Drought Stress Affects Earthworms in a Biosolids Amended Colorado Soil.

The distribution of earthworms is limited in many areas in the semi-arid western US by the availability of water.  However, earthworms have been shown to adapt to periods of low soil moisture by making small chambers and entering diapause to protect the earthworm from the 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.  The hypothesis was that the survival of earthworms would be unaffected by drought periods two weeks or less in length but longer time periods would affect the survival of the earthworms.  The earthworms were exposed to constant water content or a one, two, or three-week, drought stress period.  The pots were destructively sampled at 22, 43, and 64 days.  At sampling, the earthworms were either classified as active, in diapause, or dead.  The percent change in mass of the earthworms was also measured.  The results of the study showed that there was a significant effect of treatment on the number of earthworms in diapause after two weeks of drought.  The longer drought periods also significantly increased the mortality rate.  A mortality rate of 14% was observed following a three-week drought period, with all other periods having a negligible rate.  The result of the study indicated that earthworms have the potential to survive up to three-week drought periods in Colorado soil.