Root functional strategies as drivers of the functional composition of soil fungal communities

Plant functioning heavily relies on roots. Many root functions, however, are intrinsically linked with fungal mutualists or can be reduced by fungal antagonists. Consequently, a better knowledge of the factors shaping fungal community composition is essential for our understanding of plant and ecosystem functioning. Beyond abiotic factors, the identity and composition of the plant community itself influence the soil fungal community. Depending on their root functional strategies, plants may engage differently with the soil microbial community. The root economics space (RES) has advanced our understanding of these root functional strategies, and the close association of the collaboration gradient with mycorrhizal fungi indicates that root traits could provide insights into soil fungal communities.We hypothesize that root trait strategies along the collaboration and conservation axes of the RES, as well as plant diversity, influence the composition of soil fungal communities. For instance, we hypothesize a decrease in the abundance of plant pathogenic fungi in more diverse plant communities and those with well-defended species. Higher plant defense is typically associated with root traits related to high mycorrhization ("outsourcing") and high tissue density ("slow"). Arbuscular mycorrhizal fungi are expected to exhibit contrasting trends compared to pathogens. We expect saprotrophic fungi to be mostly affected by the change in litter quality along the conservation gradient (‘fast-slow’). Hence, our study explores the interactive and additive effects of root trait gradients and plant species richness on soil fungal communities.In plots of the Jena Experiment with varying levels of plant species richness, we measured root traits at the community level and sampled soil microbial communities. Using amplicon sequencing, PLFA analyses, and microbial respiration measurements, we determined relative abundances of mycorrhizal mutualists, plant pathogens, and saprotrophs as well as absolute fungal and microbial biomass. Our findings indicate a significant decrease in the diversity and relative abundance of plant pathogenic fungi in plant communities with outsourcing root strategies. This highlights the central role of the root collaboration axis in shaping soil fungal communities beyond the direct link with arbuscular mycorrhiza. Changes in fungal and microbial biomass, however, are strongly determined by plant diversity and not driven by root traits.