Aridity and grazing regulate the effects of grassland biodiversity on the synergies/tradeoffs among ecosystem functions in drylands

Grassland is the crucial ecosystem that supports biodiversity and multiple ecosystem functions in drylands, with their relationship heavily regulated by both aridity and grazing. However, it is not clear under which aridity conditions and what grazing intensity can maximize the positive effect of biodiversity on enhancing multiple ecosystem functions. Here, we related biodiversity (plant and soil microbial diversity) to ecosystem functions (ecosystem multifunctionality [EMF], individual ecosystem function, and the tradeoffs and synergies among ecosystem functions) across 91 sites along aridity and grazing gradients in natural grasslands in drylands. We showed that the positive correlation between plant diversity and ecosystem functions and their synergies was overall stronger in arid regions than in semi-arid regions. Aridity and grazing interactively affected the biodiversity-ecosystem function (BEF) relationships. In arid regions, ecosystem functions and their synergies were positively correlated with plant diversity under low and moderate grazing intensity, and were negatively related to microbial diversity under high grazing intensity. In arid regions, the tradeoff between productivity and carbon storage was positively related to soil microbial diversity under low and moderate grazing intensity, but showed the opposite change under high grazing intensity. In semi-arid regions, the tradeoff was positively related to soil microbial diversity under high grazing intensity. Moreover, we found that greater aridity and increased grazing intensity directly and indirectly affected the tradeoffs and synergies among ecosystem functions mainly by altering soil properties (e.g., soil clay content and pH) and plant diversity. Together, we reveal that under the predicted intensified aridity scenario, the conservation of plant diversity is pivotal for facilitating the synergistic enhancement of multiple ecosystem functions under less stress conditions, whereas the regulation of soil microbial diversity becomes increasingly critical under more stress conditions in drylands with long grazing history. We emphasize the importance of designing targeted management plans based on aridity levels and grazing intensity to enhance ecosystem functions. It provides a reasonable basis for decision-making on how to optimize the grassland management to coordinate ecosystem functions.