Plant diversity and water regime have distinct effects on soil fungal communities that are linked to plant productivity

The soil microbiome both shapes and is shaped by the ecological consequences of drought and plant diversity loss. Sequencing soil fungal communities in a biodiversity experiment, we found that water addition, plant species richness, and plant phylogenetic diversity all impacted soil fungal composition, and increased the arbuscular mycorrhizal fungi (AMF) richness in particular. The richness of different fungal functional guilds all increased with plant functional trait diversity but were related to different above- and below-ground plant functional traits -- AMF richness was negatively associated with root traits that indicate mycorrhizal dependence, whereas diverse soil pathogens generally co-occurred with plant traits linked to faster life histories. Unexpectedly, the effect of plant diversity loss on fungal diversity did not interact with the effects of either the watering treatment or the community weighted mean of plant traits. Shifts in AMF richness and soil fungal taxa due to plant diversity loss were further associated with reduced plant productivity and diversity effects. Overall, our results indicate that changes in water input and loss in plant taxonomic, phylogenetic, and functional diversity can negatively impact soil fungal communities, and through changes in soil microbiome, may also reduce plant productivity. This negative effect of plant diversity loss is consistent across water regimes and plant community-level traits, suggesting a general vulnerability of soil microbiome and ecosystem productivity towards the biodiversity crisis.