Impacts of earthworm invasion on soils of the Canadian boreal forest
Creators: Lejoly, Justine Daniele Marthe
Publisher: name: University of Alberta Library
Year: 2022

<p>Exotic species of earthworms are invading North American forests, where native earthworms were extirpated by the last glaciation. The invasion of these ecosystem engineers can alter soil organic matter (SOM) dynamics, carbon (C) persistence, and ecosystem functioning. While the topic has been widely studied in temperate forests, the understanding of the consequences of this invasion in boreal forests is still insufficient. I aimed to determine how invasive earthworms are affecting boreal forest soils, their C stocks, SOM dynamics, and associated microbial communities. To ad-dress these objectives, I selected sites to encompass the three most common soil types across the Canadian boreal forest habitable by earthworms: Luvisols, Podzols, and Brunisols. Within each site, sampling zones were delimited to only differ by invasion status while keeping other environ-mental and pedological factors as similar as possible. I described soil morphological features and estimated the C stocks for the forest floors and mineral soils of invaded and non-invaded soils. I then compared their bacterial and fungal communities using phospholipid fatty acid (PLFA) analysis and metabarcoding of the 16S rRNA gene and ITS2 region. Finally, I estimated the labile (i.e. mineralizable) C using laboratory incubations, characterized the chemical composition of SOM by pyrolysis-gas chromatography-mass spectrometry, and determined C distribution in different density and size fractions of the mineral soil.<br>
The presence of invasive geoengineering earthworms resulted in the thinning of forest floors and the development of novel Ahu horizons, enriched in C and clay. For the forest floors, there was a net loss of C stocks, although the proportion of labile C remained unchanged. While fungal com-munities of forest floors were unaffected by earthworm invasion, their bacterial communities did shift, notably with increased Gram(+):Gram(-) bacteria ratios, suggesting decreased C availability. In the novel Ahu horizons, the proportion of labile C was higher than in non-invaded mineral soils and C content was significantly higher across all &lt; 2 mm soil fractions, with a greater proportion of C found in the occluded light fraction. The proportion of root-derived SOM, identified from suberin markers, decreased after earthworm invasion, while that of microbially degraded SOM increased. Increased microbial decomposition was further supported by higher C oxidation states observed in earthworm-invaded soils, and increased C content of the silt and clay-sized fractions, predominantly containing microbially degraded C. In mineral soils, earthworm invasion favoured fungi over bacteria, mainly by an increased relative abundance of ectomycorrhizal fungi at the expense of saprotrophic fungi. For bacterial communities, increased Proteobacteria:Acidobacteriota and decreased Gram(+):Gram(-) bacteria ratios indicated higher nutrient availability in earth-worm-invaded mineral soils. These findings show that earthworm invasion significantly affects C dynamics and microbial communities in boreal forest soils, notably through a faster degradation of fresh SOM. This could alter C sequestration and other ecosystem services of boreal forests in the long term.</p>