1. Plant populations can exhibit local adaptation to their abiotic environment, such as climate and soil properties, as well as biotic components such as the chemical signatures of dominant plant species and mutualistic and pathogenic microbial populations. While patterns of local adaptation in individual species are widely recorded, the importance of microevolutionary processes for plant community assembly and function is poorly understood.2. Here, we examined how a history of long-term co-existence, and thus potential for local co-adaptation, influenced the process of plant community assembly.Soil inocula and seeds of eight plant species were collected from three calcareous grasslands with a long history of grazing within a single geographical region.Mesocosm communities were established using local genotypes from a single site or an artificial mixture of genotypes from two different sites. To investigate the role of root exudates and local ('home') and non-local ('away') soil biota as mediators of plant species co-existence, the population origin treatment was combined with the addition of activated carbon, which is known to adsorb exudates from soil, and sterilization of soil inocula. Individual-, species-and mesocosm-level responses were measured over the course of three growing seasons.3. We found that root exudates promoted seedling survival, species co-existence and productivity in assemblages of genotypes originating from the same community but had a weak impact in mixed, novel communities. Soil biota promoted the growth of subordinate forbs and restrained the growth of dominant graminoids, particularly in communities composed of local genotypes. The effects of population origin were significant in the first 2 years of the experiment but were not detectable in the third year when interbreeding and new seedling establishment took place. Plant genotypes coupled with 'home' microbial inoculum experienced a stronger reduction in growth compared with genotypes exposed to 'away' inoculum, indicating that plants experienced home-field disadvantage in interactions with soil biota. Synthesis.Our study demonstrates that the mechanisms of initial grassland community assembly depend on community history, with below-ground chemical 2612 | Journal of Ecology SEMCHENKO Et al.

Question Plant community assembly has traditionally been viewed from the perspective of plant resource niches and competition and, more recently, facilitation in stressful environments. However, plants also engage in biotic interactions with neighbours and soil microbiome by producing chemically diverse root exudates. Root exudates have the potential to affect plant community productivity and composition by mediating plant interactions, fostering beneficial microbial interactions and affecting nutrient cycling. However, empirical evidence for the role of root exudates in plant community dynamics is still lacking, particularly under field conditions. Location Two calcareous grasslands in Western Estonia, Europe. Methods We tested the role of root exudates in shaping plant community productivity and composition by applying activated carbon powder, known to adsorb root exudates from soil solution, to two intact species‐rich grasslands. The experiment continued for four years and was combined with fertiliser application. Results We found that the application of activated carbon triggered significant changes in root morphology, plant productivity and growth form composition that were distinct from the effects of fertilisation. Community responses were site‐specific and varied with the duration of activated carbon application. Activated carbon addition increased the production of finer roots in a community with shallow soil in the first year of the experiment and significantly reduced above‐ground productivity at both sites by the end of the fourth growth season. The application of activated carbon promoted legume abundance at both sites and suppressed forb dominance at the site with shallow soil. Conclusion Our findings suggest that chemical interactions in the plant rhizosphere may play an important role in shaping grassland productivity and plant community composition. We also show that activated carbon can be used as an effective tool for manipulating plant interactions in intact grasslands occupied by long‐lived individuals.