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

Grasslands are integral to maintaining biodiversity and key ecosystem services worldwide. Plant and soil biodiversity, and their interactions, support the provision of multiple ecosystem functions (multifunctionality). However, whether plant and soil biodiversity can explain a unique portion of variation in ecosystem functions as their relative contribution in supporting multifunctionality across global grasslands remains virtually unknown. This lack of knowledge hampers our capacity to predict and manage the impact of biodiversity losses on ecosystem functions under climate change in grasslands worldwide. Here, we report results from a global field survey of 101 grasslands, and a novel microcosm study, controlling for both plant and soil microbial diversity, aimed to identify the individual and interactive contribution of above- and belowground diversity to support multifunctionality under contrasting climatic conditions. Our combined approach provides strong evidence that, after accounting for global aridity levels and experimental drought, plant and soil biodiversity independently predict a unique portion of variation in above- and belowground ecosystem functioning, suggesting that both types of biodiversity complement each other to explain ecosystem functioning across global grasslands, suggesting that both types of biodiversity complement to explain function across global grasslands. We further deliver empirical evidence that interactions between above- and belowground diversity regulate specific ecosystem services such as primary productivity and, nutrient storage and plant productivity. We also found some context dependencies in the relationships of plant and soil biodiversity with multifunctionality. Soil fungal diversity was strongly associated to multifunctionality in less arid regions whilst 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 and indicate that climate change may shift the relative contribution of plant and soil biodiversity to multifunctionality across global grasslands.