Earthworm burrows affect vertical distribution of springtails in soil

Extreme climatic events, such as prolonged dry spells, are causing more intense soil droughts, which can be a major threat to soil life. Soil animals in general are rather sensitive to strong fluctuations in soil moisture content but may be able to escape from drought by moving deeper into the soil. Bioturbation, for example by burrowing activity of earthworms, may facilitate such vertical movement and hence moderate the consequences of drought for soil animals. Here, we investigated if earthworm burrows enable soil-dwelling Collembola to move deeper into the soil and escape drought conditions. We also tested if drought affects bioturbation activity of earthworms, and measured evaporation from soil under drought conditions. Using transparent 2D-terraria, we analyzed the effect of four burrow treatments (i.e. burrows from an anecic earthworm species, burrows from an endogeic earthworm species, artificially made burrows, no burrows), each subjected to either drought or normal soil moisture conditions. We added 40 euedaphic springtails (Folsomia candida) per terrarium. After two weeks, we recorded survival of the springtails and their vertical localization in the soil. We used computer vision to estimate the cover and average depth of bioturbated area from photographs of the 2D-terraria. We found that the presence of Aporrectodea caliginosa (endogeic) increased the survival of springtails. Under normal moisture conditions, springtails were found deeper in the soil in the presence of A. longa (anecic). Aporrectodea longa strongly increased evaporation under normal soil moisture conditions. Our experiment showed that earthworms may moderate the impact of drought on euedaphic springtails, which opens up the hypothesis that other soil fauna may benefit as well from earthworm burrowing activity.