Drying by worming: deciphering how earthworms dry tundra soil

Deep-burrowing earthworms (anecic and endogeic species) can dry soils by reworking mineral soil layers. Although this ‘drying effect’ has been reported across many ecosystems, including the Fennoscandian tundra, little is known about the driving processes. In this study, we measure plant transpiration in combination with controlled experiments of water holding capacity and evaporation to assess drivers of soil–water losses in tundra soil as the result of endogeic and anecic earthworms. Our experimental system was a common garden experiment with shrub-dominated (heath) and forb-dominated (meadow) vegetation (N = 48), where long-term monitoring revealed drier soils due to the addition of earthworms. Although we found that tundra plant transpiration was highest during the peak growing season and that meadow soil had a higher field capacity, our earthworm treatment did not strongly affect these two parameters. Evaporation, on the other hand, was on average 14 % higher in the meadow with earthworms although no such effect was observed in the heath soil. Using a network model of macropore vapor transfer that measures evaporation effects, we found an increase in macropore conductance between the subsoil and the atmosphere and that the vaporization rate in relation to the diffusion rate controls the strength of the evaporation effect. Our findings underscore the need to account for evaporation due to the reworking of pore architectures by soil biota when predicting changes in soil–water availability.