Earthworms are arguably the best known soil bioturbator, yet their impacts on soil biogeochemistry are difficult to quantify as a function of their roles in physically mixing soils. In glaciated regions of North America, northern hardwood forests have evolved without native earthworms since the last glacial retreat. However, earthworms have invaded northern hardwood forests owing 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, yet their impacts on soil biogeochemistry remain largely unknown. An earthworm invasion chronosequence near Leech Lake in Northern Minnesota provides an ideal outdoor laboratory to quantify the interactions between biogeochemical and physical processes associated with different earthworm species and biomasses. Across the earthworm invasion transect, the A horizon elemental chemistry profiles show that earthworms have vertically relocated minerals, which is consistent with 210-Pb activity profiles. While soil elemental depth profiles confirm increased mixing with earthworm invasion, the depth profiles cannot be solely explained by mixing. I used a geochemical mass balance model to examine soils' biogeochemical responses to invasive earthworms. Fractional and absolute mass losses/gains of biologically important elements such as Ca, P, K, Fe, and Si, relative to the parent material, are substantially altered by invasive earthworm species. The arrival of A-horizon-mixing, endogeic earthworms most dramatically reduces the level of the elemental enrichments in the A horizons. The declined elemental enrichments are likely derived from the consumption of particulate organic matter by endogeic species, which leads to the mineralization and leaching of Ca, P, K. The dramatic losses of the enrichments also suggest that the newly mineralized nutrients are in excess of the nutrient demand from understory plants. Our results indicate the significant and potentially negative impacts of invasive earthworms on the soil nutrient cycling and consequently the sustainability of the hardwood forests in the Great Lakes Region.
University of Minnesota M.S. thesis. June 2013. Major: Soil Science. Advisor: Kyungsoo Yoo. 1 computer file (PDF); v, 165 pages.
Resner, Kathryn "Kit" Elizabeth.
Impacts of earthworm bioturbation on elemental cycles in soils: An application of a geochemical mass balance to an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota..
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