Soil–Arthropod Interactions Across Forest Types in a Mediterranean Mountain Landscape

Ground-dwelling arthropods contribute to decomposition, nutrient cycling, and biological control, but their relationships with soil properties remain insufficiently understood in Mediterranean mountain forests. We evaluated taxonomic and functional structure, seasonal variation, and associations with edaphic gradients across three forest stands—one holm oak (Quercus rotundifolia), one Pyrenean oak (Quercus pyrenaica), and one maritime pine (Pinus pinaster)—in Montesinho Natural Park, northeastern Portugal. Arthropods were sampled with pitfall traps during four campaigns from July 2023 to April 2024, and soil properties were characterized at each trap. We recorded 5511 arthropods, comprising 18 orders, 54 taxa, and six functional groups. Total abundance was lowest in February, highest in April, and lower in the Pyrenean oak stand than in the other stands. Taxon richness, predator abundance, and the abundance of taxa with unresolved or multiple trophic functions varied differently among stands across campaigns. The sampling campaign explained 28.7% of the variation in taxonomic composition. Total abundance and omnivore abundance, the latter dominated by Formicidae, increased along PC1, associated with finer-textured soils, higher pH and C:N ratio, and lower available potassium. Omnivores decreased along PC2, associated mainly with organic matter, roots, and clay-bound nitrogen. PC1 and PC2 jointly explained 33.2% of the variation in taxonomic composition. Overall, the results show that seasonal and edaphic variation are important in structuring the taxonomic and functional components of ground-dwelling arthropod communities in Mediterranean mountain forests.