Biodiversity underpins critical ecological processes, yet its relationship with phosphorus (P) remains poorly understood globally. In particular, it is essential to explore how plant diversity and soil microbial diversity respond distinctly to changes in P availability on a global scale. This study combines meta-analysis and natural gradient analysis to assess the responses of plant and soil microbial diversity to altered P supply at a global scale. Specifically, we conducted a meta-analysis using 393 observations from 128 field P addition experiments, complemented by a natural gradient analysis of forest tree diversity and vascular plant diversity. Our results showed that P additions reduced plant richness by 8.5% and the Shannon index by 1.3% in global grasslands, while exerting minimal effects on soil bacterial and fungal diversity across major terrestrial ecosystems worldwide. Natural gradient analysis showed that both forest tree richness and vascular plant richness were significantly related to soil total P concentration, either positively or negatively, depend on the environmental variables controlled for partial correlation analyses. These findings collectively suggest that plant diversity exhibits greater sensitivity to altered soil P availability than soil microbial diversity. Therefore, improved understanding of the distinct responses of above- and below-ground biodiversity to altered nutrient supply provides a scientific foundation for implementing sound management practices of terrestrial functions and processes.