title: How do earthworms and soil microbial communities interact to determine the fate of organic matter amendments to soil?
Creators: name: Omosigho, Henny Oyindamola
Publisher: name: University of Reading
Year: 2021

description: The soil ecosystem harbours most of the global ecosystem’s biodiversity. The soil ecosystem 
contains a dynamic complex of microorganisms such as bacteria, microfauna (such as 
nematodes) and macrofauna (such as earthworms) and the non-living environment interacting 
as one functional unit. The biodiversity of this ecosystem plays a critical role in the provision 
of ecosystem functions and services. These ecosystem functions and services benefit humans. 
However, the rise in global challenges due to the rapid increase in human population, food 
shortage, the increase in our ecological footprint, rapid changes in the earth’s climate, the 
introduction of invasive exotic organisms capable of changing ecosystem biodiversity and the 
increasing incidences of droughts and other natural disasters, has resulted in a ‘trade off’ of 
ecosystem services and a reduction in soil biodiversity that is pivotal to the ecosystem 
functions and services. 
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 the provision of assimilable nutrients via depolymerisation. 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. Also, we must improve our 
understanding of soil functioning especially given the pressures on the soil to deliver the 
services related to addressing the global challenges and the potential feedback between soil and the climate system into the future. The overall aim of this thesis was to increase our 
understanding about the importance of the earthworm - earthworm microbiome relationship 
in supporting critical soil ecosystem process such as organic matter mineralisation, explicitly 
paying attention to the feedback of the presence of the earthworm microbiome to the health 
and function of earthworms in supporting ecosystem processes.
I used a novel antibiotic-based procedure to suppress Lumbricus terrestris earthworms’ gut 
microbiome in the pilot study. The use of antibiotics significantly reduced the abundance of 
L. terrestris-associated culturable microorganisms (P &lt; 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.
The influence of the earthworm microbiome (antibiotic-treated or intact) and the soil 
microbiome (autoclaved or intact), and their interaction on L. terrestris feeding on, and 
preference for, three plant species litters (Lolium multiflorum (ryegrass), Quercus robur (oak) 
and Fraxinus excelsior(ash)) was then investigated. Across all earthworm microbiome x soil 
microbiome treatments, L. terrestris showed a greater preference for ash litter (P &lt; 0.05) 
when compared to ryegrass and oak litter: a preference that 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 resulted in 
significantly (P &lt; 0.05) reduced overall consumption of litter and a shift in litter preference to 
consume less oak litter.
Finally, in a soil microbial diversity manipulation experiment, the research attempted to 
examine whether an increase in OM mineralisation in the presence of earthworms would
depend on soil microbial diversity and could be linked to microbial diversity impacts on 
earthworm health. The aim to use the created soil with a gradient of microbial richness and L. terrestris as a test species to examine whether changes in soil microbial richness influence the 
effect of earthworms on organic matter decomposition to respired CO2 and whether any 
effects were linked to earthworm health status that may feedback to ecosystem functioning. 
Health status was evaluated by assessment of earthworm energy reserves through lipid, 
carbohydrate, and protein analysis. There was no effect of microbial diversity on earthworm 
functional impacts on SOM mineralization or earthworm health and therefore does not 
support the overall hypothesis for this thesis. Taken together, the findings currently suggest 
that the correlated properties of the soil microbiome that are important for earthworm health 
and functional role are abundance (biomass) and activity rather than the richness of species 
present or the presence of specific species or combinations of species. The insensitivity of 
both earthworm health and function to bacterial species loss suggests that the nature of the 
function provided by the ingested soil bacterial microbiome to its host is a functionally 
redundant one, because the functions provided are very generic, as many other microbial 
groups can provide similar function.