Species Differences in Vertical Distribution and Seasonality of Non-Native Earthworm Activity in the Houghton Rhizotron Facility.

Non-native earthworms can affect the biological, physical and geochemical properties of soil in northern forests, potentially contributing to loss and spatial reorganization of soil carbon.  Aporrectodea feed in continuously in-filled burrows, whereas Lumbricus terrestris use putatively permanent burrows. We studied the seasonal activity and vertical distribution of earthworms and the longevity of L. terrestris burrows in two contrasting soil/plant communities at the USFS Rhizotron facility in Houghton, Michigan. From 2009-2015 we recorded soil fauna at 7 depth classes bi-weekly, tracked L. terrestris burrows, logged soil temperature and moisture, and captured hourly above-and below-ground images.  We hypothesized soil moisture and temperature would influence the distribution of earthworms, with cold and dry soils driving earthworms away from the top soil horizons and increases in soil moisture during the fall and spring bringing earthworms closer to soil surface.

We found the activity and vertical distribution of earthworms and soil organisms to be closely linked to soil temperature and moisture.  L. terrestris remain active through the winter, whereas some but not all Aporrectodea aestivate during the coldest months. Aporrectodea and Lumbricus activity was greatest at 10-20cm depth when surface soil temperature is coldest (<2°C).  As soil temperature and moisture increase in April, both species become most active at 0-10cm depth, with 57% of Aporrectodea near the surface when soil temperatures are between 2-8°C and 67% near the surface when temperatures are greater than 8°C.  Activity of all earthworms decreases substantially in July and August when soil temperature is at its highest and soil moisture at its lowest for the year.  L. terrestris burrows can remain active for over 5 years, and hence become permanent hotspots of soil resources in the stable subsoil environment.  Understanding the activity of non-native earthworms can provide further insight into their impact on nutrient and carbon cycling.