Temperature and phosphorus gradients shape the trophic structure and basal resource use of soil microarthropods as indicated by bulk and amino acid stable isotope analyses

Temperature and soil phosphorus strongly influence microarthropod communities and may affect their trophic structure and energy channeling. In this study, we investigated the two most abundant detritivore microarthropod taxa, Collembola and Oribatida, and examined changes in their trophic structure and basal resource use across temperature and phosphorus gradients. We applied two state-of-the-art approaches at the community level: bulk stable isotope analysis and compound-specific stable isotope analysis of amino acids. Samples were taken at 12 forest sites in central Europe along three temperature and four soil phosphorus levels. Estimates of trophic positions from both amino acid-specific stable isotope analysis (TP CSIA ) and bulk tissue Δ 15 N values (calculated by calibrating δ 15 N values to litter as baseline) showed significant responses to temperature and phosphorus availability, but this depended on microarthropod groups. Generally, TP CSIA of Collembola (3.00 ± 0.25; mean ± SD) was higher than that of Oribatida (2.66 ± 0.24) but this was not pronounced at sites with low temperature and rich phosphorus. Bayesian mixing models identified fungi as the main source of essential amino acids (eAAs) in Collembola (37.3–59.6%), while in Oribatida it was bacteria (39.9–50.2%). With increasing temperature, the contribution of fungal and bacterial eAAs increased, while the contribution of plant eAAs declined. At low temperature, plants comprised the second most important basal resource in both Collembola and Oribatida, accounting for 17.2 – 46.1% and 26.0 – 46.2% of total basal resources, respectively. Our findings reveal how Collembola and Oribatida partition resources and adjust trophic positions in response to environmental gradients. Collembola occupy higher but less flexible trophic positions at low and intermediate temperature with strong fungal reliance, whereas Oribatida are more plastic and rely more on bacterial pathways. Both groups are able to shift towards plant-derived resources when microbial processing or nutrient availability is limited. The results advance our understanding of soil microarthropod ecology and highlight the interplay between environmental conditions, resource use and trophic structure in shaping soil food webs.