Earthworm Burrows in Glacial Till: Spatial Distribution and Macropore Function

Earthworms contribute to clean groundwater through their presence in agricultural soil on glacial till, a common parent material in the Northern Hemisphere. This study maps the spatial structure of earthworm macropores in a Danish field under winter wheat and annual crop rotation during 2011–2014. Macropore connectivity and preferential flow potential were assessed using, horizontal excavation and macropore mapping, Brilliant Blue dye infiltration and surface smoke tracer tests. The community was dominated by anecic species (Lumbricus terrestris and Aporrectodea longa) that maintain deep vertical burrows contributing to hydrological connectivity. Stable isotope analysis confirmed their distinct ecological roles and trophic strategies. Following a shift from conventional to reduced tillage, we observed a marked increase in surface-connected macropores and burrow density, indicating recovery of earthworm-mediated infiltration pathways. The density of BB-stained earthworm burrows remained stable down to 2.0 m depth, while unstained burrows declined with depth, supporting the interpretation that earthworm burrows acted as dominant preferential flow paths. These findings highlight the long-term functional legacy of earthworm activity in structured glacial till soils, with implications for groundwater recharge and transport of solutes.