Plant responses to diversity‐driven selection and associated rhizosphere microbial communities

Plant diversity loss can alter plant–plant and plant–rhizosphere microbiome interactions. These altered interactions, in turn, may exert diversity‐driven selection pressure to which plants respond with phenotypic changes. Diverse plant communities may favour the survival and fitness of individuals with traits that avoid competition. Conversely, monocultures may accumulate species‐specific pests favouring greater investment in defence traits. Yet, it is unknown how altered plant rhizosphere interactions influence the plant diversity‐driven selection for altered plant phenotypes. We tested for plant diversity‐driven selection on plant above‐ground traits and how these traits are modified by their rhizosphere microbial communities after 11 years in experimental plant monocultures and mixtures. Plants propagated from monocultures or mixtures were grown in combination with their ‘home’ versus ‘away’ arbuscular mycorrhizal fungi (AMF) or non‐AMF microbes in two separate experiments using five and eight plant species, respectively. We hypothesized that plants in monocultures may be selected for better defence and better performance in association with rhizosphere microbial communities compared with plants in mixtures. Monoculture and mixture plants significantly differed in their above‐ground phenotypes. As predicted, plant traits related to defence (greater leaf mass per area and leaf dry matter content, reduced leaf damage) were more pronounced in monoculture plants in both experiments. Effects of the rhizosphere microbial communities, which generally enhanced plant growth, tended to be species‐specific. Significant three‐way interactions between diversity‐driven selection, AMF treatment and plant species showed that home versus away effects could be positive or negative, depending on plant species. We conclude that long‐term differences in plant diversity lead to selection for altered plant phenotypes. Such differences may be further modified in association with the AMF microbial communities derived from the different plant diversity treatments, but often outcomes are species‐specific. This suggests that plant species differ in their capacity to respond to diversity loss and associated changes in rhizosphere microbial communities, making it complicated to predict community‐level responses to such loss. A free Plain Language Summary can be found within the Supporting Information of this article.