Litter chemistry of common European tree species drives the feeding preference and consumption rate of soil invertebrates, and shapes the diversity and structure of gut and faecal microbiomes

Terrestrial isopods and millipedes are key drivers of a litter decomposition in terrestrial ecosystems but the effects of litter chemistry on feeding preference and litter consumption rate as well as on the diversity and composition of gut and faecal microbiome still entails several challenges. We established a mesocosm experiment with terrestrial isopods (Oniscus asellus) and millipedes (Glomeris marginata) fed by leaf litter from six common European tree species (ash, maple, lime, beech, oak and Norway spruce) to reveal the effect of litter chemistry on consumption rate and feeding preference as well as on the compositions of gut and faecal microbiomes. The total percentage of consumed litter showed that O. asellus preferred nutrient-rich and labile-C litter of ash over more recalcitrant litter of oak, beech, and Norway spruce, while G. marginata preferred calcium-rich ash, maple and lime litter over beech and Norway spruce. Consumption of litter by O. asellus and G. marginata increased with concentrations of magnesium, sulphur and potassium but decreased with concentrations of iron, phosphorus, lignin, cellulose and TOC. The millipede G. marginata harboured higher bacterial OTU richness (73.5 ± 12.5) than the isopod O. asellus (49.1 ± 15.9), but fungal OTU richness was similar with 25.8 ± 6.7 in O. asellus and 25.7 ± 2.7 in G. marginata. In total, faeces of both animals hosted higher diversity than gut. In gut and faeces of O. asellus, the fungal OTU richness was highest for individuals fed by litter of Norway spruce, while lowest OTU richness was recorded for individuals fed by litter of more palatable ash. In contrast, the highest diversity of the fungal community in gut and faeces of G. marginata was recorded for individuals fed by palatable lime litter, while the lowest OTUs richness was recorded when millipedes were fed by maple and spruce. The structures of bacterial and fungal communities generally separated between O. asellus and G. marginata. The fungal community structure in gut and faeces differed between animals fed by different foliar litters, while the bacterial community structure mainly differed between gut and faeces regardless of the offered type of litter. The fungal community structure in gut and faeces of O. asellus and G. marginata were shaped by concentrations of magnesium, sulphur, lignin and cellulose. The bacterial communities in gut and faeces of both O. asellus and G. marginata were dominated by copiotrophic bacteria, while fungal communities were dominated by unspecified saprotrophs. Our study suggest that litter quality is a strong driver of feeding preference and consumption rates as well as composition of bacterial and fungal communities in gut and faeces of two species representing the main groups of litter feeding soil fauna in European forests.