Vertical stratification and trophic interactions among organisms of a soil decomposer food web – a field experiment using 15N as a tool

In this field study, we explored the spatial segregation between the litter- and humus-inhabiting organisms of the detrital food web using 15N-isotope technique. The study was established in 11 × 11 m plots fertilized with 15N-labelled urea. Ten years after urea application, soil samples were taken, both from the litter layer and the combined F+H layer. The samples were analysed for N content and the proportion of 15N in (i) the residual organic matter in the litter and F+H layer (excluding microbes), (ii) microbial biomass, and (iii) various feeding guilds of soil fauna. The basal resource, soil microbes, and the fauna were more enriched with 15N in the F+H layer than in the litter layer. In the litter layer, the 15N enrichment of the expected food source equalled the one of the consumers, whereas in the F+H layer all trophic groups, except microbes and small microbi-detritivores, showed a significantly lower 15N enrichment than their expected food source. The results indicate that large and mobile humus-inhabiting decomposers exploit the overlying litter layer as a feeding site, whereas the feeding of the more sedentary smaller organisms is restricted to the humus layer.