Precipitation alters the relationship between biodiversity and multifunctionality of grassland ecosystems.

Precipitation changes largely influence the relationship between biodiversity and ecosystem multifunctionality (EMF). Our understanding of how biodiversity at multiple trophic levels regulates EMF under different precipitation conditions and how the relative importance of biodiversity at these different trophic levels to EMF changes dynamically along the precipitation gradient still needs to be improved. This study evaluated how the relationship between plant diversity, soil biodiversity, and EMF responds to precipitation changes using information obtained on biomes (including plants and soil organisms) and ecological functional traits. We collected 120 samples at eight representative stations along a 3177 km precipitation gradient (mean annual precipitation from 268.4 to 722.9 mm) in the northeastern Tibetan Plateau. We investigated the relationship between plant diversity, soil bacterial and fungal diversity, soil ciliate diversity, and EMF along the precipitation gradient. The results showed that across the precipitation gradient, the functional richness of plant diversity was the strongest predictor of EMF, effectively driving EMF over a wide threshold interval from 10% to 99%, with a maximum effect size of 0.27. The relative importance of plant diversity and soil biodiversity on EMF changes around a mean annual precipitation (MAP) of 450 mm. Plant diversity has a significant positive effect on EMF when MAP is above 463 mm. Soil biodiversity is more critical for EMF when MAP is below 428 mm. Our study shows that the impact of plant and soil biomes on EMF changes dynamically along a precipitation gradient. We identified a critical precipitation threshold of approximately 450 mm MAP, the dividing line between semi-arid and sub-humid climates. Our study highlights that the loss of plant and soil biodiversity may have severe consequences under low and high precipitation conditions, respectively, calling for developing biodiversity conservation strategies in response to climate change to avoid impacts on grassland ecosystem services.