Three-dimensional mapping of earthworm (Lumbricus terrestris) seed transport

Abstract

Transport by earthworms contributes to the secondary dispersal of seeds and influences patterns of plant community structure and function. To date, however, research on earthworm seed dispersal has focused primarily on vertical transport and the collection of seeds into middens at the soil surface, with no efforts to map horizontal distribution after burial. This study used an emergence depth assay and depth-stratified seedling emergence from soil microcosm cross-sections to map earthworm seed transport in three dimensions using commercial grass seed and the anecic earthworm Lumbricus terrestris L. While specific seed burial profiles should not be considered representative of burial under field conditions (due primarily to high earthworm densities under artificially limiting microcosm conditions), we created what we believe to be the first three-dimensional and most detailed map of earthworm seed burial, visualizing and quantifying the ability of L. terrestris to redistribute seed both vertically and horizontally throughout the soil profile. L. terrestris buried 6.5 to 17.4% of the total seeds and reduced aboveground grass biomass by 59–65 %. Vertically, L. terrestris buried seeds evenly across the 13 cm deep microcosms. Horizontally, L. terrestris aggregated seeds into small clusters aligned obliquely across soil depths, which suggest seeds being deposited in burrow walls (variance to mean ratio: 3.39 ± 2.49). Seeds were aggregated regardless of L. terrestris density but covered a greater cross-sectional area of the soil profile when more earthworms were present. Spatial patterning of shallow buried seed still capable of emergence is a newly described mechanism by which L. terrestris might influence the spatial structure of plant communities.