Climate and soil variables driving the distribution of invasive earthworms in cropping systems of the Inland Pacific Northwest
Earthworms play an important role in agricultural soils, incorporating crop residue, enhancing aggregation, aeration and infiltration, and contributing to nutrient cycling. Earthworms also affect both soil organic carbon (SOC) and soil greenhouse gas (GHG) emissions via complicated and contradictory mechanisms. Earthworms break up organic matter and stimulate microbial activity, potentially increasing GHG emissions and decreasing SOC. Conversely, earthworms have been shown to increase plant growth and stabilize carbon in their casts leading to decreased GHG emissions and increased residence time of SOC. The overall impact of earthworms depends on their density and period of activity, which are in turn dependent on moisture and, to a lesser extent, soil temperature. Other biophysical factors also play secondary roles in modifying earthworm tolerance to changes in moisture and temperature. By improving our understanding of earthworm distribution under current environmental and climatic conditions, we can gain insight into potential impacts of climate change on these important soil organisms. Earthworms were collected from 40 sites across the wheat growing regions of Oregon, Washington and Northern Idaho during the springs of 2011 and 2012. Sampling will continue in 2013. Sites occurred across a climatic gradient consisting of 6 different recognized bioclimatic zones.
Earthworm populations were dominated by the invasive species, Aporrectodea trapezoides, which represented 85% of adult specimens. Earthworms were found in 5 of the 6 agroecological zones with densities varying between and within zones, sites and years. Mean earthworm density for all sites was 113 earthworms m-2 in 2011 and 333 earthworms m-2 in 2012. Across sites and years, densities ranged from 2 to 1498 earthworms m-2. Similar patterns among agroecological zones occurred in both years. Earthworms were found at 100% of sites within zones receiving at least 350 mm mean annual precipitation. These higher rainfall zones also had the highest mean earthworm densities. In zones receiving 250-450 mm annual precipitation earthworms were found at less than one quarter of sites, with densities ranging from 14 to 241 earthworms m-2. Earthworms were not found in the lowest rainfall zone (less than 350 mm per year) even when irrigation was utilized. Trends in earthworm density did not follow strict precipitation gradients and suggest that factors other than mean annual precipitation play a role in determining earthworm density.