Volumetric change and losses of nutrient elements along an earthworm invasion chronosequence in a northern hardwood forest in Minnesota

Background/Question/Methods

Earthworms are arguably the best known soil bioturbator, yet their impacts on soil biogeochemistry are difficult to quantify separately from their role in physically mixing soils. In glaciated regions of North America, forests have evolved without native earthworms since the last glacial retreat. However, earthworms have invaded these forests due to agricultural expansion, fishing, recreational, and logging activities. Earthworm consumption of the organic horizon in Minnesota hardwood forests has resulted in dramatically changing forest floor ecology and soil morphology. The earthworm invasion chronosequence in Northern Minnesota provides an ideal outdoor laboratory to quantify the interactions between biogeochemical and physical processes associated with different earthworm species and biomasses. We combined total elemental chemistry (Ca, Mg, K, P, C, Fe, Al, Si) with a geochemical mass balance model, in conjunction with radioisotope 137-Cs activities, to calculate element specific mixing and leaching rates. These estimates are further discussed with quantitative XRD, selective elemental extractions (Fe, Al), exchangeable chemistry (Ca, Mg, K, Na), and earthworm species/biomass data, to elucidate the effects of invasive earthworms on soil biogeochemical cycles.    

Results/Conclusions

Soils populated with litter dwelling earthworm species show high 137-Cs activity in the upper 5cm which quickly disappears with increasing depth, while soils with A horizon mixers and deep burrowing species have deeper and homogenized profiles of 137-Cs activity. Although earthworms mix organic matter into the mineral soil, heavily invaded soils have a greater bulk density, and when compared to the parent material, and are less dilated than the soils under initial stage of earthworm invasion. While soil elemental depth profiles confirms increased mixing with earthworm invasion, they cannot be explained by mixing alone. Mass balance calculations show significant losses of Ca, Mg, and P in the biologically active 0-7cm depths in heavily invaded soils. These losses and measured depth profiles of exchange capacity of base cations suggest that earthworm consumption of organic matter may lead to soil nutrients leaching.  Loss of Ca from the rhizosphere may have ecological consequences for the Ca dependent sugar maple trees that dominate out study site. Understanding how invasive earthworms affect the extent that various elements are vertically mixed and are mobilized for biological consumption or hydrological leaching will help predict the future nutrient cycles in the glaciated northern hardwood forests in N. America.