Trophic niche variation in springtails across soil depth

Soil invertebrates move vertically through the soil to forage and avoid environmental stress. However, how their diet shifts with depth remains poorly understood, limiting our understanding of their trophic plasticity. Trophic consistency across depths could result from similar trophic niches existing at the microscale within different soil layers (the micro-scale feeding hypothesis). To test this, we conducted a microcosm experiment incubating springtails (Ceratophysella denticulata) in six separate forest soil layers (O_L, and O_F/H, and 0–3, 3–6, 6–9 and 9–12 cm depth of the mineral soil) and analysed changes in Collembola stable isotope ratios (¹³C/¹²C, ¹⁵N/¹⁴N). As expected, ¹³C/¹²C and ¹⁵N/¹⁴N ratios in litter and soil organic matter increased with depth, whereas ¹³C/¹²C ratios of Collembola did not significantly differ across layers suggesting consistent basal resource use supporting the micro-scale feeding hypothesis. By contrast, ¹⁵N/¹⁴N ratios of Collembola increased with depth, following the trend of organic matter from O_L to 0–3 cm soil, but not beyond. These results suggest that carbon and nitrogen nutrition of springtails is decoupled, and that the use of litter to calibrate ¹⁵N/¹⁴N values for estimating trophic positions of soil animals requires careful interpretation. Our results highlight the importance of soil depth as determinant of trophic positions of soil animals and point to principle differences in nitrogen resource acquisition between litter and soil in soil animal decomposers. Overall, the vertical structure of soils and a microscale view of trophic interactions needs closer attention to better understand niche differentiation and resource acquisition of soil animals.