Background and aims <p>Plant-associated microbiota play a key role in mediating the effect of plant diversity on productivity. However, previous studies have generally focused on either above- or belowground microbiota. As a result, the interplay between above- and belowground plant-associated microbiota in shaping plant diversity-productivity relationships has rarely been investigated.</p> Methods <p>We conducted microbial inoculation experiment in plant communities of varying diversity to determine how above- and belowground plant-associated microbiota interact to influence plant diversity-productivity relationships. We established herbaceous plant communities with 15 different compositions at four species richness levels in the greenhouse, and applied microbial inoculum collected from the field onto plant leaves and soil using a full factorial design.</p> Results <p>We found that leaf inoculum reduced plant productivity especially at lower plant diversity, promoting a positive diversity-productivity relationship. In contrast, soil inoculum counteracted leaf inoculation effects at lower diversity, thus weakening the positive diversity-productivity relationship. We showed that the diversity and relative abundance of arbuscular mycorrhizal fungi in soil increased at lower plant diversity when leaf inoculum was applied.</p> Conclusion <p>Our findings suggest that plant diversity promotes productivity through the suppression of foliar pathogens, but this positive diversity effect can be compromised by belowground plant-beneficial microbiota. This result highlights that simultaneous consideration of plant diversity, aboveground and belowground microbiota is required to enhance ecosystem productivity.</p>