Complementary effects of above- and belowground biodiversity on ecosystem functions across global grasslands

Grasslands are integral to maintaining biodiversity and key ecosystem services under climate change. Plant and soil biodiversity, and their interactions, support the provision of multiple ecosystem functions (multifunctionality). However, whether plant and soil biodiversity explain unique, or shared, contributions to supporting multifunctionality across global grasslands remains virtually unknown. Here, we combine results from a global survey of 101 grasslands with a novel microcosm study, controlling for both plant and soil microbial diversity to identify their individual and interactive contribution to support multifunctionality under aridity and experimental drought. We found that, plant and soil microbial diversity independently predict a unique portion of variation in above- and belowground functioning, suggesting both types of biodiversity complement each other. Interactions between plant and soil microbial diversity regulated primary productivity, nutrient storage, and plant productivity. Our findings were also context dependent, since soil fungal diversity was strongly associated to multifunctionality in less arid regions, while plant diversity was strongly linked to multifunctionality in more arid regions. Our results highlight the need to conserve both above- and belowground diversity to sustain grassland multifunctionality in a drier world and indicate climate change may shift the relative contribution of plant and soil biodiversity to multifunctionality across global grasslands.