Seed performances from casts of different earthworm species and different soil types

Background/Question/Methods

It has been largely shown that the modification of soil properties through the action of earthworms has indirect consequences on plant growth. Some studies also suggest that earthworms could have a direct impact on seed germination and plant seedling growth through their ingestion and rejection of seeds within their nutrient-rich casts. The relationship between physical, chemical and biological properties of casts and a modification of seed germination is yet still to be studied.

The goal of this study is to link seed/seedling performances with the properties of earthworm casts. These properties include physical properties (aggregate wettability and water content), chemical properties (pH, CEC, C, N, cations, OM) and biological properties (fungal and microbial populations). Casts of three earthworm species (Allolobophora cholorotica, Aporrectodea rosea and Lumbricus terrestris) were collected over a six-month period and seeds of five species were studied (Festuca lemanii, Origanum vulgare, Trifolium campestre, Trifolium repens, Urtica dioica). Casts were produced on three different grassland soils that followed a humidity gradient. The twelve treatments (3 Earthworm species x 3 soils + 3 soil controls) were repeated five times in mesocosms.

Results/Conclusions

Our results showed significant differences between earthworm casts and soil properties. Casts of L. terrestris showed higher water content than the surrounding soil in two of the three soils. Casts of the three earthworm species tended to neutralize the pH in all soils. They all had significantly higher NH₄⁺ and NO₃^- contents than the soils. NH₄⁺ was higher for L. terrestris whereas NO₃^- was higher for A. rosea. Higher Na^+, K⁺, Mn²⁺ and Mg²⁺ contents were generally found successively in casts of L. terrestris, A. chlorotica, A. rosea and controls, except for higher K⁺ and Mg²⁺ contents found in casts of A. chlorotica for one soil type. Biological tests showed a higher functional diversity in casts, especially of L. terrestris. First germination results in artificial casts showed no significant difference in germinated seeds between all soil types. Less seeds germinated in casts of A. rosea. T. campestre showed low germination rates compared to the other species. Germination patterns and complementary analyses are still under process. Discussion will consist in analyzing the impact of earthworm casts composition on the dynamics of plant communities. These results can potentially lead to applications in restoration ecology and the use of earthworms to restore vegetation.