The role of soil bacterial community diversity and composition in earthworm health and function

Earthworms are dominant members of soil invertebrate communities that play a key role in soil ecosystems' functioning directly through impacts on soil structure and through the stimulation of soil microbial decompositional activities in bipores and as a result of soil ingestion and gut passage. The earthworm gut microbiome, mainly derived from ingested soil, is hypothesised to influence host physiology, for example, by enhancing nutrition through provision of assimilable nutrients via depolymerisation, or alleviation of chemical stress by detoxification. However, few studies have examined the nature of the relationship between earthworm health and function and their soil-derived gut microbiome's diversity and composition.

 We first used a novel antibiotic-based procedure to suppress Lumbricus terrestris earthworms' gut microbiome. We then investigated the influence of earthworm microbiome (antibiotic-treated or untreated) and soil microbiome (autoclaved or unautoclaved), and their interaction on L. terrestris feeding on, and preference for, three plant species litters (Lolium multiflorum, Quercus robur and Fraxinus excelsior). Finally, in a soil microbial diversity manipulation experiment, we examined if more subtle perturbations to microbial community richness and structure influenced earthworm health and function by determining the fate of crop residue C added to the soil.  

 The use of antibiotics significantly reduced the abundance of L. terrestris-associated culturable microorganisms (P < 0.05), but 16S rRNA gene amplicon analysis showed no effect on earthworm microbiome alpha diversity and only subtle effects on beta diversity despite the pronounced knockdown of bacterial colony-forming units. Across all earthworm microbiome x soil microbiome treatments, L. terrestris showed a greater preference for F. excelsior litter (P < 0.05) when compared to L. multiflorum and Q. robur litter: a preference which may relate to differences in litter quality parameters (C: N and polyphenol content). However, disruption of either the soil microbiome, earthworm microbiome or soil and earthworm microbiome all resulted in significantly (P < 0.05) reduced overall consumption of litter and a shift in litter preference to consume less Q. robur litter.

 The outcome of the diversity manipulation experiment suggested that only the soil treatment with the most eroded microbial diversity (by one order of magnitude compared to the intact soil) impacted earthworm energy reserves ( protein, carbohydrate and lipid), which were lowered by 0.307 %, 0.22% and 0.265% respectively.

Examining the effect of the presence of earthworm and soil microbial diversity revealed an influence on the rate of soil respiration. The diversity*earthworm interaction also revealed an influence on soil respiration rate. Our results highlight the importance of the soil microbiome for earthworm function; particularly organic matter decomposition. Further experiments examine whether residue C's fate is altered by earthworm presence and/or the soil microbial community's diversity.

This presentation will highlight the evidence underpinning the detailed effects of earthworm microbiome and soil microbial diversity on earthworm physiology and function.  

Keywords: earthworm, microbiome, Fraxinus excelsior, Lumbricus terrestris, preference,16S rRNA, health and function, litter decomposition, diversity.