The coupling of bacterial and fungal diversity under land-use conversion in global grassland regions is limited by perturbation intensity

The dynamics of bacterial and fungal communities in grasslands are strongly affected by land-use conversion, but their large-scale diversity responses remain unclear. We quantified the effects of grassland-use conversion (i.e., native grassland converted to cropland, forest, or artificial grassland) on soil microbial alpha diversity (Chao1 richness and Shannon diversity) using 422 observations from 94 publications. Overall, grassland-use conversion significantly reduced fungal Shannon diversity (5.1 %), whereas it marginally increased bacterial Shannon diversity (2.0 %) and Chao1 richness (6.0 %). These effects were driven by conversion types associated with high perturbation, namely conversion to cropland or forest, and by cultivation or succession management. The responses of bacterial and fungal Chao1 richness were positively coupled under land-use conversion but decoupled in cropland ecosystems and under cultivation management. The positive coupling of bacterial and fungal Shannon diversity was not statistically significant until the perturbation associated with conversion converged at a lower level: when the bacterial diversity change induced by land-use conversion was within ± 30 %. This coupling was regulated by nutrient availability in soil (nitrate and available phosphorus). Our findings highlight the limited and coupled responses of bacterial and fungal diversity to grassland-use conversion, in a manner regulated by perturbation intensity. Land-use conversion and management should be undertaken cautiously with the goals of conserving biodiversity and soil function potential in grassland regions.