Earthworm cast microbiomes differ across soil types in northern forests

Abstract

Earthworms influence soil properties and element cycling, notably through the formation of casts. These biogenic structures are hotspots of microbial activity and play a role in soil carbon and nutrient dynamics. Previous research revealed that earthworm casts harbour distinct microbiomes from mineral soils under controlled laboratory conditions. But field studies are scarce, especially in northern forests currently undergoing earthworm invasion. Identifying drivers of earthworm cast microbiomes in the field and their potential differences with mineral soils and forest floors is therefore crucial to advance our understanding of their impact on soil functioning and nutrient cycling. Using a combined PLFA and DNA (ITS and 16S) approach, we characterized the fungal and bacterial communities of field-collected earthworm casts from three soil types: Luvisols, Brunisols (WRB: Cambisols), and Podzols. We compared them with those of surrounding mineral soils and forest floors. We did not find evidence of community filtering effects following gut transit and mixing of mineral soil and forest floor, as α- diversity and species richness were not reduced in casts. Cast microbiomes were dominated by taxa also found in forest floors and mineral soils. Yet they presented key differences in that they were systematically enriched in saprotrophic fungi and Bacteroidota and depleted in ectomycorrhizal fungi. Further, they were more similar to forest floor than mineral soil microbiomes. We also found that, although cast microbiomes were dominated by taxa common to casts of all sites, they differed across soil types. This finding highlights the prevailing role of pedogenesis in explaining the microbiome composition of field-aged casts. Our results also underscore the importance of sampling both forest floors and mineral soils when investigating the effects of earthworms on carbon and microbial dynamics in forest ecosystems.