Earthworm-microbe interactions: what can we learn from controlled earthworm introduction into boreal forest soil?

Earthworms are considered ecosystem engineers due to their remarkable influence on the soil system. While creating the drilosphere in the soil, they interact with microorganisms both directly and indirectly and thereby greatly affecting soil carbon and nutrient cycling. Earthworm activity may reshape soil microbial communities in several ways. Amongst others, earthworms may cause shifts in microbial communities and activities/processes by damaging hyphal networks, selectively feeding on substrates hosting certain bacteria and fungi, and by redistributing nutrients in the litter-soil continuum of the soil. However, interactions between earthworms and soil microbes, especially fungi, are poorly understood, and the mechanisms by which earthworms affect microorganisms are challenging to study. First, given the widespread presence of earthworms, finding soils that have not been previously affected by earthworms is difficult. Second, controlled laboratory incubation experiments generally exclude certain functionally important groups of fungi such as plant-associated ectomycorrhizal (EcM) fungi. In our recently initiated project, we aim to study earthworm influence on soil fungal communities and associated soil biogeochemical processes by introducing soil-dwelling earthworms into to date yet uncolonised northern forest soils. Therefore, we established mesocosm boxes filled with soil turfs including tree saplings from northern boreal forests and placed them in an experimental forest in southern Sweden. Later on, we will introduce earthworms (Aporrectodea and Lumbricus spp) into the mesocosms and measure (depth specific) changes in microbial communities and genes using RNA and DNA sequencing. Potential microbial changes will be related to measurements of carbon and nitrogen cycling, such as carbon-dioxide flux measurements and soil mineral nitrogen content analysis in the growing season after earthworm introduction.