Survivability of Aporrectodea caliginosa in Response to Drought Stress in a Colorado Soil

The distribution of earthworms is limited in many areas in the semiarid western United States by the availability of water and low soil organic matter; however, earthworms have been shown to adapt to periods of low soil moisture by making small chambers and entering estivation to protect themselves from declining soil moisture. The objective of this study was to investigate the effects of varying lengths of drought stress on the survival of earthworms in a low organic matter soil from eastern Colorado that was amended with biosolids to increase organic matter. The earthworms were exposed to constant water content or cycles of 1-, 2-, or 3-wk drought stress periods, which resulted in average soil water matric potentials of -0.061, -0.085, -0.13, and -0.19 MPa, respectively. Replicate pots were destructively sampled at 21, 42, and 63 d. At sampling, the earthworms were classified as either active, in estivation, or dead. Drought did not affect earthworm mass, but drought lasting 2 or 3 wk increased the number of A. caliginosa in estivation. Three weeks of drought resulted in a mortality rate of 14%, with all other drought treatments having negligible mortality rates. The average number of A. caliginosa in estivation decreased with time, from 1.71 at 21 and 42 d to 0.75 at 63 d. Based on these results, A. caliginosa has the potential to survive up to 3-wk drought periods and after introduction, can adapt to this Colorado soil with time, but field studies would need to confirm these results.