Effects of cast properties and passage through the earthworm gut on seed germination and seedling growth

• Seed germination and seedling growth were monitored in artificial casts.
• Cast material was produced by three earthworm species in three soil types.
• Seed ingestion by earthworms decreased seed germination success.
• Cast properties influenced seed germination and seedling growth.
• Plant performances were higher in the nutrient-poor soil and control samples.

Success of seed germination and seedling establishment is potentially affected by interactions with earthworms. Two of the possible mechanisms that might explain such impact are the selective ingestion of seeds by earthworms that might break seed dormancy, and germination in their nutrient-rich casts. The aim of this study was to disentangle the effect of seed passage through the earthworm gut and the effect of cast alone, as a germination medium, on the germination and growth of four herbaceous species. We hypothesized that the presence of seeds in casts facilitates their germination and seedling growth and that the passage of seeds through gut favors seed germination. Non-ingested seeds were placed in artificial earthworm casts shaped from cast material of 3 soil types × 3 earthworm species combinations (plus control). Seed germination after seed ingestion and excretion was tested in a Petri dish experiment in the presence of each earthworm species. Contrary to our expectations, we found that passage of seeds through the gut of Lumbricus terrestris decreased the germination of Festuca lemanii and that all seeds of Origanum vulgare and Urtica dioica were digested. Total seed germination of non-ingested seeds placed in casts was affected by cast properties, i.e., by the interaction between the soil type and the earthworm species. Seedlings germinating from the control material had a higher relative growth rate than in material from Allolobophora chlorotica cast. Our results suggest that seed ingestion alters seed germination success of specific plant species. The cases of lower germination and seedling growth induced by cast effects are discussed with regards to their physical, chemical and microbiological properties.