Relationships between plant diversity and soil microbial diversity vary across taxonomic groups and spatial scales

Plant diversity has long been assumed to predict soil microbial diversity. However, contradictory results have been found when examining their relationships, particularly at broad spatial scales. To address this issue, we conducted a meta-analysis to evaluate the patterns in the correlation between plant diversity and soil microbial diversity and the underlying factors driving the relationship. We collected correlation data from 84 studies covering more than 3900 natural terrestrial samples globally. Using the hierarchical mixed-effects model, we investigated factors including targeted taxonomic group, microbial examination method, sampling extent, biome type, soil type, and environmental factors to assess the patterns of the plant-microbial correlation and the determinants of their variations. We found that microbial richness displayed a modest but positive correlation with plant diversity (r = 0.333, CI = 0.220-0.437). In spite of variability among taxonomic groups and their relationship with plant diversity, positive correlations were more pronounced in the intermediate sampling extent of latitude and elevation coverage, and tropical forests. Among examined environmental factors, soil pH was negatively associated with the plant and soil microbial relationships at large spatial scales. The plant-microbial correlation appears more sensitive to edaphic factor variation in the poor nutrients and soil less compact systems. Collectively, our results point to key differences across taxonomic groups, spatial scales and biomes, and the modulating effects of climate and soil. The findings shed light on our deep understanding in plant-microbial diversity relationships at broad spatial scales and ecosystem sensitivity to biodiversity loss and environmental change.