Seasonal influence over earthworm populations exposed to long-term elevated atmospheric carbon dioxide in a sweetgum plantation

Background/Question/Methods

Possible effects of increasing atmospheric carbon dioxide concentrations on soil macrofauna have been largely understudied despite their influence on ecosystem processes.  Earthworms are important members of the soil macrofauna for altering soil physical properties and processing of organic matter.  Our objective was to characterize and compare earthworm populations during and after long-term exposure to elevated carbon dioxide (e[CO₂]) and current carbon dioxide (c[CO₂]) atmospheric CO₂ concentrations.  We measured earthworm species composition, density and fresh weight during exposure to elevated CO₂ from fall 2007 through fall 2008.  After termination of the CO₂ exposure in 2009, we measured species composition, density and fresh weight on spring and fall 2010.  We sampled earthworms in replicated (n = 2) e[CO₂] and c[CO₂] sweetgum (Liquidambar styraciflua) plots in an eleven-year FACE (Free Air CO₂ Enrichment) ecosystem experiment at the Oak Ridge National Laboratory (ORNL). 

Results/Conclusions

Over 94% of the earthworms collected belonged to the genus Diplocardia, which is native to the eastern and southeastern United States.  The native species Diplocardia singularis and two new Diplocardia species were found.  The exotic earthworm species were Aporrectodea rosea, Amynthas corticis and Lumbricus rubellus.  No statistical difference was detected for overall mean earthworm density (p = 0.32) and overall mean fresh weight (p = 0.26) between e[CO₂] and c[CO₂] treatments. Overall earthworm density was 139 ind. m^-2 and 96 ind. m^-2 at e[CO₂] and c[CO₂] treatments, respectively.  Overall earthworm fresh weights for e[CO₂] and c[CO₂] treatments were 36 g m^-2 and 21 g m^-2, respectively.  However, earthworm densities responded to seasonal changes with higher densities at the e[CO₂] than at c[CO₂] treatments during the spring seasons (p = 0.03).  Our results suggest that an interaction between seasonal changes and CO₂ treatments may be affecting earthworm densities at the ORNL FACE site.  Larger earthworms found at e[CO₂], indicated by higher fresh weights, may suggest positive effects of e[CO₂] treatment on earthworm biomass due to increased organic inputs to the soil.