Habitat filtering in soil food webs
As anticipated, habitat filtering strongly influenced soil food web structure (Crotty et al. 2014; Arribas et al. 2021; Seppet al. 2021). Differences in abiotic conditions and resource availability between habitats could explain the observed differences in the proportions of trophic groups. Grassland soils exhibit lower acidity, higher P and N availability and a lower C/N ratio (Joimelet al. 2016, Fig.S5), and have a higher below-ground plant biomass, which is more easily decomposed, leading to significant inputs of readily decomposable organic matter into the soil (Mason & Zanner 2005; Heděnec et al. 2022). These factors likely enhanced resource availability for detritus-based trophic groups, resulting in the higher dominance of copiotroph decomposers (e.g., saprophytic fungi and bacteria), micro- or meso- detritivores (i.e., Enchytraeidae) and microbivores in grasslands. In contrast, plant detritus in forests, which is more challenging to decompose, serves as a direct resource for litter and wood saprotrophic fungi and provide a habitat for macro-detritivores, which feed on plant litter and associated fungi (David & Handa 2010; Zuo et al. 2014). Light availability in grasslands may favour the presence of photoautotrophic bacteria that use light as their primary resource, and facilitate interactions, such as arbuscular mycorrhiza (Konvalinková & Jansa 2016). Moreover, grasslands had a higher proportion of phytoparasites, likely due to the higher biomass of fine roots in this habitat (Jackson et al. 1997; Heděnec et al. 2022), which are more easy to colonise by symbionts, while forests had a higher proportion of zooparasites, benefiting from a greater abundance of potential hosts, predominantly arthropods, molluscs or earthworms, for the taxa belonging to our zooparasite groups. These findings align with our results showing that trophic interactions within the soil food web contributed to explain food web dissimilarities between habitats (Fig. 4) and are consistent with previous studies examining changes in the trophic composition of protists (de Araujo et al. 2018; Fiore-Donno et al. 2020) and nematodes (Zhao & Neher 2014) across habitats. The distinct identities of soil antagonists in the soil food webs from grassland to forests warrant further investigation as they could provide insights for land management strategies in mountain ecosystems (Wall et al.2015).