Cynthia Albracht

The effects of plant species richness on soil microbial diversity and the associated ecosystem functions strengthen over time. However, the mechanisms underlying the plant diversity effects on fungal diversity and community composition along the plant-soil continuum remain largely unknown. Here, we established a plant diversity gradient (species richness level 1, 2, 3 and 6 plant species), factorially crossed with plant community-specific soil- and plant history treatments derived from a long-term field biodiversity experiment, in an Ecotron experiment. We studied the effect of plant species richness on fungal community diversity and composition across different compartments: bulk soil, rhizosphere, and roots. Using ITS amplicon sequencing, we determined fungal diversity, community composition, and assembly processes across the compartments. Fungal diversity increased significantly with plant species diversity, to varying degrees according to the compartments and history treatments. While plant history did not affect fungal community composition, shared soil history markedly increased fungal richness and affected community composition. Notably, the effect of soil and plant history on the relationship between plant species richness and the fungal diversity depended on the studied compartment, with the strongest positive plant diversity effect on fungal diversity in roots and rhizosphere soil in treatments with shared soil history. Key environmental variables were considered for exploring possible mechanisms responsible for plant species richness and history treatment effects on fungal community diversity and composition, with only plant biomass being a significant mediator. Our findings indicate that plant species richness leaves a legacy in soil, and this plant community-specific soil legacy is a significant driver of fungal diversity and community composition, but only in the compartments directly influenced by roots.

Studies have investigated the interactions between plants through competition and resource sharing to understand the mechanisms behind the positive effects of plant diversity on productivity. Volatile organic compounds (VOCs) are important info-chemicals in plant–plant interactions, but they have so far rarely been considered in this context. Here, we measured VOC emissions at the community scale and for one species ( Plantago lanceolata ) in experimental plant communities of varying diversity (The Jena Experiment) to understand the role of VOCs in driving biodiversity-ecosystem functioning relationships. We show that plant diversity determines the release of plant VOCs at both scales. At the community level, plant species richness directly enhanced VOC emission and increased VOC richness both directly and indirectly by altering leaf area index. At the species level, plant diversity did not directly affect the VOC emissions of P. lanceolata but indirectly affected it by influencing the VOC emissions from the surrounding community. P. lanceolata individuals in communities with high concentrations of green leaf volatiles decreased their VOC emission, while those in communities with high concentrations of terpenoids increased their VOC diversity. Our results provide evidence that plant diversity shapes community-level plant VOC emission and thus influences focal plant VOC emission inside the community.

Summary: Increasing frequencies of severe summer droughts and plant diversity loss disrupt ecosystem functioning and stability of European grasslands. Understanding how these factors interact with pathogens is crucial. We investigated the effects of plant diversity and repeated summer drought on soil‐borne parasites within a grassland biodiversity experiment. The experiment included plant communities ranging from 1 to 60 species, with a sub‐experiment simulating annual droughts for 6 wk in summer over 9 yr. One year after the final drought period, we analyzed the diversity and community composition of two parasitic protistan taxa with many plant‐pathogenic members, Oomycota (Stramenopila) and Phytomyxea (Rhizaria), as well as protistan consumers in the Cercozoa (Rhizaria) using amplicon sequencing. Both Oomycota and Cercozoa, including Phytomyxea, responded to plant species richness and drought, but not uniformly. Plant species‐specific Oomycota were enriched under drought, while Phytomyxea and cercozoan consumers exhibited shifts in both enriched and reduced operational taxonomic units. No mitigating effect of plant diversity against the effects of drought was observed. Our findings suggest that repeated summer droughts weaken plant defense against protistan plant parasites, causing long‐lasting soil legacy effects across plant communities varying in diversity and community composition, potentially threatening ecosystem stability and functioning under future climate conditions.

Studies have investigated the interactions between plants through competition and resource sharing to understand the mechanisms behind the positive effects of plant diversity on productivity. Volatile organic compounds (VOCs) are important info-chemicals in plant-plant interactions, but they have so far rarely been considered in this context. Here, we measured VOC emissions at both community and species levels ( Plantago lanceolata ) in experimental plant communities of varying diversity (The Jena Experiment) to understand the role of VOCs in driving biodiversity-ecosystem functioning relationships. We show that plant diversity determines the release of plant VOCs at both levels. At the community level, plant species richness directly enhanced VOC emission and increased VOC richness both directly and indirectly by altering herbivore damage and LAI. At the species level, plant diversity did not directly affect the VOC emissions of P. lanceolata but indirectly affected it by influencing the VOC emissions from the surrounding community. Plantago lanceolata individuals in communities with high concentrations of green leaf volatiles decreased their VOC emission, while those in communities with high concentrations of terpenoids increased their VOC diversity. Our results provide first evidence that plant diversity shapes community-level plant VOC emission and thus influences focal plant VOC emission inside the community.

Summary: The increased positive impact of plant diversity on ecosystem functioning is often attributed to the accumulation of mutualists and dilution of antagonists in diverse plant communities. While increased plant diversity alters traits related to resource acquisition, it remains unclear whether it reduces defence allocation, whether this reduction differs between roots and leaves, or varies among species. To answer these questions, we assessed the effect of plant species richness, plant species identity and their interaction on the expression of 23 physical and chemical leaf and fine root defence traits of 16 plant species in a 19‐yr‐old biodiversity experiment. Only leaf mass per area, leaf and root dry matter content and root nitrogen, traits associated with both, resource acquisition and defence, responded consistently to species richness. However, species richness promoted a decoupling of these defences in leaves and fine roots, possibly in response to resource limitations in diverse communities. Species‐specific responses were rare and related to chemical defence and mutualist collaboration, likely responding to species‐specific antagonists' dilution and mutualists' accumulation. Overall, our study suggests that resource limitation in diverse communities might mediate the relationship between plant defence traits and antagonist dilution.

