Quantifying changes in soil bioporosity in subarctic soils after earthworm invasions

Abstract: Pores provide important hotspots for chemical and biological processes in soils. Earthworm burrows affect the macropore structure and their actions may create new preferential pathways for water and gas flow within soils. This, in turn, indirectly affect plants, nutrient cycling, hydraulic conductivity, gas exchange, and soil organisms. While the effects of invasive earthworms on soil properties has been well-documented in temperate and boreal ecosystems, we know little how these organism may affect tundra soils. In this study, I assessed how the three-dimensional network of soil-macropores are affected by earthworm species (Aporrectodea sp. and Lumbricus sp). I hypothesized: i) that earthworms increase the frequency of macropores with a likely biological origin (biopores); ii) effects of biopores are dependent on tundra vegetation type (meadow or heath); and iii) the macropore network properties are altered by earthworms.  The hypotheses were tested using a common garden experiment with 48 mesocosms. The pore structure of each mesocosm was analyzed using X-ray CT tomography. I found that biopores increased in the tundra from on 0.05 ±0.01 % (mean ± standard deviation) in the control to about 0.59 ± 0.07 % in the earthworm treatments. However, in contrast to my second hypothesis, I found no vegetation dependent effect. Interestingly, I found that earthworms decreased the complexity and directionality of macropores. My findings strongly indicate that burrowing can severely impact the pore properties of previously uninhabited subarctic soils.