One of the most widespread introductions of novel biodiversity in eastern North America has been the invasion of exotic earthworms. Without practical options for prevention or control, we require a better understanding of how these organisms interact with land management in invaded ecosystems. One example is wood ash, a pulp and paper byproduct proposed as a soil amendment for improving pH and nutrient deficiencies in North American forests. However, the interactions of ash amendment with key soil fauna such as earthworms remain poorly understood.
To investigate how ash impacts the common exotic earthworm Lumbricus terrestris, earthworms were placed in mesocosms amended with four different ash types and under control conditions. After two weeks, we assessed earthworm survivorship, biomass, and foraging activity. To determine ash effects on habitat preference/avoidance, we provided earthworms with a choice between bare soil and one of four ash types. Earthworm location was noted after 48 hours. To explore how earthworms influence ash burial, a fluorescent powder was mixed with ash and applied in mesocosms inoculated with increasing earthworm densities. After 1 month, mesocosms were sampled in cross sections and ultraviolet photographs taken at sequential depths to track ash movement into the soil.
Results/Conclusions
Ash amendment had no significant impact on earthworm survivorship or growth, with an overall survivorship of 94% and a biomass increase of 15%. However, foraging activity was reduced by 29% in the presence of one higher pH ash (pH ≈ 12). Earthworms showed no strong preference for or avoidance of most ash types except for the same high pH ash, which was avoided by 97% of earthworms. In the burial experiment, ash was moved belowground through earthworm burrows, with 27 times as much ash recovered below the surface and a 12 cm increase in maximum recovery depth when earthworms were present.
Overall, ash amendment was found to have limited negative impact on our focal earthworm species, though higher pH materials may be more likely to have negative effects. Notably, the presence of earthworms may facilitate the belowground integration of surface-applied ash, increasing the utility of ash in forests where amendments cannot be manually mixed into the soil. Our work provides an example of how novel biodiversity can both affect and be affected by anthropogenic land management in invaded ecosystems, including interactions which may be contextually beneficial and deserving of further investigation.