The legacy effects of earthworm gut modification with different diets on soil microbial communities during cast ageing process under dry-rewet cycles
Earthworm casts are nutrient-enriched faecal structures with suspected high microbial activity. However, little is known about the ecological succession of the cast microbial community along their ageing process, as the available nutrients and organic substrates are depleting. In this study, we investigated changes in total microbial abundances and compared the compositional dynamics of active (RNA-based) and total (DNA-based) prokaryotic communities in casts of earthworms (Lumbricus terrestris. L) and bulk soil up to six months. The co-influence of varied food-type and moisture was also examined through mesocosms and microcosms experiments setup under laboratory conditions, by using qPCR and 16S rRNA amplicon-sequencing. Gut passage significantly modified soil microbial abundances and prokaryotic community structures, with anaerobic and gut-associated taxa flourishing after transiting the anoxic gut environment. The effects of gut modification on soil microbial community exerted in casts were gradually weakened by ageing, during which the shifting of dominant phyla was not only based on their trophic characteristics but also their ecological functions involved in litter-derived nutrient turnover. Keystone taxa identified in the co-occurrence networks of cast microbiomes suggested that the temporal patterns were nutrient-driven, particularly by carbon-related decomposers. The cast microbial community dynamics were negligibly affected by dry-rewet cycles during ageing, without notable alternative compositional dynamics and keystone taxa. Our results revealed that earthworms can selectively reassemble soil prokaryotic community compositions by gut transformation, resulting in a resource-driven temporal succession of the cast microbiome during ageing. Additionally, we found that the active (RNA-based) cast prokaryotic communities were slightly more sensitive to experimental factors, suggesting that the RNA fraction of communities may reflect real changes.