Melissa McCormick: Mycorrhizal Ecology

Earthworms are considered keystone species for a very good reason. By mixing the soil and eating and incorporating surface leaf litter into the soil earthworms alter the forest floor environment for microbes, plants and other invertebrates. Through these activities earthworms can affect nutrient cycling and the succession and composition of the plant community. I currently have two major ongoing projects that examine the effects of invasive earthworms on mycorrhizal associations. In collaboration with Katalin Szlavecz (Johns Hopkins University), Richard Pouyat (USFS and UMBC) and Dennis Whigham I am looking at the effects of abundant invasive earthworms on arbuscular and ectomycorrhizal tree seedlings and the mycorrhizal fungi with which they associate.


This research is funded by the USDA. We have established a set of experimental plots in the forest at SERC where we are manipulating earthworm abundance and leaf litter in an effort to tease apart the separate effects of forest stand age (and canopy composition) and abundance of non-native earthworms. We established half of these plots in intermediate forest stands (50-70 years post abandonment), which support abundant non-native earthworms, and half in mature forest stands (120-150 years post abandonment), which support fewer
 
earthworms, some of which are native species. In half of the plots in each forest stand we are removing earthworms by electroshocking and in half we are adding earthworms to establish a uniform ratio of different earthworm ecological types (those that feed primarily in the litter and humus layers and those that feed primarily deeper in the soil).
 
We have planted each plot with seedlings of two ectomycorrhizal and two arbuscular mycorrhizal trees and will be monitoring seedling growth and mycorrhizal colonization as well as abundance of different fungal groups in the soil in response to different earthworm abundances.

We are also conducting a series of manipulative experiments to understand how earthworm activities affect the abundance of mycorrhizal fungi both in the soil and also on host plant roots. This experiment will tease apart the separate influences of earthworms eating and mixing leaves into the soil and making more labile carbon available, so favoring bacteria and specific groups of fungi.


In a second project, in collaboration with Timothy Filley (Purdue University), Cliff Johnston (Purdue University), and Katalin Szlavecz (Johns Hopkins University), I am examining the effects of abundant invasive earthworms on soil fungal communities and the amounts and types of carbon that are stored in the soil. This work is receiving attention for the effects of earthworms on CO2 sequestration and was recently featured in a news release by Purdue University. My role in this project is to examine the impacts of invasive earthworms on the soil fungal community, both as a result of different forms of carbon being available as food for fungi and resulting in altered extracellular enzyme dynamics that, in turn, affect the forms and amounts of carbon that are incorporated into the soil.


We have found that earthworms alter the amount and types of carbon that are incorporated into the soil from leaf litter (Filley et al. 2008, Crow et al. in press). Earthworms likely also alter how accessible the incorporated carbon is and so how long it is likely to remain in the soil rather than being released to the atmosphere. We are currently examining exactly what pools of carbon are altered by earthworm activities and how protected those pools are.


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