The vast majority of earthworms in many parts of North America are not native, and northern parts of the continent have no native earthworms. At the same time, earthworms are considered keystone species because they dramatically change nutrient cycling, litter decomposition, air and water filtration into the soils and how soil characteristics vary with depth.
In collaboration with Timothy Filley (Purdue University), Cliff Johnston (Purdue University) and Katalin Szlavecz (Johns Hopkins University), we are examining the effects of abundant non-native earthworms on soil fungal communities and the amounts and types of carbon that are stored in the soil. This work, funded by NSF, has received attention for the effects of earthworms on carbon sequestration and was featured in a news release by Purdue University. Our role in the project is to examine the impacts of non-native earthworms on fungal and bacterial communities in the soil and how these communities impact the extracellular enzymes produced which, in turn, affect the forms and amounts of carbon incorporated into the soil and how long they stay there.
We have found that earthworms alter the amount and type of carbon that is incorporated into the soil from leaf litter and wood (Filley et al. 2008, Crow et al. 2009, Ma et al. in review). Earthworms likely also change how accessible the incorporated carbon is and therefore how long it remains in the soil before being decomposed and released into the atmosphere. We are currently examining which pools of carbon are altered by earthworm activities and how protected those pools are.
Earthworm effects on fungi go beyond soil carbon decomposition. Over 80 percent of land plants rely on mycorrhizal fungi to obtain nutrients and water. Mycorrhizal fungi are a critically important component of the microbial community that determines how plants interact with the soil and with other plants. Because they form such a critical connection between above- and below-ground environments and between plants, anything that changes the mycorrhizal fungus community can also change the plants that grow there. Earthworms mix the soil, eat and incorporate surface leaf litter into the soil and dramatically change the environment for microbes, plants and other invertebrates. In collaboration with Katalin Szlavecz (Johns Hopkins University), Richard Pouyat (USFS) and Dennis Whigham (SERC), we are looking at the effects of abundant invasive earthworms on arbuscular and ectomycorrhizal tree seedlings and the mycorrhizal fungi with which they associate.
In this research, funded by the USDA, we established a set of experimental plots in the forest at SERC, where we manipulated earthworm abundance and leaf litter to tease apart the effects of land use history (and forest canopy composition) and abundance of non-native earthworms. We established half of our experimental plots in intermediate forest stands (50-70 years post-abandonment) that supported abundant non-native earthworms, and half in mature forest stands (120-150 years post-abandonment) that supported fewer earthworms, some of which were native species. We then planted each plot with seedlings of trees associating with two different types of mycorrhizae (two with ectomycorrhizae and two with arbuscular mycorrhizae). Then we monitored seedling growth and mycorrhizal colonization, as well as the abundance of different groups of mycorrhizal fungi in the soil in response to different earthworm abundances. We found that earthworms dramatically decreased the abundance of both arbuscular and ectomycorrhizal fungi in the soil, but these decreases had variable effects on the tree seedlings. We are currently preparing this work for publication.
We also conducted a series of manipulative experiments to understand the mechanism by which earthworm activities affected the abundance of mycorrhizal fungi in the soil. This experiment separated effects of different types of earthworm activities (disrupting hyphae, mixing soil, and increasing availability of nitrogen and labile carbon) to determine how earthworms changed decomposition and the abundance of mycorrhizal fungi. The initial findings of this work were published in Biological Invasions.