply of these nutrients has been related to tectonically driven erosion of soil minerals, our results show that the initial colonization of bioturbators may have a dramatic influence over the enrichment and depletion of soil elements on a time scale of years.