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 the processing and storage of organic matter in soils. Our objectives were (1) to characterize and compare earthworm populations under elevated (e[CO2]) and current (c[CO2]) atmospheric CO2 concentrations, and (2) to explore the effect of e[CO2] on the relative utilization by earthworms of leaf litter and soil organic matter as a resource. In September 2007, we measured earthworm species composition, density and fresh weight in replicated (n = 2) e[CO2] and c[CO2] sweetgum (Liquidambar styraciflua) plots in a nine-year FACE (Free Air CO2 Enrichment) ecosystem experiment at the Oak Ridge National Laboratory (ORNL). To study resource utilization we analyzed the stable isotopic composition of carbon and nitrogen in earthworms, soils and forest floor litter.
All earthworms collected belonged to the genus Diplocardia, which is native to the eastern and southeastern United States. Mean earthworm density was higher in e[CO2] than c[CO2] treatments (173 m-2 and 59 m-2, respectively; F1,2 = 23.61, p = 0.04); earthworm fresh weights, however, did not differ significantly between e[CO2] and c[CO2] treatments (22 g m-2 and 15g m-2, respectively; F1,2 = 2.19, p = 0.28). In e[CO2] treatments, mean δ13C values were -32‰, -38‰, and -30‰ for earthworm, leaf litter, and soil, respectively. In c[CO2] treatments, respective mean δ13C values were -24‰, -29‰, and -26‰. Our preliminary data show that native earthworm densities are responding to e[CO2] conditions at the ORNL FACE site. Based on the signatures of δ13C and δ15N in earthworms, leaf litter and soil, it appears that Diplocardia feed primarily on soil and associated organic matter rather than leaf litter. The feeding behavior of these native earthworms at the ORNL FACE plots suggest that they could have a stronger influence over soil organic matter dynamics than on leaf litter consumption and early decomposition.