Microhabitat more than ecosystem type determines the trophic position of springtail species

Trophic plasticity may intensify competition among soil animal species and reduce their high diversity, which is often maintained by trophic niche differentiation and vertical microstratification in soils. Soil decomposers such as Collembola can shift their trophic niche with changing microhabitats across soil depths (vertical variation) and between different ecosystem types (horizontal variation), but these variations need further investigation. Here, we compared the stable isotope values of 27 Collembola species between litter and soil layers in 40 forest stands, comprising monocultures of European beech, Norway spruce, Douglas fir, or mixtures of European beech with either of the two conifer species. The results showed that the δ¹⁵N values of Collembola within species were uniformly higher in soil than in the litter layer irrespective of forest type. This increase correlated with the biomass of Gram-negative bacteria but not with that of fungi. The δ¹³C values of Collembola were significantly enriched in beech forests, but were similar in Douglas fir and Norway spruce forests indicating similar effects of the two coniferous species on the basal resource use of Collembola. Overall, the results suggest that Collembola shift their diet to consume local resources available in the respective soil layer they colonize, with bacteria playing a more important role in driving variations in their trophic positions than previously assumed. By contrast, the consistent trophic position of Collembola between ecosystem types suggest access to similar food sources in the soil microhabitat, despite large changes in aboveground vegetation type. Overall this study suggests that changes in ecosystem types has little influence on the resources accessible to Collembola which presumably selectively feed on specific resources in the soil microhabitat. By contrast, variations in resource types and soil microhabitats across soil depth more strongly affect the trophic niche of Collembola, which requires plasticity to adapt to their diet.