Linking millipede trophic niche to litter–millipede elemental composition across an altitudinal gradient

• Combined isotope and stoichiometry analyses reveal detritivore niche shifts. • Millipede resource use and trophic position change significantly with increasing altitude. • Calcium concentration plays a key role in shaping millipede trophic niches. • Altitude impacts millipede element uptake, influencing nutritional strategy. • Functional traits of millipedes adapt to changes in trophic niches. Knowledge on a species trophic niche is critically important for understanding species diversity and ecosystem functions. Nitrogen (N) and carbon (C) stable isotopes are effective tools for characterizing trophic niches, but may be complemented by stoichiometric characteristics. However, the potential of combining these two complementary approaches remains little explored, especially in belowground communities. Here, we combined stable isotope analysis and stoichiometric characteristics to analyze the niches of five taxonomic groups of soil detritivores (millipedes; Diplopoda) across an altitudinal gradient from 800 to 1850 m on Changbai Mountain, northeastern China. The results indicated significant differences in litter-calibrated 15 N values as well as in, at least in some taxonomic groups, 13 C values of millipedes with increasing altitude, reflecting changes in the use of basal resources and trophic positions. These variations were closely related to stoichiometric mismatches between millipedes and litter resources. Additionally, element concentrations of millipedes reflected shifts in their trophic position and nutritional status, with primary decomposers exhibiting higher concentrations of calcium and elevated 13 C values. Overall, our study highlights that combining stable isotope analysis with stoichiometric characteristics offers a powerful approach for elucidating the complex interactions in soil food webs, thereby enhancing our understanding of biodiversity and ecosystem functions.