Recently, biomass of invertebrates has declined substantially at many locations with the implications of this biodiversity loss for ecosystems yet unknown. Through multitrophic interactions, plant‐ and soil‐associated microbiomes might be altered, causing a cascade of changes on diverse ecosystem processes. We simulated aboveground invertebrate decline in grassland ecosystems with two levels of invertebrate biomass (36% and 100% of current ambient conditions), plus a control with no invertebrates present. Each standardised grassland mesocosm additionally contained one clonal Quercus robur L. sapling to investigate the extent of invertebrate decline effects exceeding grasslands. We investigated oak biomass partitioning and mycorrhiza formation, oak leaf transcriptome and microbiome composition of leaves, roots and rhizosphere. While invertebrate decline did not significantly affect oak performance and herbivory‐related gene expression, fungal communities presented an increase of saprotrophs and pathogens, especially in leaves. Among leaf‐inhabiting bacteria, Proteobacteria and Actinobacteria increased under invertebrate decline. The belowground microbiome was only little affected. But, invertebrate decline came along with a reduced influence on predators leading to an elevated aphids infestation that proofed able to alter microbiota. Our findings establish a strong difference between above‐ and belowground, with the impacts of invertebrate decline being more pronounced in the leaf microbiome.

Trait‐based approaches have been increasingly used to relate plants to soil microbial communities. Using the recently described root economics space as an approach to explain the structure of soil‐borne fungal communities, our study in a grassland diversity experiment reveals distinct root trait strategies at the plant community level. In addition to significant effects of plant species richness, we show that the collaboration and conservation gradient are strong drivers of the composition of the different guilds of soil fungi. Saprotrophic fungi are most diverse in species‐rich plant communities with ‘slow’ root traits, whereas plant pathogenic fungi are most diverse and abundant in communities with ‘fast’ and ‘DIY’ root traits. Fungal biomass is strongly driven by plant species richness. Our results illustrate that the root economics space and plant species richness jointly determine the effects of plants on soil fungal communities and their potential role in plant fitness and ecosystem functioning.

Ecosystem functioning depends on biodiversity at multiple trophic levels, yet relationships between multitrophic diversity and ecosystem multifunctionality have been poorly explored, with studies often focusing on individual trophic levels and functions and on specific ecosystem types. Here, we show that plant diversity can affect ecosystem functioning both directly and by affecting other trophic levels. Using data on 13 trophic groups and 13 ecosystem functions from two large biodiversity experiments—one representing temperate grasslands and the other subtropical forests—we found that plant diversity increases multifunctionality through elevated multitrophic diversity. Across both experiments, the association between multitrophic diversity and multifunctionality was stronger than the relationship between the diversity of individual trophic groups and multifunctionality. Our results also suggest that the role of multitrophic diversity is greater in forests than in grasslands. These findings imply that, to promote sustained ecosystem multifunctionality, conservation planning must consider the diversity of both plants and higher trophic levels.

The relationship between biodiversity and ecosystem functioning strengthens with ecosystem age. However, the interplay between the plant diversity - ecosystem functioning relationship and Glomeromycotinian arbuscular mycorrhizal fungi (AMF) community assembly has not yet been scrutinized in this context, despite AMF’s role in plant survival and niche exploration. We study the development of AMF communities by disentangling soil- and plant-driven effects from calendar year effects. Within a long-term grassland biodiversity experiment, the pre-existing plant communities of varying plant diversity were re-established as split plots with combinations of common plant and soil histories: split plots with neither common plant nor soil history, with only soil but no plant history, and with both common plant and soil history. We found that bulk soil AMF communities were primarily shaped by common soil history, and additional common plant history had little effect. Further, the steepness of AMF diversity and plant diversity relationship did not strengthen over time, but AMF community evenness increased with common history. Specialisation of AMF towards plant species was low throughout, giving no indication of AMF communities specialising or diversifying over time. The potential of bulk soil AMF as mediators of variation in plant and microbial biomass over time and hence as drivers of biodiversity and ecosystem relationships was low. Our results suggest that soil processes may be key for the build-up of plant community-specific mycorrhizal communities with likely feedback effects on ecosystem productivity, but the plant-available mycorrhizal pool in bulk soil itself does not explain the strengthening of biodiversity and ecosystem relationships over time.

Trait-based approaches have been increasingly used to relate plants to soil microbial communities. However, the plant organs mediating this plant-microbe interaction – the roots – have been largely overlooked. The recent discovery of the root economics space offers a predictive framework for the structure of soil microbial communities, and specifically soil-borne fungal communities. Applying this novel approach, our study in a grassland plant diversity experiment reveals distinct root trait strategies at the level of the plant community. In addition to significant effects of plant species richness, we show that both axes of the root economics space – the collaboration and conservation gradient – are strong drivers of the composition of the different guilds of soil fungi, including saprotrophic, plant pathogenic, and mycorrhizal fungi. Our results illustrate that the root economics space and plant species richness jointly determine the effects of plants on fungal communities and their potential role in plant health and ecosystem functioning.