Belowground Eukaryotes Dominate Biodiversity–Multifunctionality Relationships Across Coastal and Other Ecosystems Globally

Aim Belowground biodiversity is key to ecosystem multifunctionality, yet the strength and taxonomic drivers of biodiversity–multifunctionality (BMF) relationships likely vary across ecosystems. Whether ecosystem type modulates these relationships, however, remains largely untested due to a lack of cross‐ecosystem evidence. Coastal ecosystems comprising mudflats, salt marshes and mangroves provide a natural gradient of aboveground plant coverage, offering an ideal opportunity to address this knowledge gap. Location Mudflats, salt marshes and mangroves along the entire coastline of China. Time Period Current. Major Taxa Studied Bacteria, fungi, protists and metazoa. Method We combined a large‐scale survey along China's coastline with a meta‐analytic synthesis of 43 datasets across six ecosystems. We quantified 17 functions underpinning four key ecosystem services: plant production, carbon storage, nutrient supply and soil organic matter decomposition, and assessed the contributions of bacteria, fungi, protists and metazoans in maintaining the functions. Using beta regression, piecewise structural equation models and multilevel meta‐regressions we examined BMF relationships both in coastal ecosystems and across six other ecosystems globally. Results Despite supporting the highest multifunctionality, mangroves exhibited relatively weak positive BMF coupling (59% of functions positively supported). In contrast, salt marshes and mudflats exhibited stronger positive relationships (75%–88% of functions). Fungal diversity was linked to the broadest range of functions, while bacterial diversity showed variable and often negative associations. Globally, we found that eukaryote‐dominated positive BMF relationships were more prevalent in ecosystems with limited vegetation cover (e.g., grasslands, drylands) than in dense forests, mirroring the patterns observed in coastal ecosystems. Main Conclusions Our findings demonstrate that the contribution of belowground biodiversity to ecosystem multifunctionality varies systematically across ecosystems, with eukaryotes dominating positive BMF relationships in coastal and multiple other ecosystems globally. This highlights the importance of conserving belowground eukaryotic diversity, especially in ecosystems with low vegetation cover, to sustain multifunctionality.