Forest floors are vertically structured systems in which successive stages of leaf-litter decomposition alter both the stable isotope composition and availability of basal resources to consumers.Yet it remains unresolved whether trophic differences along this vertical profile arise from taxonomic turnover or isotopic shifts within taxa shared among forest-floor layers. To address this, we sampled three forest-floor layers (OL: fresh litter; OF: fragmented litter; A: 0-5 cm of soil) in a temperate beech forest and analysed three major microarthropod groups representing different trophic roles. Oribatida and Collembola were identified to species or genus level, whereas Mesostigmata to family level. We quantified vertical variation in basal δ¹³C and δ¹⁵N to account for depth-related shifts in resource isotope composition. We hypothesized that baseline-corrected community isotopic position would differ among layers, that OL–OF changes would arise from group-specific combinations of taxonomic turnover and within-taxon variation, and that isotopic niche breadth would contract toward mineral soil. Basal δ¹³C and δ¹⁵N increased by approximately 3‰ and 4‰, respectively, from OL to A layer. Community mean Δ¹³C and Δ¹⁵N decreased with depth in Oribatida, whereas only Δ¹³C decreased in Collembola and Mesostigmata. Taxonomic composition differed among layers in all three groups, while trophic-guild structure changed in Oribatida and Collembola. From OL to OF, abundance-weighted community mean Δ¹³C declined by 0.73‰ in Oribatida, 1.16‰ in Collembola, and 1.30‰ in Mesostigmata; within-taxon components represented approximately 77%, 79%, and 93% of these declines, respectively. In Oribatida and Mesostigmata, opposing turnover and within-taxon effects partly cancelled one another, producing smaller overall changes in community mean Δ¹⁵N. Contrary to our prediction, isotopic niche size did not contract toward mineral soil. Vertical stratification therefore reorganized carbon pathways and relative trophic positions through interacting and compensating processes, without consistently narrowing isotopic niche breadth across forest-floor compartments.