There is widespread concern that cessation of grazing in historically grazed ecosystems is causing biotic homogenization and biodiversity loss. Here, we used 12 montane grassland sites along an 800-km north-south gradient across the United Kingdom, to test whether cessation of grazing affects local ɑ- and β-diversity of belowground food webs. We show that cessation of grazing leads to strongly decreased ɑ-diversity of both soil microbial and faunal diversity. In contrast, the β-diversity varied between groups of soil organisms. While soil microbial communities exhibited increased homogenization after cessation of grazing, we observed decreased homogenization for soil fauna after cessation of grazing. Overall, our results indicate that grazer exclusion from historically grazed montane grasslands has far-ranging consequences for the diversity and composition of belowground food webs, and underscore the importance of grazers for maintaining the diversity of belowground communities, which play a central role in ecosystem functioning.