The effects of invasive earthworms on soil properties and ectomycorrhizal fungi

Background/Question/Methods

In forested regions of North America with no native earthworms, invasive earthworms from Eurasia have a large impact on the below- and aboveground environment. These invasions potentially alter carbon and nutrient cycling, decrease native plant germination and diversity, and facilitate nonnative plant invasion. Although ectomycorrhizal fungi are essential symbionts of many important North American trees, the effect of earthworm invasions on ectomycorrhizal fungal ecology is unknown.  We predict that the homogenization of soil horizons by earthworm invasions will result in more uniform soil nutrient availability, thereby decreasing the number of niches available to ectomycorrhizal fungi.  In 2012, we sampled four partially-invaded sites in New York (Heiberg Memorial Forest, Mianus River Gorge Preserve, Mohonk Preserve) and New Hampshire (White Mountains National Forest) to measure the impact of earthworm invasion on soil properties and ectomycorrhizal diversity. At 10 plots per site (5 invaded, 5 uninvaded), we extracted earthworms and took soil cores.  Cores were separated by horizon for soil property (C, N, P, Ca, Mg, K, pH, moisture) and ectomycorrhizal root-tip analyses.

Results/Conclusions

Earthworm-invaded organic soil had significantly lower Ca, Mg, C, and C:N than uninvaded soil. Invaded mineral soil had significantly higher Ca and Mg, and significantly lower C than uninvaded soil.  In addition, invaded soil cores had a higher similarity index across all soil variables, suggesting that earthworms are homogenizing the soil profile. A decrease in available niches could result in a subsequent decrease in biodiversity and functional diversity, for both the belowground fungal community and their associated plant partners. Ectomycorrhizal richness was highly variable between plots, making it difficult to measure the effects of earthworm invasion. Our results suggest that while earthworms drastically modify the soil environment, they may not affect overall ectomycorrhizal richness. Future research will elucidate whether certain ectomycorrhizal functional types are more or less sensitive to earthworm invasions.